ASP.NET Core Web API Help Pages using Swagger

By Shayne Boyer and Scott Addie

Understanding the various methods of an API can be a challenge for a developer when building a consuming application.

Generating good documentation and help pages for your Web API, using Swagger with the .NET Core implementation Swashbuckle.AspNetCore, is as easy as adding a couple of NuGet packages and modifying the Startup.cs.

  • Swashbuckle.AspNetCore is an open source project for generating Swagger documents for ASP.NET Core Web APIs.

  • Swagger is a machine-readable representation of a RESTful API that enables support for interactive documentation, client SDK generation, and discoverability.

This tutorial builds on the sample on Building Your First Web API with ASP.NET Core MVC and Visual Studio. If you'd like to follow along, download the sample at https://github.com/aspnet/Docs/tree/master/aspnetcore/tutorials/first-web-api/sample.

Getting Started

There are three main components to Swashbuckle:

  • Swashbuckle.AspNetCore.Swagger: a Swagger object model and middleware to expose SwaggerDocument objects as JSON endpoints.

  • Swashbuckle.AspNetCore.SwaggerGen: a Swagger generator that builds SwaggerDocument objects directly from your routes, controllers, and models. It's typically combined with the Swagger endpoint middleware to automatically expose Swagger JSON.

  • Swashbuckle.AspNetCore.SwaggerUI: an embedded version of the Swagger UI tool which interprets Swagger JSON to build a rich, customizable experience for describing the Web API functionality. It includes built-in test harnesses for the public methods.

NuGet Packages

Swashbuckle can be added with the following approaches:

  • From the Package Manager Console window:

    Install-Package Swashbuckle.AspNetCore
    
  • From the Manage NuGet Packages dialog:

    • Right-click your project in Solution Explorer > Manage NuGet Packages
    • Set the Package source to "nuget.org"
    • Enter "Swashbuckle.AspNetCore" in the search box
    • Select the "Swashbuckle.AspNetCore" package from the Browse tab and click Install

Add and configure Swagger to the middleware

Add the Swagger generator to the services collection in the ConfigureServices method of Startup.cs:

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

    // Register the Swagger generator, defining one or more Swagger documents
    services.AddSwaggerGen(c =>
    {
        c.SwaggerDoc("v1", new Info { Title = "My API", Version = "v1" });
    });
}

Add the following using statement for the Info class:

using Swashbuckle.AspNetCore.Swagger;

In the Configure method of Startup.cs, enable the middleware for serving the generated JSON document and the SwaggerUI:

public void Configure(IApplicationBuilder app)
{
    // Enable middleware to serve generated Swagger as a JSON endpoint.
    app.UseSwagger();

    // Enable middleware to serve swagger-ui (HTML, JS, CSS, etc.), specifying the Swagger JSON endpoint.
    app.UseSwaggerUI(c =>
    {
        c.SwaggerEndpoint("/swagger/v1/swagger.json", "My API V1");
    });

    app.UseMvc();
}

Launch the app, and navigate to http://localhost:<random_port>/swagger/v1/swagger.json. The generated document describing the endpoints appears.

Note: Microsoft Edge, Google Chrome, and Firefox display JSON documents natively. There are extensions for Chrome that format the document for easier reading. The following example is reduced for brevity.

{
   "swagger": "2.0",
   "info": {
       "version": "v1",
       "title": "API V1"
   },
   "basePath": "/",
   "paths": {
       "/api/Todo": {
           "get": {
               "tags": [
                   "Todo"
               ],
               "operationId": "ApiTodoGet",
               "consumes": [],
               "produces": [
                   "text/plain",
                   "application/json",
                   "text/json"
               ],
               "responses": {
                   "200": {
                       "description": "Success",
                       "schema": {
                           "type": "array",
                           "items": {
                               "$ref": "#/definitions/TodoItem"
                           }
                       }
                   }
                }
           },
           "post": {
               ...
           }
       },
       "/api/Todo/{id}": {
           "get": {
               ...
           },
           "put": {
               ...
           },
           "delete": {
               ...
   },
   "definitions": {
       "TodoItem": {
           "type": "object",
            "properties": {
                "id": {
                    "format": "int64",
                    "type": "integer"
                },
                "name": {
                    "type": "string"
                },
                "isComplete": {
                    "default": false,
                    "type": "boolean"
                }
            }
       }
   },
   "securityDefinitions": {}
}

This document drives the Swagger UI, which can be viewed by navigating to http://localhost:<random_port>/swagger:

Swagger UI

Each public action method in TodoController can be tested from the UI. Click a method name to expand the section. Add any necessary parameters, and click "Try it out!".

Example Swagger GET test

Customization & Extensibility

Swagger provides options for documenting the object model and customizing the UI to match your theme.

API Info and Description

The configuration action passed to the AddSwaggerGen method can be used to add information such as the author, license, and description:

services.AddSwaggerGen(c =>
{
    c.SwaggerDoc("v1", new Info
    {
        Version = "v1",
        Title = "ToDo API",
        Description = "A simple example ASP.NET Core Web API",
        TermsOfService = "None",
        Contact = new Contact { Name = "Shayne Boyer", Email = "", Url = "https://twitter.com/spboyer" },
        License = new License { Name = "Use under LICX", Url = "https://example.com/license" }
    });
});

The following image depicts the Swagger UI displaying the version information:

Swagger UI with version information: description, author, and see more link

XML Comments

XML comments can be enabled with the following approaches:

  • Right-click the project in Solution Explorer and select Properties
  • Check the XML documentation file box under the Output section of the Build tab:

Build tab of project properties

Configure Swagger to use the generated XML file. For Linux or non-Windows operating systems, file names and paths can be case sensitive. For example, a ToDoApi.XML file would be found on Windows but not CentOS.

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

    // Register the Swagger generator, defining one or more Swagger documents
    services.AddSwaggerGen(c =>
    {
        c.SwaggerDoc("v1", new Info
        {
            Version = "v1",
            Title = "ToDo API",
            Description = "A simple example ASP.NET Core Web API",
            TermsOfService = "None",
            Contact = new Contact { Name = "Shayne Boyer", Email = "", Url = "https://twitter.com/spboyer" },
            License = new License { Name = "Use under LICX", Url = "https://example.com/license" }
        });

        // Set the comments path for the Swagger JSON and UI.
        var basePath = PlatformServices.Default.Application.ApplicationBasePath;
        var xmlPath = Path.Combine(basePath, "TodoApi.xml"); 
        c.IncludeXmlComments(xmlPath);                
    });
}

In the preceding code, ApplicationBasePath gets the base path of the app. The base path is used to locate the XML comments file. TodoApi.xml only works for this example, since the name of the generated XML comments file is based on the application name.

Adding the triple-slash comments to the method enhances the Swagger UI by adding the description to the section header:

/// <summary>
/// Deletes a specific TodoItem.
/// </summary>
/// <param name="id"></param>        
[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();
}

Swagger UI showing XML comment 'Deletes a specific TodoItem.' for the DELETE method

The UI is driven by the generated JSON file, which also contains these comments:

"delete": {
    "tags": [
        "Todo"
    ],
    "summary": "Deletes a specific TodoItem.",
    "operationId": "ApiTodoByIdDelete",
    "consumes": [],
    "produces": [],
    "parameters": [
        {
            "name": "id",
            "in": "path",
            "description": "",
            "required": true,
            "type": "integer",
            "format": "int64"
        }
    ],
    "responses": {
        "200": {
            "description": "Success"
        }
    }
}

Add a tag to the Create action method documentation. It supplements information specified in the <summary> tag and provides a more robust Swagger UI. The <remarks> tag content can consist of text, JSON, or XML.

/// <summary>
/// Creates a TodoItem.
/// </summary>
/// <remarks>
/// Sample request:
///
///     POST /Todo
///     {
///        "id": 1,
///        "name": "Item1",
///        "isComplete": true
///     }
///
/// </remarks>
/// <param name="item"></param>
/// <returns>A newly-created TodoItem</returns>
/// <response code="201">Returns the newly-created item</response>
/// <response code="400">If the item is null</response>            
[HttpPost]
[ProducesResponseType(typeof(TodoItem), 201)]
[ProducesResponseType(typeof(TodoItem), 400)]
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);
}

Notice the UI enhancements with these additional comments.

Swagger UI with additional comments shown

Data Annotations

Decorate the model with attributes, found in System.ComponentModel.DataAnnotations, to help drive the Swagger UI components.

Add the [Required] attribute to the Name property of the TodoItem class:

using System.ComponentModel;
using System.ComponentModel.DataAnnotations;

namespace TodoApi.Models
{
    public class TodoItem
    {
        public long Id { get; set; }

        [Required]
        public string Name { get; set; }

        [DefaultValue(false)]
        public bool IsComplete { get; set; }
    }
}

The presence of this attribute changes the UI behavior and alters the underlying JSON schema:

"definitions": {
    "TodoItem": {
        "required": [
            "name"
        ],
        "type": "object",
        "properties": {
            "id": {
                "format": "int64",
                "type": "integer"
            },
            "name": {
                "type": "string"
            },
            "isComplete": {
                "default": false,
                "type": "boolean"
            }
        }
    }
},

Add the [Produces("application/json")] attribute to the API controller. Its purpose is to declare that the controller's actions support a return a content type of application/json:

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

The Response Content Type drop-down selects this content type as the default for the controller's GET actions:

Swagger UI with default response content type

As the usage of data annotations in the Web API increases, the UI and API help pages become more descriptive and useful.

Describing Response Types

Consuming developers are most concerned with what is returned — specifically response types and error codes (if not standard). These are handled in the XML comments and data annotations.

The Create action returns 201 Created on success or 400 Bad Request when the posted request body is null. Without proper documentation in the Swagger UI, the consumer lacks knowledge of these expected outcomes. That problem is fixed by adding the highlighted lines in the following example:

/// <summary>
/// Creates a TodoItem.
/// </summary>
/// <remarks>
/// Sample request:
///
///     POST /Todo
///     {
///        "id": 1,
///        "name": "Item1",
///        "isComplete": true
///     }
///
/// </remarks>
/// <param name="item"></param>
/// <returns>A newly-created TodoItem</returns>
/// <response code="201">Returns the newly-created item</response>
/// <response code="400">If the item is null</response>            
[HttpPost]
[ProducesResponseType(typeof(TodoItem), 201)]
[ProducesResponseType(typeof(TodoItem), 400)]
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);
}

The Swagger UI now clearly documents the expected HTTP response codes:

Swagger UI showing POST Response Class description 'Returns the newly created Todo item' and '400 - If the item is null' for status code and reason under Response Messages

Customizing the UI

The stock UI is both functional and presentable; however, when building documentation pages for your API, you want it to represent your brand or theme. Accomplishing that task with the Swashbuckle components requires adding the resources to serve static files and then building the folder structure to host those files.

If targeting .NET Framework, add the Microsoft.AspNetCore.StaticFiles NuGet package to the project:

<PackageReference Include="Microsoft.AspNetCore.StaticFiles" Version="2.0.0" />

Enable the static files middleware:

public void Configure(IApplicationBuilder app)
{
    app.UseStaticFiles();

    // Enable middleware to serve generated Swagger as a JSON endpoint.
    app.UseSwagger();

    // Enable middleware to serve swagger-ui (HTML, JS, CSS, etc.), specifying the Swagger JSON endpoint.
    app.UseSwaggerUI(c =>
    {
        c.SwaggerEndpoint("/swagger/v1/swagger.json", "My API V1");
    });

    app.UseMvc();
}

Acquire the contents of the dist folder from the Swagger UI GitHub repository. This folder contains the necessary assets for the Swagger UI page.

Create a wwwroot/swagger/ui folder, and copy into it the contents of the dist folder.

Create a wwwroot/swagger/ui/css/custom.css file with the following CSS to customize the page header:

.swagger-section #header
{
    border-bottom: 1px solid #000000;
    font-style: normal;
    font-weight: 400;
    font-family: "Segoe UI Light","Segoe WP Light","Segoe UI","Segoe WP",Tahoma,Arial,sans-serif;
    background-color: black;
}

.swagger-section #header h1
{
    text-align: center;
    font-size: 20px;
    color: white;
}

Reference custom.css in the index.html file:

<link href='css/custom.css' media='screen' rel='stylesheet' type='text/css' />

Browse to the index.html page at http://localhost:<random_port>/swagger/ui/index.html. Enter http://localhost:<random_port>/swagger/v1/swagger.json in the header's textbox, and click the Explore button. The resulting page looks as follows:

Swagger UI with custom header title

There is much more you can do with the page. See the full capabilities for the UI resources at the Swagger UI GitHub repository.