Create a Web API with ASP.NET Core MVC and Visual Studio for Mac

By Rick Anderson and Mike Wasson

In this tutorial, you’ll build a web API for managing a list of "to-do" items. You won’t build a UI.

There are 3 versions of this tutorial:

Overview

Here is the API that you’ll create:

API Description Request body Response body
GET /api/todo Get all to-do items None Array of to-do items
GET /api/todo/{id} Get an item by ID None To-do item
POST /api/todo Add a new item To-do item To-do item
PUT /api/todo/{id} Update an existing item   To-do item None
DELETE /api/todo/{id}     Delete an item     None None


The following diagram shows the basic design of the app.

The client is represented by a box on the left and submits a request and receives a response from the application, a box drawn on the right. Within the application box, three boxes represent the controller, the model, and the data access layer. The request comes into the application's controller, and read/write operations occur between the controller and the data access layer. The model is serialized and returned to the client in the response.

  • The client is whatever consumes the web API (mobile app, browser, etc). We aren’t writing a client in this tutorial. We'll use Postman or curl to test the app.

  • A model is an object that represents the data in your application. In this case, the only model is a to-do item. Models are represented as C# classes, also know as Plain Old C# Object (POCOs).

  • A controller is an object that handles HTTP requests and creates the HTTP response. This app will have a single controller.

  • To keep the tutorial simple, the app doesn’t use a persistent database. The sample app stores to-do items in an in-memory database.

Prerequisites

Install the following:

Create the project

From Visual Studio, select File > New Solution.

macOS New solution

Select .NET Core App > ASP.NET Core Web API > Next.

macOS New project dialog

Enter TodoApi for the Project Name, and then select Create.

config dialog

Launch the app

In Visual Studio, select Run > Start With Debugging to launch the app. Visual Studio launches a browser and navigates to http://localhost:port, where port is a randomly chosen port number. You get an HTTP 404 (Not Found) error. Change the URL to http://localhost:port/api/values. The ValuesController data will be displayed:

["value1","value2"]

Add support for Entity Framework Core

Install the Entity Framework Core InMemory database provider. This database provider allows Entity Framework Core to be used with an in-memory database.

  • From the Project menu, select Add NuGet Packages.

    • Alternately, you can right-click Dependencies, and then select Add Packages.
  • Enter EntityFrameworkCore.InMemory in the search box.

  • Select Microsoft.EntityFrameworkCore.InMemory, and then select Add Package.

Add a model class

A model is an object that represents the data in your application. In this case, the only model is a to-do item.

Add a folder named Models. In Solution Explorer, right-click the project. Select Add > New Folder. Name the folder Models.

new folder

Note: You can put model classes anywhere in your project, but the Models folder is used by convention.

Add a TodoItem class. Right-click the Models folder and select Add > New File > General > Empty Class. Name the class TodoItem, and then select New.

Replace the generated code with:

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

The database generates the Id when a TodoItem is created.

Create the database context

The database context is the main class that coordinates Entity Framework functionality for a given data model. You create this class by deriving from the Microsoft.EntityFrameworkCore.DbContext class.

Add a TodoContext class to the Models folder.

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 order to inject the database context into the controller, we need to register it with the dependency injection container. Register the database context with the service container using the built-in support for dependency injection. Replace the contents of the Startup.cs file with the following:

using Microsoft.AspNetCore.Builder;
using Microsoft.EntityFrameworkCore;
using Microsoft.Extensions.DependencyInjection;
using TodoApi.Models;

namespace TodoApi
{
    public class Startup
    {       
        public void ConfigureServices(IServiceCollection services)
        {
            services.AddDbContext<TodoContext>(opt => opt.UseInMemoryDatabase());
            services.AddMvc();
        }

        public void Configure(IApplicationBuilder app)
        {
            app.UseMvc();
        }
    }
}

The preceding code:

  • Removes the code we're not using.
  • Specifies an in-memory database is injected into the service container.

Add a controller

In Solution Explorer, in the Controllers folder, add the class TodoController.

Replace the generated code with the following (and add closing braces):

using System.Collections.Generic;
using Microsoft.AspNetCore.Mvc;
using TodoApi.Models;
using System.Linq;

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

        public TodoController(TodoContext context)
        {
            _context = context;

            if (_context.TodoItems.Count() == 0)
            {
                _context.TodoItems.Add(new TodoItem { Name = "Item1" });
                _context.SaveChanges();
            }
        }       
    }
}

The preceding code:

  • Defines an empty controller class. In the next sections, we'll add methods to implement the API.
  • The constructor uses Dependency Injection to inject the database context (TodoContext) into the controller. The database context is used in each of the CRUD methods in the controller.
  • The constructor adds an item to the in-memory database if one doesn't exist.

Getting to-do items

To get to-do items, add the following methods to the TodoController class.

[HttpGet]
public IEnumerable<TodoItem> GetAll()
{
    return _context.TodoItems.ToList();
}

[HttpGet("{id}", Name = "GetTodo")]
public IActionResult GetById(long id)
{
    var item = _context.TodoItems.FirstOrDefault(t => t.Id == id);
    if (item == null)
    {
        return NotFound();
    }
    return new ObjectResult(item);
}

These methods implement the two GET methods:

  • GET /api/todo
  • GET /api/todo/{id}

Here is an example HTTP response for the GetAll method:

HTTP/1.1 200 OK
   Content-Type: application/json; charset=utf-8
   Server: Microsoft-IIS/10.0
   Date: Thu, 18 Jun 2015 20:51:10 GMT
   Content-Length: 82

   [{"Key":"1", "Name":"Item1","IsComplete":false}]

Later in the tutorial I'll show how you can view the HTTP response using Postman or curl.

Routing and URL paths

The [HttpGet] attribute specifies an HTTP GET method. The URL path for each method is constructed as follows:

  • Take the template string in the controller’s route attribute:
namespace TodoApi.Controllers
{
    [Route("api/[controller]")]
    public class TodoController : Controller
    {
        private readonly TodoContext _context;
  • Replace "[Controller]" with the name of the controller, which is the controller class name minus the "Controller" suffix. For this sample, the controller class name is TodoController and the root name is "todo". ASP.NET Core routing is not case sensitive.
  • If the [HttpGet] attribute has a route template (such as [HttpGet("/products")], append that to the path. This sample doesn't use a template. See Attribute routing with Http[Verb] attributes for more information.

In the GetById method:

[HttpGet("{id}", Name = "GetTodo")]
public IActionResult GetById(long id)

"{id}" is a placeholder variable for the ID of the todo item. When GetById is invoked, it assigns the value of "{id}" in the URL to the method's id parameter.

Name = "GetTodo" creates a named route and allows you to link to this route in an HTTP Response. I'll explain it with an example later. See Routing to Controller Actions for detailed information.

Return values

The GetAll method returns an IEnumerable. MVC automatically serializes the object to JSON and writes the JSON into the body of the response message. The response code for this method is 200, assuming there are no unhandled exceptions. (Unhandled exceptions are translated into 5xx errors.)

In contrast, the GetById method returns the more general IActionResult type, which represents a wide range of return types. GetById has two different return types:

  • If no item matches the requested ID, the method returns a 404 error. This is done by returning NotFound.

  • Otherwise, the method returns 200 with a JSON response body. This is done by returning an ObjectResult

Launch the app

In Visual Studio, select Run > Start With Debugging to launch the app. Visual Studio launches a browser and navigates to http://localhost:port, where port is a randomly chosen port number. You get an HTTP 404 (Not Found) error. Change the URL to http://localhost:port/api/values. The ValuesController data will be displayed:

["value1","value2"]

Navigate to the Todo controller athttp://localhost:port/api/todo:

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

Implement the other CRUD operations

We'll add Create, Update, and Delete methods to the controller. These are variations on a theme, so I'll just show the code and highlight the main differences. Build the project after adding or changing code.

Create

[HttpPost]
public IActionResult Create([FromBody] TodoItem item)
{
    if (item == null)
    {
        return BadRequest();
    }

    _context.TodoItems.Add(item);
    _context.SaveChanges();

    return CreatedAtRoute("GetTodo", new { id = item.Id }, item);
}

This is an HTTP POST method, indicated by the [HttpPost] attribute. The [FromBody] attribute tells MVC to get the value of the to-do item from the body of the HTTP request.

The CreatedAtRoute method returns a 201 response, which is the standard response for an HTTP POST method that creates a new resource on the server. CreatedAtRoute also adds a Location header to the response. The Location header specifies the URI of the newly created to-do item. See 10.2.2 201 Created.

Use Postman to send a Create request

  • Start the app (Run > Start With Debugging).
  • Start Postman.

Postman console

  • Set the HTTP method to POST
  • Select the Body radio button
  • Select the raw radio button
  • Set the type to JSON
  • In the key-value editor, enter a Todo item such as
{
    "name":"walk dog",
    "isComplete":true
}
  • Select Send

  • Select the Headers tab in the lower pane and copy the Location header:

Headers tab of the Postman console

You can use the Location header URI to access the resource you just created. Recall the GetById method created the "GetTodo" named route:

[HttpGet("{id}", Name = "GetTodo")]
public IActionResult GetById(string id)

Update

[HttpPut("{id}")]
public IActionResult Update(long id, [FromBody] TodoItem item)
{
    if (item == null || item.Id != id)
    {
        return BadRequest();
    }

    var todo = _context.TodoItems.FirstOrDefault(t => t.Id == id);
    if (todo == null)
    {
        return NotFound();
    }

    todo.IsComplete = item.IsComplete;
    todo.Name = item.Name;

    _context.TodoItems.Update(todo);
    _context.SaveChanges();
    return new NoContentResult();
}

Update is similar to Create, but uses HTTP PUT. The response is 204 (No Content). According to the HTTP spec, a PUT request requires the client to send the entire updated entity, not just the deltas. To support partial updates, use HTTP PATCH.

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

Postman console showing 204 (No Content) response

Delete

[HttpDelete("{id}")]
public IActionResult Delete(long id)
{
    var todo = _context.TodoItems.FirstOrDefault(t => t.Id == id);
    if (todo == null)
    {
        return NotFound();
    }

    _context.TodoItems.Remove(todo);
    _context.SaveChanges();
    return new NoContentResult();
}

The response is 204 (No Content).

Postman console showing 204 (No Content) response

Next steps