Get started with NSwag and ASP.NET Core

By Christoph Nienaber, Rico Suter, and Dave Brock

View or download sample code (how to download)

NSwag offers the following capabilities:

• The ability to utilize the Swagger UI and Swagger generator.
• Flexible code generation capabilities.

With NSwag, you don't need an existing API—you can use third-party APIs that incorporate Swagger and generate a client implementation. NSwag allows you to expedite the development cycle and easily adapt to API changes.

Register the NSwag middleware

Register the NSwag middleware to:

• Generate the Swagger specification for the implemented web API.
• Serve the Swagger UI to browse and test the web API.

To use the NSwag ASP.NET Core middleware, install the NSwag.AspNetCore NuGet package. This package contains the middleware to generate and serve the Swagger specification, Swagger UI (v2 and v3), and ReDoc UI.

Use one of the following approaches to install the NSwag NuGet package:

Visual Studio

• From the Package Manager Console window:

  • Go to View > Other Windows > Package Manager Console

  • Navigate to the directory in which the TodoApi.csproj file exists

  • Execute the following command:

    Install-Package NSwag.AspNetCore
    

• From the Manage NuGet Packages dialog:

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

Visual Studio for Mac

• Right-click the Packages folder in Solution Pad > Add Packages...
• Set the Add Packages window's Source drop-down to "nuget.org"
• Enter "NSwag.AspNetCore" in the search box
• Select the "NSwag.AspNetCore" package from the results pane and click Add Package

.NET Core CLI

Run the following command:

dotnet add TodoApi.csproj package NSwag.AspNetCore

Add and configure Swagger middleware

Add and configure Swagger in your ASP.NET Core app by performing the following steps:

• In the Startup.ConfigureServices method, register the required Swagger services:

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

    // Register the Swagger services
    services.AddSwaggerDocument();
}

• In the Startup.Configure method, enable the middleware for serving the generated Swagger specification and the Swagger UI:

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

    // Register the Swagger generator and the Swagger UI middlewares
    app.UseOpenApi();
    app.UseSwaggerUi3();

    app.UseMvc();
}

• Launch the app. Navigate to:
  • http://localhost:<port>/swagger to view the Swagger UI.
  • http://localhost:<port>/swagger/v1/swagger.json to view the Swagger specification.

Code generation

You can take advantage of NSwag's code generation capabilities by choosing one of the following options:

• NSwagStudio: A Windows desktop app for generating API client code in C# or TypeScript.
• The NSwag.CodeGeneration.CSharp or NSwag.CodeGeneration.TypeScript NuGet packages for code generation inside your project.
• NSwag from the command line.
• The NSwag.MSBuild NuGet package.
• The Unchase OpenAPI (Swagger) Connected Service: A Visual Studio Connected Service for generating API client code in C# or TypeScript. Also generates C# controllers for OpenAPI services with NSwag.

Generate code with NSwagStudio

• Install NSwagStudio by following the instructions at the NSwagStudio GitHub repository. On the NSwag release page you can download an xcopy version which can be started without installation and admin privileges.

• Launch NSwagStudio and enter the swagger.json file URL in the Swagger Specification URL text box. For example, http://localhost:44354/swagger/v1/swagger.json.

• Click the Create local Copy button to generate a JSON representation of your Swagger specification.

  

• In the Outputs area, click the CSharp Client checkbox. Depending on your project, you can also choose TypeScript Client or CSharp Web API Controller. If you select CSharp Web API Controller, a service specification rebuilds the service, serving as a reverse generation.

• Click Generate Outputs to produce a complete C# client implementation of the TodoApi.NSwag project. To see the generated client code, click the CSharp Client tab:

//----------------------
// <auto-generated>
//     Generated using the NSwag toolchain v12.0.9.0 (NJsonSchema v9.13.10.0 (Newtonsoft.Json v11.0.0.0)) (http://NSwag.org)
// </auto-generated>
//----------------------

namespace MyNamespace
{
    #pragma warning disable

    [System.CodeDom.Compiler.GeneratedCode("NSwag", "12.0.9.0 (NJsonSchema v9.13.10.0 (Newtonsoft.Json v11.0.0.0))")]
    public partial class TodoClient
    {
        private string _baseUrl = "https://localhost:44354";
        private System.Net.Http.HttpClient _httpClient;
        private System.Lazy<Newtonsoft.Json.JsonSerializerSettings> _settings;

        public TodoClient(System.Net.Http.HttpClient httpClient)
        {
            _httpClient = httpClient;
            _settings = new System.Lazy<Newtonsoft.Json.JsonSerializerSettings>(() =>
            {
                var settings = new Newtonsoft.Json.JsonSerializerSettings();
                UpdateJsonSerializerSettings(settings);
                return settings;
            });
        }

        public string BaseUrl
        {
            get { return _baseUrl; }
            set { _baseUrl = value; }
        }

        // code omitted for brevity

Tip

The C# client code is generated based on selections in the Settings tab. Modify the settings to perform tasks such as default namespace renaming and synchronous method generation.

• Copy the generated C# code into a file in the client project that will consume the API.
• Start consuming the web API:

 var todoClient = new TodoClient();

// Gets all to-dos from the API
 var allTodos = await todoClient.GetAllAsync();

 // Create a new TodoItem, and save it via the API.
var createdTodo = await todoClient.CreateAsync(new TodoItem());

// Get a single to-do by ID
var foundTodo = await todoClient.GetByIdAsync(1);

Customize API documentation

Swagger provides options for documenting the object model to ease consumption of the web API.

API info and description

In the Startup.ConfigureServices method, a configuration action passed to the AddSwaggerDocument method adds information such as the author, license, and description:

services.AddSwaggerDocument(config =>
{
    config.PostProcess = document =>
    {
        document.Info.Version = "v1";
        document.Info.Title = "ToDo API";
        document.Info.Description = "A simple ASP.NET Core web API";
        document.Info.TermsOfService = "None";
        document.Info.Contact = new NSwag.OpenApiContact
        {
            Name = "Shayne Boyer",
            Email = string.Empty,
            Url = "https://twitter.com/spboyer"
        };
        document.Info.License = new NSwag.OpenApiLicense
        {
            Name = "Use under LICX",
            Url = "https://example.com/license"
        };
    };
});

The Swagger UI displays the version's information:

XML comments

To enable XML comments, perform the following steps:

Visual Studio

• Right-click the project in Solution Explorer and select Edit <project_name>.csproj.
• Manually add the highlighted lines to the .csproj file:

<PropertyGroup>
  <GenerateDocumentationFile>true</GenerateDocumentationFile>
  <NoWarn>$(NoWarn);1591</NoWarn>
</PropertyGroup>

Visual Studio for Mac

• From the Solution Pad, press control and click the project name. Navigate to Tools > Edit File.
• Manually add the highlighted lines to the .csproj file:

<PropertyGroup>
  <GenerateDocumentationFile>true</GenerateDocumentationFile>
  <NoWarn>$(NoWarn);1591</NoWarn>
</PropertyGroup>

.NET Core CLI

Manually add the highlighted lines to the .csproj file:

<PropertyGroup>
  <GenerateDocumentationFile>true</GenerateDocumentationFile>
  <NoWarn>$(NoWarn);1591</NoWarn>
</PropertyGroup>

Data annotations

Because NSwag uses Reflection, and the recommended return type for web API actions is ActionResult<T>, it can only infer the return type defined by T. You can't automatically infer other possible return types.

Consider the following example:

[HttpPost]
public ActionResult<TodoItem> Create(TodoItem item)
{
    _context.TodoItems.Add(item);
    _context.SaveChanges();

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

The preceding action returns ActionResult<T>. Inside the action, it's returning CreatedAtRoute. Since the controller has the [ApiController] attribute, a BadRequest response is possible, too. For more information, see Automatic HTTP 400 responses. Use data annotations to tell clients which HTTP status codes this action is known to return. Mark the action with the following attributes:

[ProducesResponseType(StatusCodes.Status201Created)]     // Created
[ProducesResponseType(StatusCodes.Status400BadRequest)]  // BadRequest

In ASP.NET Core 2.2 or later, you can use conventions instead of explicitly decorating individual actions with [ProducesResponseType]. For more information, see Use web API conventions.

The Swagger generator can now accurately describe this action, and generated clients know what they receive when calling the endpoint. As a recommendation, mark all actions with these attributes.

For guidelines on what HTTP responses your API actions should return, see RFC 9110: HTTP Semantics (Section 9.3. Method Definitions).