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Tutorial: Create a web API with ASP.NET Core

• Article
• 11/08/2022
• 63 minutes to read

In this article

By Rick Anderson and Kirk Larkin

This tutorial teaches the basics of building a web API using a database.

Overview

This tutorial creates the following API:

The following diagram shows the design of the app.

Prerequisites

Create a web project

• Visual Studio
• Visual Studio Code
• Visual Studio for Mac

Test the project

The project template creates a WeatherForecast API with support for Swagger.

• Visual Studio
• Visual Studio Code
• Visual Studio for Mac

Press Ctrl+F5 to run without the debugger.

Visual Studio displays the following dialog when a project is not yet configured to use SSL:

Select Yes if you trust the IIS Express SSL certificate.

The following dialog is displayed:

Select Yes if you agree to trust the development certificate.

For information on trusting the Firefox browser, see Firefox SEC_ERROR_INADEQUATE_KEY_USAGE certificate error.

Visual Studio launches the default browser and navigates to https://localhost:<port>/swagger/index.html, where <port> is a randomly chosen port number.

The Swagger page /swagger/index.html is displayed. Select GET > Try it out > Execute. The page displays:

• The Curl command to test the WeatherForecast API.
• The URL to test the WeatherForecast API.
• The response code, body, and headers.
• A drop-down list box with media types and the example value and schema.

If the Swagger page doesn't appear, see this GitHub issue.

Swagger is used to generate useful documentation and help pages for web APIs. This tutorial focuses on creating a web API. For more information on Swagger, see ASP.NET Core web API documentation with Swagger / OpenAPI.

Copy and paste the Request URL in the browser: https://localhost:<port>/weatherforecast

JSON similar to the following example is returned:

[
    {
        "date": "2019-07-16T19:04:05.7257911-06:00",
        "temperatureC": 52,
        "temperatureF": 125,
        "summary": "Mild"
    },
    {
        "date": "2019-07-17T19:04:05.7258461-06:00",
        "temperatureC": 36,
        "temperatureF": 96,
        "summary": "Warm"
    },
    {
        "date": "2019-07-18T19:04:05.7258467-06:00",
        "temperatureC": 39,
        "temperatureF": 102,
        "summary": "Cool"
    },
    {
        "date": "2019-07-19T19:04:05.7258471-06:00",
        "temperatureC": 10,
        "temperatureF": 49,
        "summary": "Bracing"
    },
    {
        "date": "2019-07-20T19:04:05.7258474-06:00",
        "temperatureC": -1,
        "temperatureF": 31,
        "summary": "Chilly"
    }
]

Add a model class

A model is a set of classes that represent the data that the app manages. The model for this app is the TodoItem class.

• Visual Studio
• Visual Studio Code
• Visual Studio for Mac

namespace TodoApi.Models;

public class TodoItem
{
    public long Id { get; set; }
    public string? Name { get; set; }
    public bool IsComplete { get; set; }
}

The Id property functions as the unique key in a relational database.

Model classes can go anywhere in the project, but the Models folder is used by convention.

Add a database context

The database context is the main class that coordinates Entity Framework functionality for a data model. This class is created by deriving from the Microsoft.EntityFrameworkCore.DbContext class.

• Visual Studio
• Visual Studio Code / Visual Studio for Mac

• Enter the following code:

  using Microsoft.EntityFrameworkCore;
  
  namespace TodoApi.Models;
  
  public class TodoContext : DbContext
  {
      public TodoContext(DbContextOptions<TodoContext> options)
          : base(options)
      {
      }
  
      public DbSet<TodoItem> TodoItems { get; set; } = null!;
  }
  

Register the database context

In ASP.NET Core, services such as the DB context must be registered with the dependency injection (DI) container. The container provides the service to controllers.

Update Program.cs with the following highlighted code:

using Microsoft.EntityFrameworkCore;
using TodoApi.Models;

var builder = WebApplication.CreateBuilder(args);

builder.Services.AddControllers();
builder.Services.AddDbContext<TodoContext>(opt =>
    opt.UseInMemoryDatabase("TodoList"));
builder.Services.AddEndpointsApiExplorer();
builder.Services.AddSwaggerGen();

var app = builder.Build();

if (app.Environment.IsDevelopment())
{
    app.UseSwagger();
    app.UseSwaggerUI();
}

app.UseHttpsRedirection();

app.UseAuthorization();

app.MapControllers();

app.Run();

The preceding code:

• Adds using directives.
• Adds the database context to the DI container.
• Specifies that the database context will use an in-memory database.

Scaffold a controller

• Visual Studio
• Visual Studio Code / Visual Studio for Mac

dotnet add package Microsoft.VisualStudio.Web.CodeGeneration.Design -v 7.0.0-*
dotnet add package Microsoft.EntityFrameworkCore.Design -v 7.0.0-*
dotnet add package Microsoft.EntityFrameworkCore.SqlServer -v 7.0.0-*
dotnet tool uninstall -g dotnet-aspnet-codegenerator
dotnet tool install -g dotnet-aspnet-codegenerator

dotnet-aspnet-codegenerator controller -name TodoItemsController -async -api -m TodoItem -dc TodoContext -outDir Controllers

The generated code:

• Marks the class with the [ApiController] attribute. This attribute indicates that the controller responds to web API requests. For information about specific behaviors that the attribute enables, see Create web APIs with ASP.NET Core.
• Uses DI to inject the database context (TodoContext) into the controller. The database context is used in each of the CRUD methods in the controller.

The ASP.NET Core templates for:

• Controllers with views include [action] in the route template.
• API controllers don't include [action] in the route template.

When the [action] token isn't in the route template, the action name (method name) isn't included in the endpoint. That is, the action's associated method name isn't used in the matching route.

Update the PostTodoItem create method

Update the return statement in the PostTodoItem to use the nameof operator:

[HttpPost]
public async Task<ActionResult<TodoItem>> PostTodoItem(TodoItem todoItem)
{
    _context.TodoItems.Add(todoItem);
    await _context.SaveChangesAsync();

    //    return CreatedAtAction("GetTodoItem", new { id = todoItem.Id }, todoItem);
    return CreatedAtAction(nameof(GetTodoItem), new { id = todoItem.Id }, todoItem);
}

The preceding code is an HTTP POST method, as indicated by the [HttpPost] attribute. The method gets the value of the TodoItemfrom the body of the HTTP request.

For more information, see Attribute routing with Http[Verb] attributes.

The CreatedAtAction method:

• Returns an HTTP 201 status code if successful. HTTP 201 is the standard response for an HTTP POST method that creates a new resource on the server.
• Adds a Location header to the response. The Location header specifies the URI of the newly created to-do item. For more information, see 10.2.2 201 Created.
• References the GetTodoItem action to create the Location header's URI. The C# nameof keyword is used to avoid hard-coding the action name in the CreatedAtAction call.

Test PostTodoItem

• Press Ctrl+F5 to run the app.

• In the Swagger browser window, select POST /api/TodoItems, and then select Try it out.

• In the Request body input window, update the JSON. For example,

  {
    "name": "walk dog",
    "isComplete": true
  }
  

• Select Execute

  

Test the location header URI

In the preceding POST, the Swagger UI shows the location header under Response headers. For example, location: https://localhost:7260/api/TodoItems/1. The location header shows the URI to the created resource.

To test the location header:

• In the Swagger browser window, select GET /api/TodoItems/{id}, and then select Try it out.

• Enter 1 in the id input box, and then select Execute.

  

Examine the GET methods

Two GET endpoints are implemented:

• GET /api/todoitems
• GET /api/todoitems/{id}

The previous section showed an example of the /api/todoitems/{id} route.

Follow the POST instructions to add another todo item, and then test the /api/todoitems route using Swagger.

This app uses an in-memory database. If the app is stopped and started, the preceding GET request will not return any data. If no data is returned, POST data to the app.

Routing and URL paths

The [HttpGet] attribute denotes a method that responds to an HTTP GET request. The URL path for each method is constructed as follows:

• Start with the template string in the controller's Route attribute:

  [Route("api/[controller]")]
  [ApiController]
  public class TodoItemsController : ControllerBase
  

• Replace [controller] with the name of the controller, which by convention is the controller class name minus the "Controller" suffix. For this sample, the controller class name is TodoItemsController, so the controller name is "TodoItems". ASP.NET Core routing is case insensitive.

• If the [HttpGet] attribute has a route template (for example, [HttpGet("products")]), append that to the path. This sample doesn't use a template. For more information, see Attribute routing with Http[Verb] attributes.

In the following GetTodoItem method, "{id}" is a placeholder variable for the unique identifier of the to-do item. When GetTodoItem is invoked, the value of "{id}" in the URL is provided to the method in its id parameter.

[HttpGet("{id}")]
public async Task<ActionResult<TodoItem>> GetTodoItem(long id)
{
    var todoItem = await _context.TodoItems.FindAsync(id);

    if (todoItem == null)
    {
        return NotFound();
    }

    return todoItem;
}

Return values

The return type of the GetTodoItems and GetTodoItem methods is ActionResult<T> type. ASP.NET Core automatically serializes the object to JSON and writes the JSON into the body of the response message. The response code for this return type is 200 OK, assuming there are no unhandled exceptions. Unhandled exceptions are translated into 5xx errors.

ActionResult return types can represent a wide range of HTTP status codes. For example, GetTodoItem can return two different status values:

• If no item matches the requested ID, the method returns a 404 status NotFound error code.
• Otherwise, the method returns 200 with a JSON response body. Returning item results in an HTTP 200 response.

The PutTodoItem method

Examine the PutTodoItem method:

[HttpPut("{id}")]
public async Task<IActionResult> PutTodoItem(long id, TodoItem todoItem)
{
    if (id != todoItem.Id)
    {
        return BadRequest();
    }

    _context.Entry(todoItem).State = EntityState.Modified;

    try
    {
        await _context.SaveChangesAsync();
    }
    catch (DbUpdateConcurrencyException)
    {
        if (!TodoItemExists(id))
        {
            return NotFound();
        }
        else
        {
            throw;
        }
    }

    return NoContent();
}

PutTodoItem is similar to PostTodoItem, except it uses HTTP PUT. The response is 204 (No Content). According to the HTTP specification, a PUT request requires the client to send the entire updated entity, not just the changes. To support partial updates, use HTTP PATCH.

Test the PutTodoItem method

This sample uses an in-memory database that must be initialized each time the app is started. There must be an item in the database before you make a PUT call. Call GET to ensure there's an item in the database before making a PUT call.

Using the Swagger UI, use the PUT button to update the TodoItem that has Id = 1 and set its name to "feed fish". Note the response is HTTP 204 No Content.

The DeleteTodoItem method

Examine the DeleteTodoItem method:

[HttpDelete("{id}")]
public async Task<IActionResult> DeleteTodoItem(long id)
{
    var todoItem = await _context.TodoItems.FindAsync(id);
    if (todoItem == null)
    {
        return NotFound();
    }

    _context.TodoItems.Remove(todoItem);
    await _context.SaveChangesAsync();

    return NoContent();
}

Test the DeleteTodoItem method

Use the Swagger UI to delete the TodoItemthat has Id = 1. Note the response is HTTP 204 No Content.

Test with http-repl, Postman, or curl

http-repl, Postman, and curl are often used to test API's. Swagger uses curl and shows the curl command it submitted.

For instructions on these tools, see the following links:

• Test APIs with Postman
• Install and test APIs with http-repl

For more information on http-repl, see Test web APIs with the HttpRepl.

Prevent over-posting

Currently the sample app exposes the entire TodoItem object. Production apps typically limit the data that's input and returned using a subset of the model. There are multiple reasons behind this, and security is a major one. The subset of a model is usually referred to as a Data Transfer Object (DTO), input model, or view model. DTO is used in this tutorial.

A DTO may be used to:

• Prevent over-posting.
• Hide properties that clients are not supposed to view.
• Omit some properties in order to reduce payload size.
• Flatten object graphs that contain nested objects. Flattened object graphs can be more convenient for clients.

To demonstrate the DTO approach, update the TodoItem class to include a secret field:

namespace TodoApi.Models
{
    public class TodoItem
    {
        public long Id { get; set; }
        public string? Name { get; set; }
        public bool IsComplete { get; set; }
        public string? Secret { get; set; }
    }
}

The secret field needs to be hidden from this app, but an administrative app could choose to expose it.

Verify you can post and get the secret field.

Create a DTO model:

namespace TodoApi.Models;

public class TodoItemDTO
{
    public long Id { get; set; }
    public string? Name { get; set; }
    public bool IsComplete { get; set; }
}

Update the TodoItemsController to use TodoItemDTO:

using Microsoft.AspNetCore.Mvc;
using Microsoft.EntityFrameworkCore;
using TodoApi.Models;

namespace TodoApi.Controllers;

[Route("api/[controller]")]
[ApiController]
public class TodoItemsController : ControllerBase
{
    private readonly TodoContext _context;

    public TodoItemsController(TodoContext context)
    {
        _context = context;
    }

    // GET: api/TodoItems
    [HttpGet]
    public async Task<ActionResult<IEnumerable<TodoItemDTO>>> GetTodoItems()
    {
        return await _context.TodoItems
            .Select(x => ItemToDTO(x))
            .ToListAsync();
    }

    // GET: api/TodoItems/5
    // <snippet_GetByID>
    [HttpGet("{id}")]
    public async Task<ActionResult<TodoItemDTO>> GetTodoItem(long id)
    {
        var todoItem = await _context.TodoItems.FindAsync(id);

        if (todoItem == null)
        {
            return NotFound();
        }

        return ItemToDTO(todoItem);
    }
    // </snippet_GetByID>

    // PUT: api/TodoItems/5
    // To protect from overposting attacks, see https://go.microsoft.com/fwlink/?linkid=2123754
    // <snippet_Update>
    [HttpPut("{id}")]
    public async Task<IActionResult> PutTodoItem(long id, TodoItemDTO todoDTO)
    {
        if (id != todoDTO.Id)
        {
            return BadRequest();
        }

        var todoItem = await _context.TodoItems.FindAsync(id);
        if (todoItem == null)
        {
            return NotFound();
        }

        todoItem.Name = todoDTO.Name;
        todoItem.IsComplete = todoDTO.IsComplete;

        try
        {
            await _context.SaveChangesAsync();
        }
        catch (DbUpdateConcurrencyException) when (!TodoItemExists(id))
        {
            return NotFound();
        }

        return NoContent();
    }
    // </snippet_Update>

    // POST: api/TodoItems
    // To protect from overposting attacks, see https://go.microsoft.com/fwlink/?linkid=2123754
    // <snippet_Create>
    [HttpPost]
    public async Task<ActionResult<TodoItemDTO>> PostTodoItem(TodoItemDTO todoDTO)
    {
        var todoItem = new TodoItem
        {
            IsComplete = todoDTO.IsComplete,
            Name = todoDTO.Name
        };

        _context.TodoItems.Add(todoItem);
        await _context.SaveChangesAsync();

        return CreatedAtAction(
            nameof(GetTodoItem),
            new { id = todoItem.Id },
            ItemToDTO(todoItem));
    }
    // </snippet_Create>

    // DELETE: api/TodoItems/5
    [HttpDelete("{id}")]
    public async Task<IActionResult> DeleteTodoItem(long id)
    {
        var todoItem = await _context.TodoItems.FindAsync(id);
        if (todoItem == null)
        {
            return NotFound();
        }

        _context.TodoItems.Remove(todoItem);
        await _context.SaveChangesAsync();

        return NoContent();
    }

    private bool TodoItemExists(long id)
    {
        return _context.TodoItems.Any(e => e.Id == id);
    }

    private static TodoItemDTO ItemToDTO(TodoItem todoItem) =>
       new TodoItemDTO
       {
           Id = todoItem.Id,
           Name = todoItem.Name,
           IsComplete = todoItem.IsComplete
       };
}

Verify you can't post or get the secret field.

Call the web API with JavaScript

See Tutorial: Call an ASP.NET Core web API with JavaScript.

Web API video series

See Video: Beginner's Series to: Web APIs.

In this tutorial, you learn how to:

• Create a web API project.
• Add a model class and a database context.
• Scaffold a controller with CRUD methods.
• Configure routing, URL paths, and return values.
• Call the web API with http-repl.

At the end, you have a web API that can manage "to-do" items stored in a database.

Overview

This tutorial creates the following API:

The following diagram shows the design of the app.

Prerequisites

Create a web project

• Visual Studio
• Visual Studio Code
• Visual Studio for Mac

dotnet add package Microsoft.EntityFrameworkCore.InMemory

Test the project

The project template creates a WeatherForecast API with support for Swagger.

• Visual Studio
• Visual Studio Code
• Visual Studio for Mac

Press Ctrl+F5 to run without the debugger.

Visual Studio displays the following dialog when a project is not yet configured to use SSL:

Select Yes if you trust the IIS Express SSL certificate.

The following dialog is displayed:

Select Yes if you agree to trust the development certificate.

For information on trusting the Firefox browser, see Firefox SEC_ERROR_INADEQUATE_KEY_USAGE certificate error.

Visual Studio launches the default browser and navigates to https://localhost:<port>/swagger/index.html, where <port> is a randomly chosen port number.

The Swagger page /swagger/index.html is displayed. Select GET > Try it out > Execute. The page displays:

• The Curl command to test the WeatherForecast API.
• The URL to test the WeatherForecast API.
• The response code, body, and headers.
• A drop-down list box with media types and the example value and schema.

If the Swagger page doesn't appear, see this GitHub issue.

Swagger is used to generate useful documentation and help pages for web APIs. This tutorial focuses on creating a web API. For more information on Swagger, see ASP.NET Core web API documentation with Swagger / OpenAPI.

Copy and paste the Request URL in the browser: https://localhost:<port>/weatherforecast

JSON similar to the following example is returned:

[
    {
        "date": "2019-07-16T19:04:05.7257911-06:00",
        "temperatureC": 52,
        "temperatureF": 125,
        "summary": "Mild"
    },
    {
        "date": "2019-07-17T19:04:05.7258461-06:00",
        "temperatureC": 36,
        "temperatureF": 96,
        "summary": "Warm"
    },
    {
        "date": "2019-07-18T19:04:05.7258467-06:00",
        "temperatureC": 39,
        "temperatureF": 102,
        "summary": "Cool"
    },
    {
        "date": "2019-07-19T19:04:05.7258471-06:00",
        "temperatureC": 10,
        "temperatureF": 49,
        "summary": "Bracing"
    },
    {
        "date": "2019-07-20T19:04:05.7258474-06:00",
        "temperatureC": -1,
        "temperatureF": 31,
        "summary": "Chilly"
    }
]

Update the launchUrl

In Properties\launchSettings.json, update launchUrl from "swagger" to "api/todoitems":

"launchUrl": "api/todoitems",

Because Swagger will be removed, the preceding markup changes the URL that is launched to the GET method of the controller added in the following sections.

Add a model class

A model is a set of classes that represent the data that the app manages. The model for this app is a single TodoItem class.

• Visual Studio
• Visual Studio Code
• Visual Studio for Mac

namespace TodoApi.Models
{
    public class TodoItem
    {
        public long Id { get; set; }
        public string? Name { get; set; }
        public bool IsComplete { get; set; }
    }
}

The Id property functions as the unique key in a relational database.

Model classes can go anywhere in the project, but the Models folder is used by convention.

Add a database context

The database context is the main class that coordinates Entity Framework functionality for a data model. This class is created by deriving from the Microsoft.EntityFrameworkCore.DbContext class.

• Visual Studio
• Visual Studio Code
• Visual Studio for Mac

• Enter the following code:

  using Microsoft.EntityFrameworkCore;
  using System.Diagnostics.CodeAnalysis;
  
  namespace TodoApi.Models
  {
      public class TodoContext : DbContext
      {
          public TodoContext(DbContextOptions<TodoContext> options)
              : base(options)
          {
          }
  
          public DbSet<TodoItem> TodoItems { get; set; } = null!;
      }
  }
  

Register the database context

In ASP.NET Core, services such as the DB context must be registered with the dependency injection (DI) container. The container provides the service to controllers.

Update Program.cs with the following code:

using Microsoft.EntityFrameworkCore;
using TodoApi.Models;


var builder = WebApplication.CreateBuilder(args);

// Add services to the container.

builder.Services.AddControllers();
builder.Services.AddDbContext<TodoContext>(opt =>
    opt.UseInMemoryDatabase("TodoList"));
//builder.Services.AddSwaggerGen(c =>
//{
//    c.SwaggerDoc("v1", new() { Title = "TodoApi", Version = "v1" });
//});

var app = builder.Build();

// Configure the HTTP request pipeline.
if (builder.Environment.IsDevelopment())
{
    app.UseDeveloperExceptionPage();
    //app.UseSwagger();
    //app.UseSwaggerUI(c => c.SwaggerEndpoint("/swagger/v1/swagger.json", "TodoApi v1"));
}

app.UseHttpsRedirection();

app.UseAuthorization();

app.MapControllers();

app.Run();

The preceding code:

• Removes the Swagger calls.
• Removes unused using directives.
• Adds the database context to the DI container.
• Specifies that the database context will use an in-memory database.

Scaffold a controller

• Visual Studio
• Visual Studio Code
• Visual Studio for Mac

dotnet add package Microsoft.VisualStudio.Web.CodeGeneration.Design
dotnet add package Microsoft.EntityFrameworkCore.Design
dotnet add package Microsoft.EntityFrameworkCore.SqlServer
dotnet tool install -g dotnet-aspnet-codegenerator
dotnet-aspnet-codegenerator controller -name TodoItemsController -async -api -m TodoItem -dc TodoContext -outDir Controllers

The generated code:

• Marks the class with the [ApiController] attribute. This attribute indicates that the controller responds to web API requests. For information about specific behaviors that the attribute enables, see Create web APIs with ASP.NET Core.
• Uses DI to inject the database context (TodoContext) into the controller. The database context is used in each of the CRUD methods in the controller.

The ASP.NET Core templates for:

• Controllers with views include [action] in the route template.
• API controllers don't include [action] in the route template.

When the [action] token isn't in the route template, the action name is excluded from the route. That is, the action's associated method name isn't used in the matching route.

Update the PostTodoItem create method

Update the return statement in the PostTodoItem to use the nameof operator:

[HttpPost]
public async Task<ActionResult<TodoItem>> PostTodoItem(TodoItem todoItem)
{
    _context.TodoItems.Add(todoItem);
    await _context.SaveChangesAsync();

    //return CreatedAtAction("GetTodoItem", new { id = todoItem.Id }, todoItem);
    return CreatedAtAction(nameof(GetTodoItem), new { id = todoItem.Id }, todoItem);
}

The preceding code is an HTTP POST method, as indicated by the [HttpPost] attribute. The method gets the value of the to-do item from the body of the HTTP request.

For more information, see Attribute routing with Http[Verb] attributes.

The CreatedAtAction method:

• Returns an HTTP 201 status code if successful. HTTP 201 is the standard response for an HTTP POST method that creates a new resource on the server.
• Adds a Location header to the response. The Location header specifies the URI of the newly created to-do item. For more information, see 10.2.2 201 Created.
• References the GetTodoItem action to create the Location header's URI. The C# nameof keyword is used to avoid hard-coding the action name in the CreatedAtAction call.

Install http-repl

This tutorial uses http-repl to test the web API.

• Run the following command at a command prompt:

  dotnet tool install -g Microsoft.dotnet-httprepl
  

• If you don't have the .NET 6.0 SDK or runtime installed, install the .NET 6.0 runtime.

Test PostTodoItem

• Press Ctrl+F5 to run the app.

• Open a new terminal window, and run the following commands. If your app uses a different port number, replace 5001 in the httprepl command with your port number.

  httprepl https://localhost:5001/api/todoitems
  post -h Content-Type=application/json -c "{"name":"walk dog","isComplete":true}"
  

  Here's an example of the output from the command:

  HTTP/1.1 201 Created
  Content-Type: application/json; charset=utf-8
  Date: Tue, 07 Sep 2021 20:39:47 GMT
  Location: https://localhost:5001/api/TodoItems/1
  Server: Kestrel
  Transfer-Encoding: chunked
  
  {
    "id": 1,
    "name": "walk dog",
    "isComplete": true
  }
  

Test the location header URI

To test the location header, copy and paste it into an httprepl get command.

The following example assumes that you're still in an httprepl session. If you ended the previous httprepl session, replace connect with httprepl in the following commands:

connect https://localhost:5001/api/todoitems/1
get

Here's an example of the output from the command:

HTTP/1.1 200 OK
Content-Type: application/json; charset=utf-8
Date: Tue, 07 Sep 2021 20:48:10 GMT
Server: Kestrel
Transfer-Encoding: chunked

{
  "id": 1,
  "name": "walk dog",
  "isComplete": true
}

Examine the GET methods

Two GET endpoints are implemented:

• GET /api/todoitems
• GET /api/todoitems/{id}

You just saw an example of the /api/todoitems/{id} route. Test the /api/todoitems route:

connect https://localhost:5001/api/todoitems
get

Here's an example of the output from the command:

HTTP/1.1 200 OK
Content-Type: application/json; charset=utf-8
Date: Tue, 07 Sep 2021 20:59:21 GMT
Server: Kestrel
Transfer-Encoding: chunked

[
  {
    "id": 1,
    "name": "walk dog",
    "isComplete": true
  }
]

This time, the JSON returned is an array of one item.

This app uses an in-memory database. If the app is stopped and started, the preceding GET request will not return any data. If no data is returned, POST data to the app.

Routing and URL paths

The [HttpGet] attribute denotes a method that responds to an HTTP GET request. The URL path for each method is constructed as follows:

• Start with the template string in the controller's Route attribute:

  [Route("api/[controller]")]
  [ApiController]
  public class TodoItemsController : ControllerBase
  

• Replace [controller] with the name of the controller, which by convention is the controller class name minus the "Controller" suffix. For this sample, the controller class name is TodoItemsController, so the controller name is "TodoItems". ASP.NET Core routing is case insensitive.

• If the [HttpGet] attribute has a route template (for example, [HttpGet("products")]), append that to the path. This sample doesn't use a template. For more information, see Attribute routing with Http[Verb] attributes.

In the following GetTodoItem method, "{id}" is a placeholder variable for the unique identifier of the to-do item. When GetTodoItem is invoked, the value of "{id}" in the URL is provided to the method in its id parameter.

[HttpGet("{id}")]
public async Task<ActionResult<TodoItem>> GetTodoItem(long id)
{
    var todoItem = await _context.TodoItems.FindAsync(id);

    if (todoItem == null)
    {
        return NotFound();
    }

    return todoItem;
}

Return values

The return type of the GetTodoItems and GetTodoItem methods is ActionResult<T> type. ASP.NET Core automatically serializes the object to JSON and writes the JSON into the body of the response message. The response code for this return type is 200 OK, assuming there are no unhandled exceptions. Unhandled exceptions are translated into 5xx errors.

ActionResult return types can represent a wide range of HTTP status codes. For example, GetTodoItem can return two different status values:

• If no item matches the requested ID, the method returns a 404 status NotFound error code.
• Otherwise, the method returns 200 with a JSON response body. Returning item results in an HTTP 200 response.

The PutTodoItem method

Examine the PutTodoItem method:

[HttpPut("{id}")]
public async Task<IActionResult> PutTodoItem(long id, TodoItem todoItem)
{
    if (id != todoItem.Id)
    {
        return BadRequest();
    }

    _context.Entry(todoItem).State = EntityState.Modified;

    try
    {
        await _context.SaveChangesAsync();
    }
    catch (DbUpdateConcurrencyException)
    {
        if (!TodoItemExists(id))
        {
            return NotFound();
        }
        else
        {
            throw;
        }
    }

    return NoContent();
}

PutTodoItem is similar to PostTodoItem, except it uses HTTP PUT. The response is 204 (No Content). According to the HTTP specification, a PUT request requires the client to send the entire updated entity, not just the changes. To support partial updates, use HTTP PATCH.

If you get an error calling PutTodoItem in the following section, call GET to ensure there's an item in the database.

Test the PutTodoItem method

This sample uses an in-memory database that must be initialized each time the app is started. There must be an item in the database before you make a PUT call. Call GET to ensure there's an item in the database before making a PUT call.

Update the to-do item that has Id = 1 and set its name to "feed fish":

connect https://localhost:5001/api/todoitems/1
put -h Content-Type=application/json -c "{"id":1,"name":"feed fish","isComplete":true}"

Here's an example of the output from the command:

HTTP/1.1 204 No Content
Date: Tue, 07 Sep 2021 21:20:47 GMT
Server: Kestrel

The DeleteTodoItem method

Examine the DeleteTodoItem method:

[HttpDelete("{id}")]
public async Task<IActionResult> DeleteTodoItem(long id)
{
    var todoItem = await _context.TodoItems.FindAsync(id);
    if (todoItem == null)
    {
        return NotFound();
    }

    _context.TodoItems.Remove(todoItem);
    await _context.SaveChangesAsync();

    return NoContent();
}

Test the DeleteTodoItem method

Delete the to-do item that has Id = 1:

connect https://localhost:5001/api/todoitems/1
delete

Here's an example of the output from the command:

HTTP/1.1 204 No Content
Date: Tue, 07 Sep 2021 21:43:00 GMT
Server: Kestrel

Prevent over-posting

Currently the sample app exposes the entire TodoItem object. Production apps typically limit the data that's input and returned using a subset of the model. There are multiple reasons behind this, and security is a major one. The subset of a model is usually referred to as a Data Transfer Object (DTO), input model, or view model. DTO is used in this tutorial.

A DTO may be used to:

• Prevent over-posting.
• Hide properties that clients are not supposed to view.
• Omit some properties in order to reduce payload size.
• Flatten object graphs that contain nested objects. Flattened object graphs can be more convenient for clients.

To demonstrate the DTO approach, update the TodoItem class to include a secret field:

namespace TodoApi.Models
{
    public class TodoItem
    {
        public long Id { get; set; }
        public string? Name { get; set; }
        public bool IsComplete { get; set; }
        public string? Secret { get; set; }
    }
}

The secret field needs to be hidden from this app, but an administrative app could choose to expose it.

Verify you can post and get the secret field.

Create a DTO model:

namespace TodoApi.Models
{
    public class TodoItemDTO
    {
        public long Id { get; set; }
        public string? Name { get; set; }
        public bool IsComplete { get; set; }
    }
}

Update the TodoItemsController to use TodoItemDTO:

using Microsoft.AspNetCore.Mvc;
using Microsoft.EntityFrameworkCore;
using TodoApi.Models;

namespace TodoApi.Controllers
{
    [Route("api/[controller]")]
    [ApiController]
    public class TodoItemsController : ControllerBase
    {
        private readonly TodoContext _context;

        public TodoItemsController(TodoContext context)
        {
            _context = context;
        }

        // GET: api/TodoItems
        [HttpGet]
        public async Task<ActionResult<IEnumerable<TodoItemDTO>>> GetTodoItems()
        {
            return await _context.TodoItems
                .Select(x => ItemToDTO(x))
                .ToListAsync();
        }

        // GET: api/TodoItems/5
        [HttpGet("{id}")]
        public async Task<ActionResult<TodoItemDTO>> GetTodoItem(long id)
        {
            var todoItem = await _context.TodoItems.FindAsync(id);

            if (todoItem == null)
            {
                return NotFound();
            }

            return ItemToDTO(todoItem);
        }
        // PUT: api/TodoItems/5
        // To protect from overposting attacks, see https://go.microsoft.com/fwlink/?linkid=2123754
        [HttpPut("{id}")]
        public async Task<IActionResult> UpdateTodoItem(long id, TodoItemDTO todoItemDTO)
        {
            if (id != todoItemDTO.Id)
            {
                return BadRequest();
            }

            var todoItem = await _context.TodoItems.FindAsync(id);
            if (todoItem == null)
            {
                return NotFound();
            }

            todoItem.Name = todoItemDTO.Name;
            todoItem.IsComplete = todoItemDTO.IsComplete;

            try
            {
                await _context.SaveChangesAsync();
            }
            catch (DbUpdateConcurrencyException) when (!TodoItemExists(id))
            {
                return NotFound();
            }

            return NoContent();
        }
        // POST: api/TodoItems
        // To protect from overposting attacks, see https://go.microsoft.com/fwlink/?linkid=2123754
        [HttpPost]
        public async Task<ActionResult<TodoItemDTO>> CreateTodoItem(TodoItemDTO todoItemDTO)
        {
            var todoItem = new TodoItem
            {
                IsComplete = todoItemDTO.IsComplete,
                Name = todoItemDTO.Name
            };

            _context.TodoItems.Add(todoItem);
            await _context.SaveChangesAsync();

            return CreatedAtAction(
                nameof(GetTodoItem),
                new { id = todoItem.Id },
                ItemToDTO(todoItem));
        }

        // DELETE: api/TodoItems/5
        [HttpDelete("{id}")]
        public async Task<IActionResult> DeleteTodoItem(long id)
        {
            var todoItem = await _context.TodoItems.FindAsync(id);

            if (todoItem == null)
            {
                return NotFound();
            }

            _context.TodoItems.Remove(todoItem);
            await _context.SaveChangesAsync();

            return NoContent();
        }

        private bool TodoItemExists(long id)
        {
            return _context.TodoItems.Any(e => e.Id == id);
        }

        private static TodoItemDTO ItemToDTO(TodoItem todoItem) =>
            new TodoItemDTO
            {
                Id = todoItem.Id,
                Name = todoItem.Name,
                IsComplete = todoItem.IsComplete
            };
    }
}

Verify you can't post or get the secret field.

Call the web API with JavaScript

See Tutorial: Call an ASP.NET Core web API with JavaScript.

Web API video series

See Video: Beginner's Series to: Web APIs.

In this tutorial, you learn how to:

• Create a web API project.
• Add a model class and a database context.
• Scaffold a controller with CRUD methods.
• Configure routing, URL paths, and return values.
• Call the web API with Postman.

At the end, you have a web API that can manage "to-do" items stored in a database.

Overview

This tutorial creates the following API:

The following diagram shows the design of the app.

Prerequisites

Create a web project

• Visual Studio
• Visual Studio Code
• Visual Studio for Mac

• From the File menu, select New > Project.
• Select the ASP.NET Core Web API template and click Next.
• Name the project TodoApi and click Create.
• In the Create a new ASP.NET Core Web Application dialog, confirm that .NET Core and ASP.NET Core 5.0 are selected. Select the API template and click Create.

dotnet add package Microsoft.EntityFrameworkCore.InMemory

Test the project

The project template creates a WeatherForecast API with support for Swagger.

• Visual Studio
• Visual Studio Code
• Visual Studio for Mac

Press Ctrl+F5 to run without the debugger.

Visual Studio displays the following dialog when a project is not yet configured to use SSL:

Select Yes if you trust the IIS Express SSL certificate.

The following dialog is displayed:

Select Yes if you agree to trust the development certificate.

For information on trusting the Firefox browser, see Firefox SEC_ERROR_INADEQUATE_KEY_USAGE certificate error.

Visual Studio launches:

• The IIS Express web server.
• The default browser and navigates to https://localhost:<port>/swagger/index.html, where <port> is a randomly chosen port number.

The Swagger page /swagger/index.html is displayed. Select GET > Try it out > Execute. The page displays:

• The Curl command to test the WeatherForecast API.
• The URL to test the WeatherForecast API.
• The response code, body, and headers.
• A drop down list box with media types and the example value and schema.

If the Swagger page doesn't appear, see this GitHub issue.

Swagger is used to generate useful documentation and help pages for web APIs. This tutorial focuses on creating a web API. For more information on Swagger, see ASP.NET Core web API documentation with Swagger / OpenAPI.

Copy and paste the Request URL in the browser: https://localhost:<port>/weatherforecast

JSON similar to the following is returned:

[
    {
        "date": "2019-07-16T19:04:05.7257911-06:00",
        "temperatureC": 52,
        "temperatureF": 125,
        "summary": "Mild"
    },
    {
        "date": "2019-07-17T19:04:05.7258461-06:00",
        "temperatureC": 36,
        "temperatureF": 96,
        "summary": "Warm"
    },
    {
        "date": "2019-07-18T19:04:05.7258467-06:00",
        "temperatureC": 39,
        "temperatureF": 102,
        "summary": "Cool"
    },
    {
        "date": "2019-07-19T19:04:05.7258471-06:00",
        "temperatureC": 10,
        "temperatureF": 49,
        "summary": "Bracing"
    },
    {
        "date": "2019-07-20T19:04:05.7258474-06:00",
        "temperatureC": -1,
        "temperatureF": 31,
        "summary": "Chilly"
    }
]

Update the launchUrl

In Properties\launchSettings.json, update launchUrl from "swagger" to "api/todoitems":

"launchUrl": "api/todoitems",

Because Swagger will be removed, the preceding markup changes the URL that is launched to the GET method of the controller added in the following sections.

Add a model class

A model is a set of classes that represent the data that the app manages. The model for this app is a single TodoItem class.

• Visual Studio
• Visual Studio Code
• Visual Studio for Mac

namespace TodoApi.Models
{
    public class TodoItem
    {
        public long Id { get; set; }
        public string Name { get; set; }
        public bool IsComplete { get; set; }
    }
}

The Id property functions as the unique key in a relational database.

Model classes can go anywhere in the project, but the Models folder is used by convention.

Add a database context

The database context is the main class that coordinates Entity Framework functionality for a data model. This class is created by deriving from the Microsoft.EntityFrameworkCore.DbContext class.

• Visual Studio
• Visual Studio Code / Visual Studio for Mac

Add NuGet packages

• From the Tools menu, select NuGet Package Manager > Manage NuGet Packages for Solution.
• Select the Browse tab, and then enter Microsoft.EntityFrameworkCore.InMemory in the search box.
• Select Microsoft.EntityFrameworkCore.InMemory in the left pane.
• Select the Project checkbox in the right pane and then select Install.

Add the TodoContext database context

• Right-click the Models folder and select Add > Class. Name the class TodoContext and click Add.

• Enter the following code:

  using Microsoft.EntityFrameworkCore;
  
  namespace TodoApi.Models
  {
      public class TodoContext : DbContext
      {
          public TodoContext(DbContextOptions<TodoContext> options)
              : base(options)
          {
          }
  
          public DbSet<TodoItem> TodoItems { get; set; }
      }
  }
  

Register the database context

In ASP.NET Core, services such as the DB context must be registered with the dependency injection (DI) container. The container provides the service to controllers.

Update Startup.cs with the following code:

// Unused usings removed
using Microsoft.AspNetCore.Builder;
using Microsoft.AspNetCore.Hosting;
using Microsoft.Extensions.Configuration;
using Microsoft.Extensions.DependencyInjection;
using Microsoft.Extensions.Hosting;
using Microsoft.EntityFrameworkCore;
using TodoApi.Models;

namespace TodoApi
{
    public class Startup
    {
        public Startup(IConfiguration configuration)
        {
            Configuration = configuration;
        }

        public IConfiguration Configuration { get; }

        public void ConfigureServices(IServiceCollection services)
        {
            services.AddControllers();

            services.AddDbContext<TodoContext>(opt =>
                                               opt.UseInMemoryDatabase("TodoList"));
            //services.AddSwaggerGen(c =>
            //{
            //    c.SwaggerDoc("v1", new OpenApiInfo { Title = "TodoApi", Version = "v1" });
            //});
        }

        public void Configure(IApplicationBuilder app, IWebHostEnvironment env)
        {
            if (env.IsDevelopment())
            {
                app.UseDeveloperExceptionPage();
                //app.UseSwagger();
                //app.UseSwaggerUI(c => c.SwaggerEndpoint("/swagger/v1/swagger.json", "TodoApi v1"));
            }

            app.UseHttpsRedirection();
            app.UseRouting();

            app.UseAuthorization();

            app.UseEndpoints(endpoints =>
            {
                endpoints.MapControllers();
            });
        }
    }
}

The preceding code:

• Removes the Swagger calls.
• Removes unused using declarations.
• Adds the database context to the DI container.
• Specifies that the database context will use an in-memory database.

Scaffold a controller

• Visual Studio
• Visual Studio Code / Visual Studio for Mac

dotnet add package Microsoft.VisualStudio.Web.CodeGeneration.Design
dotnet add package Microsoft.EntityFrameworkCore.Design
dotnet add package Microsoft.EntityFrameworkCore.SqlServer
dotnet tool install -g dotnet-aspnet-codegenerator
dotnet aspnet-codegenerator controller -name TodoItemsController -async -api -m TodoItem -dc TodoContext -outDir Controllers

The generated code:

• Marks the class with the [ApiController] attribute. This attribute indicates that the controller responds to web API requests. For information about specific behaviors that the attribute enables, see Create web APIs with ASP.NET Core.
• Uses DI to inject the database context (TodoContext) into the controller. The database context is used in each of the CRUD methods in the controller.

The ASP.NET Core templates for:

• Controllers with views include [action] in the route template.
• API controllers don't include [action] in the route template.

When the [action] token isn't in the route template, the action name is excluded from the route. That is, the action's associated method name isn't used in the matching route.

Update the PostTodoItem create method

Update the return statement in the PostTodoItem to use the nameof operator:

// POST: api/TodoItems
[HttpPost]
public async Task<ActionResult<TodoItem>> PostTodoItem(TodoItem todoItem)
{
    _context.TodoItems.Add(todoItem);
    await _context.SaveChangesAsync();

    //return CreatedAtAction("GetTodoItem", new { id = todoItem.Id }, todoItem);
    return CreatedAtAction(nameof(GetTodoItem), new { id = todoItem.Id }, todoItem);
}

The preceding code is an HTTP POST method, as indicated by the [HttpPost] attribute. The method gets the value of the to-do item from the body of the HTTP request.

For more information, see Attribute routing with Http[Verb] attributes.

The CreatedAtAction method:

• Returns an HTTP 201 status code if successful. HTTP 201 is the standard response for an HTTP POST method that creates a new resource on the server.
• Adds a Location header to the response. The Location header specifies the URI of the newly created to-do item. For more information, see 10.2.2 201 Created.
• References the GetTodoItem action to create the Location header's URI. The C# nameof keyword is used to avoid hard-coding the action name in the CreatedAtAction call.

Install Postman

This tutorial uses Postman to test the web API.

• Install Postman
• Start the web app.
• Start Postman.
• Disable SSL certificate verification:
  • Postman for Windows: Select File > Settings (General tab), disable SSL certificate verification.
  • Postman for macOS: Select Postman > Settings (General tab), disable SSL certificate verification.

    Warning

    Re-enable SSL certificate verification after testing the controller.

Test PostTodoItem with Postman

• Create a new request.

• Set the HTTP method to POST.

• Set the URI to https://localhost:<port>/api/todoitems. For example, https://localhost:5001/api/todoitems.

• Select the Body tab.

• Select the raw radio button.

• Set the type to JSON (application/json).

• In the request body enter JSON for a to-do item:

  {
    "name":"walk dog",
    "isComplete":true
  }
  

• Select Send.

  

Test the location header URI

The location header URI can be tested in the browser. Copy and paste the location header URI into the browser.

To test in Postman:

• Select the Headers tab in the Response pane.

• Copy the Location header value:

  

• Set the HTTP method to GET.

• Set the URI to https://localhost:<port>/api/todoitems/1. For example, https://localhost:5001/api/todoitems/1.

• Select Send.

Examine the GET methods

Two GET endpoints are implemented:

• GET /api/todoitems
• GET /api/todoitems/{id}

Test the app by calling the two endpoints from a browser or Postman. For example:

• https://localhost:5001/api/todoitems
• https://localhost:5001/api/todoitems/1

A response similar to the following is produced by the call to GetTodoItems:

[
  {
    "id": 1,
    "name": "Item1",
    "isComplete": false
  }
]

Test Get with Postman

• Create a new request.
• Set the HTTP method to GET.
• Set the request URI to https://localhost:<port>/api/todoitems. For example, https://localhost:5001/api/todoitems.
• Set Two pane view in Postman.
• Select Send.

This app uses an in-memory database. If the app is stopped and started, the preceding GET request will not return any data. If no data is returned, POST data to the app.

Routing and URL paths

The [HttpGet] attribute denotes a method that responds to an HTTP GET request. The URL path for each method is constructed as follows:

• Start with the template string in the controller's Route attribute:

  [Route("api/[controller]")]
  [ApiController]
  public class TodoItemsController : ControllerBase
  {
      private readonly TodoContext _context;
  
      public TodoItemsController(TodoContext context)
      {
          _context = context;
      }
  

• Replace [controller] with the name of the controller, which by convention is the controller class name minus the "Controller" suffix. For this sample, the controller class name is TodoItemsController, so the controller name is "TodoItems". ASP.NET Core routing is case insensitive.

• If the [HttpGet] attribute has a route template (for example, [HttpGet("products")]), append that to the path. This sample doesn't use a template. For more information, see Attribute routing with Http[Verb] attributes.

In the following GetTodoItem method, "{id}" is a placeholder variable for the unique identifier of the to-do item. When GetTodoItem is invoked, the value of "{id}" in the URL is provided to the method in its id parameter.

// GET: api/TodoItems/5
[HttpGet("{id}")]
public async Task<ActionResult<TodoItem>> GetTodoItem(long id)
{
    var todoItem = await _context.TodoItems.FindAsync(id);

    if (todoItem == null)
    {
        return NotFound();
    }

    return todoItem;
}

Return values

The return type of the GetTodoItems and GetTodoItem methods is ActionResult<T> type. ASP.NET Core automatically serializes the object to JSON and writes the JSON into the body of the response message. The response code for this return type is 200 OK, assuming there are no unhandled exceptions. Unhandled exceptions are translated into 5xx errors.

ActionResult return types can represent a wide range of HTTP status codes. For example, GetTodoItem can return two different status values:

• If no item matches the requested ID, the method returns a 404 status NotFound error code.
• Otherwise, the method returns 200 with a JSON response body. Returning item results in an HTTP 200 response.

The PutTodoItem method

Examine the PutTodoItem method:

// PUT: api/TodoItems/5
[HttpPut("{id}")]
public async Task<IActionResult> PutTodoItem(long id, TodoItem todoItem)
{
    if (id != todoItem.Id)
    {
        return BadRequest();
    }

    _context.Entry(todoItem).State = EntityState.Modified;

    try
    {
        await _context.SaveChangesAsync();
    }
    catch (DbUpdateConcurrencyException)
    {
        if (!TodoItemExists(id))
        {
            return NotFound();
        }
        else
        {
            throw;
        }
    }

    return NoContent();
}

PutTodoItem is similar to PostTodoItem, except it uses HTTP PUT. The response is 204 (No Content). According to the HTTP specification, a PUT request requires the client to send the entire updated entity, not just the changes. To support partial updates, use HTTP PATCH.

If you get an error calling PutTodoItem, call GET to ensure there's an item in the database.

Test the PutTodoItem method

This sample uses an in-memory database that must be initialized each time the app is started. There must be an item in the database before you make a PUT call. Call GET to ensure there's an item in the database before making a PUT call.

Update the to-do item that has Id = 1 and set its name to "feed fish":

  {
    "Id":1,
    "name":"feed fish",
    "isComplete":true
  }

The following image shows the Postman update:

The DeleteTodoItem method

Examine the DeleteTodoItem method:

// DELETE: api/TodoItems/5
[HttpDelete("{id}")]
public async Task<IActionResult> DeleteTodoItem(long id)
{
    var todoItem = await _context.TodoItems.FindAsync(id);
    if (todoItem == null)
    {
        return NotFound();
    }

    _context.TodoItems.Remove(todoItem);
    await _context.SaveChangesAsync();

    return NoContent();
}

Test the DeleteTodoItem method

Use Postman to delete a to-do item:

• Set the method to DELETE.
• Set the URI of the object to delete (for example https://localhost:5001/api/todoitems/1).
• Select Send.

Prevent over-posting

Currently the sample app exposes the entire TodoItem object. Production apps typically limit the data that's input and returned using a subset of the model. There are multiple reasons behind this and security is a major one. The subset of a model is usually referred to as a Data Transfer Object (DTO), input model, or view model. DTO is used in this article.

A DTO may be used to:

• Prevent over-posting.
• Hide properties that clients are not supposed to view.
• Omit some properties in order to reduce payload size.
• Flatten object graphs that contain nested objects. Flattened object graphs can be more convenient for clients.

To demonstrate the DTO approach, update the TodoItem class to include a secret field:

namespace TodoApi.Models
{
    public class TodoItem
    {
        public long Id { get; set; }
        public string Name { get; set; }
        public bool IsComplete { get; set; }
        public string Secret { get; set; }
    }
}

The secret field needs to be hidden from this app, but an administrative app could choose to expose it.

Verify you can post and get the secret field.

Create a DTO model:

public class TodoItemDTO
{
    public long Id { get; set; }
    public string Name { get; set; }
    public bool IsComplete { get; set; }
}

Update the TodoItemsController to use TodoItemDTO:

// GET: api/TodoItems
[HttpGet]
public async Task<ActionResult<IEnumerable<TodoItemDTO>>> GetTodoItems()
{
    return await _context.TodoItems
        .Select(x => ItemToDTO(x))
        .ToListAsync();
}

[HttpGet("{id}")]
public async Task<ActionResult<TodoItemDTO>> GetTodoItem(long id)
{
    var todoItem = await _context.TodoItems.FindAsync(id);

    if (todoItem == null)
    {
        return NotFound();
    }

    return ItemToDTO(todoItem);
}

[HttpPut("{id}")]
public async Task<IActionResult> UpdateTodoItem(long id, TodoItemDTO todoItemDTO)
{
    if (id != todoItemDTO.Id)
    {
        return BadRequest();
    }

    var todoItem = await _context.TodoItems.FindAsync(id);
    if (todoItem == null)
    {
        return NotFound();
    }

    todoItem.Name = todoItemDTO.Name;
    todoItem.IsComplete = todoItemDTO.IsComplete;

    try
    {
        await _context.SaveChangesAsync();
    }
    catch (DbUpdateConcurrencyException) when (!TodoItemExists(id))
    {
        return NotFound();
    }

    return NoContent();
}

[HttpPost]
public async Task<ActionResult<TodoItemDTO>> CreateTodoItem(TodoItemDTO todoItemDTO)
{
    var todoItem = new TodoItem
    {
        IsComplete = todoItemDTO.IsComplete,
        Name = todoItemDTO.Name
    };

    _context.TodoItems.Add(todoItem);
    await _context.SaveChangesAsync();

    return CreatedAtAction(
        nameof(GetTodoItem),
        new { id = todoItem.Id },
        ItemToDTO(todoItem));
}

[HttpDelete("{id}")]
public async Task<IActionResult> DeleteTodoItem(long id)
{
    var todoItem = await _context.TodoItems.FindAsync(id);

    if (todoItem == null)
    {
        return NotFound();
    }

    _context.TodoItems.Remove(todoItem);
    await _context.SaveChangesAsync();

    return NoContent();
}

private bool TodoItemExists(long id) =>
     _context.TodoItems.Any(e => e.Id == id);

private static TodoItemDTO ItemToDTO(TodoItem todoItem) =>
    new TodoItemDTO
    {
        Id = todoItem.Id,
        Name = todoItem.Name,
        IsComplete = todoItem.IsComplete
    };

Verify you can't post or get the secret field.

Call the web API with JavaScript

See Tutorial: Call an ASP.NET Core web API with JavaScript.

In this tutorial, you learn how to:

• Create a web API project.
• Add a model class and a database context.
• Scaffold a controller with CRUD methods.
• Configure routing, URL paths, and return values.
• Call the web API with Postman.

At the end, you have a web API that can manage "to-do" items stored in a database.

Overview

This tutorial creates the following API:

The following diagram shows the design of the app.

Prerequisites

Create a web project

• Visual Studio
• Visual Studio Code
• Visual Studio for Mac

• From the File menu, select New > Project.
• Select the ASP.NET Core Web Application template and click Next.
• Name the project TodoApi and click Create.
• In the Create a new ASP.NET Core Web Application dialog, confirm that .NET Core and ASP.NET Core 3.1 are selected. Select the API template and click Create.

dotnet add package Microsoft.EntityFrameworkCore.InMemory

Test the API

The project template creates a WeatherForecast API. Call the Get method from a browser to test the app.

• Visual Studio
• Visual Studio Code
• Visual Studio for Mac

JSON similar to the following is returned:

[
    {
        "date": "2019-07-16T19:04:05.7257911-06:00",
        "temperatureC": 52,
        "temperatureF": 125,
        "summary": "Mild"
    },
    {
        "date": "2019-07-17T19:04:05.7258461-06:00",
        "temperatureC": 36,
        "temperatureF": 96,
        "summary": "Warm"
    },
    {
        "date": "2019-07-18T19:04:05.7258467-06:00",
        "temperatureC": 39,
        "temperatureF": 102,
        "summary": "Cool"
    },
    {
        "date": "2019-07-19T19:04:05.7258471-06:00",
        "temperatureC": 10,
        "temperatureF": 49,
        "summary": "Bracing"
    },
    {
        "date": "2019-07-20T19:04:05.7258474-06:00",
        "temperatureC": -1,
        "temperatureF": 31,
        "summary": "Chilly"
    }
]

Add a model class

A model is a set of classes that represent the data that the app manages. The model for this app is a single TodoItem class.

• Visual Studio
• Visual Studio Code
• Visual Studio for Mac

public class TodoItem
{
    public long Id { get; set; }
    public string Name { get; set; }
    public bool IsComplete { get; set; }
}

The Id property functions as the unique key in a relational database.

Model classes can go anywhere in the project, but the Models folder is used by convention.

Add a database context

The database context is the main class that coordinates Entity Framework functionality for a data model. This class is created by deriving from the Microsoft.EntityFrameworkCore.DbContext class.

• Visual Studio
• Visual Studio Code / Visual Studio for Mac

Add NuGet packages

• From the Tools menu, select NuGet Package Manager > Manage NuGet Packages for Solution.
• Select the Browse tab, and then enter Microsoft.EntityFrameworkCore.InMemory in the search box.
• Select Microsoft.EntityFrameworkCore.InMemory in the left pane.
• Select the Project checkbox in the right pane and then select Install.

Add the TodoContext database context

• Right-click the Models folder and select Add > Class. Name the class TodoContext and click Add.

• Enter the following code:

  using Microsoft.EntityFrameworkCore;
  
  namespace TodoApi.Models
  {
      public class TodoContext : DbContext
      {
          public TodoContext(DbContextOptions<TodoContext> options)
              : base(options)
          {
          }
  
          public DbSet<TodoItem> TodoItems { get; set; }
      }
  }
  

Register the database context

In ASP.NET Core, services such as the DB context must be registered with the dependency injection (DI) container. The container provides the service to controllers.

Update Startup.cs with the following highlighted code:

// Unused usings removed
using Microsoft.AspNetCore.Builder;
using Microsoft.AspNetCore.Hosting;
using Microsoft.Extensions.Configuration;
using Microsoft.Extensions.DependencyInjection;
using Microsoft.Extensions.Hosting;
using Microsoft.EntityFrameworkCore;
using TodoApi.Models;

namespace TodoApi
{
    public class Startup
    {
        public Startup(IConfiguration configuration)
        {
            Configuration = configuration;
        }

        public IConfiguration Configuration { get; }

        public void ConfigureServices(IServiceCollection services)
        {
            services.AddDbContext<TodoContext>(opt =>
               opt.UseInMemoryDatabase("TodoList"));
            services.AddControllers();
        }

        public void Configure(IApplicationBuilder app, IWebHostEnvironment env)
        {
            if (env.IsDevelopment())
            {
                app.UseDeveloperExceptionPage();
            }

            app.UseHttpsRedirection();

            app.UseRouting();

            app.UseAuthorization();

            app.UseEndpoints(endpoints =>
            {
                endpoints.MapControllers();
            });
        }
    }
}

The preceding code:

• Removes unused using declarations.
• Adds the database context to the DI container.
• Specifies that the database context will use an in-memory database.

Scaffold a controller

• Visual Studio
• Visual Studio Code / Visual Studio for Mac

dotnet add package Microsoft.VisualStudio.Web.CodeGeneration.Design
dotnet add package Microsoft.EntityFrameworkCore.Design
dotnet tool install --global dotnet-aspnet-codegenerator
dotnet tool update -g Dotnet-aspnet-codegenerator
dotnet aspnet-codegenerator controller -name TodoItemsController -async -api -m TodoItem -dc TodoContext -outDir Controllers

The generated code:

• Marks the class with the [ApiController] attribute. This attribute indicates that the controller responds to web API requests. For information about specific behaviors that the attribute enables, see Create web APIs with ASP.NET Core.
• Uses DI to inject the database context (TodoContext) into the controller. The database context is used in each of the CRUD methods in the controller.

The ASP.NET Core templates for:

• Controllers with views include [action] in the route template.
• API controllers don't include [action] in the route template.

When the [action] token isn't in the route template, the action name is excluded from the route. That is, the action's associated method name isn't used in the matching route.

Examine the PostTodoItem create method

Replace the return statement in the PostTodoItem to use the nameof operator:

// POST: api/TodoItems
[HttpPost]
public async Task<ActionResult<TodoItem>> PostTodoItem(TodoItem todoItem)
{
    _context.TodoItems.Add(todoItem);
    await _context.SaveChangesAsync();

    //return CreatedAtAction("GetTodoItem", new { id = todoItem.Id }, todoItem);
    return CreatedAtAction(nameof(GetTodoItem), new { id = todoItem.Id }, todoItem);
}

The preceding code is an HTTP POST method, as indicated by the [HttpPost] attribute. The method gets the value of the to-do item from the body of the HTTP request.

For more information, see Attribute routing with Http[Verb] attributes.

The CreatedAtAction method:

• Returns an HTTP 201 status code if successful. HTTP 201 is the standard response for an HTTP POST method that creates a new resource on the server.
• Adds a Location header to the response. The Location header specifies the URI of the newly created to-do item. For more information, see 10.2.2 201 Created.
• References the GetTodoItem action to create the Location header's URI. The C# nameof keyword is used to avoid hard-coding the action name in the CreatedAtAction call.

Install Postman

This tutorial uses Postman to test the web API.

• Install Postman
• Start the web app.
• Start Postman.
• Disable SSL certificate verification:
  • Postman for Windows: Postman for Windows File > Settings (General tab), disable SSL certificate verification.
  • Postman for macOS: Postman for Windows Postman > Settings (General tab), disable SSL certificate verification.

    Warning

    Re-enable SSL certificate verification after testing the controller.

Test PostTodoItem with Postman

• Create a new request.

• Set the HTTP method to POST.

• Set the URI to https://localhost:<port>/api/todoitems. For example, https://localhost:5001/api/todoitems.

• Select the Body tab.

• Select the raw radio button.

• Set the type to JSON (application/json).

• In the request body enter JSON for a to-do item:

  {
    "name":"walk dog",
    "isComplete":true
  }
  

• Select Send.

  

Test the location header URI with Postman

• Select the Headers tab in the Response pane.

• Copy the Location header value:

  

• Set the HTTP method to GET.

• Set the URI to https://localhost:<port>/api/todoitems/1. For example, https://localhost:5001/api/todoitems/1.

• Select Send.

Examine the GET methods

These methods implement two GET endpoints:

• GET /api/todoitems
• GET /api/todoitems/{id}

Test the app by calling the two endpoints from a browser or Postman. For example:

• https://localhost:5001/api/todoitems
• https://localhost:5001/api/todoitems/1

A response similar to the following is produced by the call to GetTodoItems:

[
  {
    "id": 1,
    "name": "Item1",
    "isComplete": false
  }
]

Test Get with Postman

• Create a new request.
• Set the HTTP method to GET.
• Set the request URI to https://localhost:<port>/api/todoitems. For example, https://localhost:5001/api/todoitems.
• Set Two pane view in Postman.
• Select Send.

This app uses an in-memory database. If the app is stopped and started, the preceding GET request will not return any data. If no data is returned, POST data to the app.

Routing and URL paths

The [HttpGet] attribute denotes a method that responds to an HTTP GET request. The URL path for each method is constructed as follows:

• Start with the template string in the controller's Route attribute:

  [Route("api/[controller]")]
  [ApiController]
  public class TodoItemsController : ControllerBase
  {
      private readonly TodoContext _context;
  
      public TodoItemsController(TodoContext context)
      {
          _context = context;
      }
  

• Replace [controller] with the name of the controller, which by convention is the controller class name minus the "Controller" suffix. For this sample, the controller class name is TodoItemsController, so the controller name is "TodoItems". ASP.NET Core routing is case insensitive.

• If the [HttpGet] attribute has a route template (for example, [HttpGet("products")]), append that to the path. This sample doesn't use a template. For more information, see Attribute routing with Http[Verb] attributes.

In the following GetTodoItem method, "{id}" is a placeholder variable for the unique identifier of the to-do item. When GetTodoItem is invoked, the value of "{id}" in the URL is provided to the method in its id parameter.

// GET: api/TodoItems/5
[HttpGet("{id}")]
public async Task<ActionResult<TodoItem>> GetTodoItem(long id)
{
    var todoItem = await _context.TodoItems.FindAsync(id);

    if (todoItem == null)
    {
        return NotFound();
    }

    return todoItem;
}

Return values

The return type of the GetTodoItems and GetTodoItem methods is ActionResult<T> type. ASP.NET Core automatically serializes the object to JSON and writes the JSON into the body of the response message. The response code for this return type is 200, assuming there are no unhandled exceptions. Unhandled exceptions are translated into 5xx errors.

ActionResult return types can represent a wide range of HTTP status codes. For example, GetTodoItem can return two different status values:

• If no item matches the requested ID, the method returns a 404 NotFound error code.
• Otherwise, the method returns 200 with a JSON response body. Returning item results in an HTTP 200 response.

The PutTodoItem method

Examine the PutTodoItem method:

// PUT: api/TodoItems/5
[HttpPut("{id}")]
public async Task<IActionResult> PutTodoItem(long id, TodoItem todoItem)
{
    if (id != todoItem.Id)
    {
        return BadRequest();
    }

    _context.Entry(todoItem).State = EntityState.Modified;

    try
    {
        await _context.SaveChangesAsync();
    }
    catch (DbUpdateConcurrencyException)
    {
        if (!TodoItemExists(id))
        {
            return NotFound();
        }
        else
        {
            throw;
        }
    }

    return NoContent();
}

PutTodoItem is similar to PostTodoItem, except it uses HTTP PUT. The response is 204 (No Content). According to the HTTP specification, a PUT request requires the client to send the entire updated entity, not just the changes. To support partial updates, use HTTP PATCH.

If you get an error calling PutTodoItem, call GET to ensure there's an item in the database.

Test the PutTodoItem method

This sample uses an in-memory database that must be initialized each time the app is started. There must be an item in the database before you make a PUT call. Call GET to ensure there's an item in the database before making a PUT call.

Update the to-do item that has Id = 1 and set its name to "feed fish":

  {
    "id":1,
    "name":"feed fish",
    "isComplete":true
  }

The following image shows the Postman update:

The DeleteTodoItem method

Examine the DeleteTodoItem method:

// DELETE: api/TodoItems/5
[HttpDelete("{id}")]
public async Task<ActionResult<TodoItem>> DeleteTodoItem(long id)
{
    var todoItem = await _context.TodoItems.FindAsync(id);
    if (todoItem == null)
    {
        return NotFound();
    }

    _context.TodoItems.Remove(todoItem);
    await _context.SaveChangesAsync();

    return todoItem;
}

Test the DeleteTodoItem method

Use Postman to delete a to-do item:

• Set the method to DELETE.
• Set the URI of the object to delete (for example https://localhost:5001/api/todoitems/1).
• Select Send.

Prevent over-posting

Currently the sample app exposes the entire TodoItem object. Production apps typically limit the data that's input and returned using a subset of the model. There are multiple reasons behind this and security is a major one. The subset of a model is usually referred to as a Data Transfer Object (DTO), input model, or view model. DTO is used in this article.

A DTO may be used to:

• Prevent over-posting.
• Hide properties that clients are not supposed to view.
• Omit some properties in order to reduce payload size.
• Flatten object graphs that contain nested objects. Flattened object graphs can be more convenient for clients.

To demonstrate the DTO approach, update the TodoItem class to include a secret field:

public class TodoItem
{
    public long Id { get; set; }
    public string Name { get; set; }
    public bool IsComplete { get; set; }
    public string Secret { get; set; }
}

The secret field needs to be hidden from this app, but an administrative app could choose to expose it.

Verify you can post and get the secret field.

Create a DTO model:

public class TodoItemDTO
{
    public long Id { get; set; }
    public string Name { get; set; }
    public bool IsComplete { get; set; }
}

Update the TodoItemsController to use TodoItemDTO:

    [HttpGet]
    public async Task<ActionResult<IEnumerable<TodoItemDTO>>> GetTodoItems()
    {
        return await _context.TodoItems
            .Select(x => ItemToDTO(x))
            .ToListAsync();
    }

    [HttpGet("{id}")]
    public async Task<ActionResult<TodoItemDTO>> GetTodoItem(long id)
    {
        var todoItem = await _context.TodoItems.FindAsync(id);

        if (todoItem == null)
        {
            return NotFound();
        }

        return ItemToDTO(todoItem);
    }

    [HttpPut("{id}")]
    public async Task<IActionResult> UpdateTodoItem(long id, TodoItemDTO todoItemDTO)
    {
        if (id != todoItemDTO.Id)
        {
            return BadRequest();
        }

        var todoItem = await _context.TodoItems.FindAsync(id);
        if (todoItem == null)
        {
            return NotFound();
        }

        todoItem.Name = todoItemDTO.Name;
        todoItem.IsComplete = todoItemDTO.IsComplete;

        try
        {
            await _context.SaveChangesAsync();
        }
        catch (DbUpdateConcurrencyException) when (!TodoItemExists(id))
        {
            return NotFound();
        }

        return NoContent();
    }

    [HttpPost]
    public async Task<ActionResult<TodoItemDTO>> CreateTodoItem(TodoItemDTO todoItemDTO)
    {
        var todoItem = new TodoItem
        {
            IsComplete = todoItemDTO.IsComplete,
            Name = todoItemDTO.Name
        };

        _context.TodoItems.Add(todoItem);
        await _context.SaveChangesAsync();

        return CreatedAtAction(
            nameof(GetTodoItem),
            new { id = todoItem.Id },
            ItemToDTO(todoItem));
    }

    [HttpDelete("{id}")]
    public async Task<IActionResult> DeleteTodoItem(long id)
    {
        var todoItem = await _context.TodoItems.FindAsync(id);

        if (todoItem == null)
        {
            return NotFound();
        }

        _context.TodoItems.Remove(todoItem);
        await _context.SaveChangesAsync();

        return NoContent();
    }

    private bool TodoItemExists(long id) =>
         _context.TodoItems.Any(e => e.Id == id);

    private static TodoItemDTO ItemToDTO(TodoItem todoItem) =>
        new TodoItemDTO
        {
            Id = todoItem.Id,
            Name = todoItem.Name,
            IsComplete = todoItem.IsComplete
        };       
}

Verify you can't post or get the secret field.

Call the web API with JavaScript

See Tutorial: Call an ASP.NET Core web API with JavaScript.

Add authentication support to a web API

ASP.NET Core Identity adds user interface (UI) login functionality to ASP.NET Core web apps. To secure web APIs and SPAs, use one of the following:

• Azure Active Directory
• Azure Active Directory B2C (Azure AD B2C)
• Duende Identity Server

Duende Identity Server is an OpenID Connect and OAuth 2.0 framework for ASP.NET Core. Duende Identity Server enables the following security features:

• Authentication as a Service (AaaS)
• Single sign-on/off (SSO) over multiple application types
• Access control for APIs
• Federation Gateway

For more information, see the Duende Identity Server documentation (Duende Software website).

Publish to Azure

For information on deploying to Azure, see Quickstart: Deploy an ASP.NET web app.

Additional resources

View or download sample code for this tutorial. See how to download.

For more information, see the following resources:

• Create web APIs with ASP.NET Core
• Tutorial: Create a minimal web API with ASP.NET Core
• ASP.NET Core web API documentation with Swagger / OpenAPI
• Razor Pages with Entity Framework Core in ASP.NET Core - Tutorial 1 of 8
• Routing to controller actions in ASP.NET Core
• Controller action return types in ASP.NET Core web API
• Deploy ASP.NET Core apps to Azure App Service
• Host and deploy ASP.NET Core
• Create a web API with ASP.NET Core