Get started with NSwag and ASP.NET Core

By Christoph Nienaber, Rico Suter, and Dave Brock

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:

  • 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 ""
    • Enter "NSwag.AspNetCore" in the search box
    • Select the "NSwag.AspNetCore" package from the Browse tab and click Install

Add and configure Swagger middleware

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

  • Import the following namespaces:
using NJsonSchema;
using NSwag.AspNetCore;
  • In the ConfigureServices method, register the required Swagger services:
public void ConfigureServices(IServiceCollection services)
    services.AddDbContext<TodoContext>(opt => 

    // Register the Swagger services
  • In the Configure method, enable the middleware for serving the generated Swagger specification and the Swagger UI:
public void Configure(IApplicationBuilder app)

    // Register the Swagger generator and the Swagger UI middlewares

  • 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:

Generate code with NSwagStudio

  • Install NSwagStudio by following the instructions at the NSwagStudio GitHub repository.

  • 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.

    Create local copy of Swagger specification

  • In the Outputs area, click the CSharp Client check box. 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)) (
// </auto-generated>

namespace MyNamespace
    #pragma warning disable

    [System.CodeDom.Compiler.GeneratedCode("NSwag", " (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();
                return settings;
        public string BaseUrl 
            get { return _baseUrl; }
            set { _baseUrl = value; }

        // code omitted for brevity


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.SwaggerContact
            Name = "Shayne Boyer",
            Email = string.Empty,
            Url = ""
        document.Info.License = new NSwag.SwaggerLicense
            Name = "Use under LICX",
            Url = ""

The Swagger UI displays the version's information:

Swagger UI with version information

XML comments

To enable XML comments, perform the following steps:

  • Right-click the project in Solution Explorer and select Edit <project_name>.csproj.
  • Manually add the highlighted lines to the .csproj file:
  • Right-click the project in Solution Explorer and select Properties
  • Check the XML documentation file box under the Output section of the Build tab

Data annotations

Because NSwag uses Reflection, and the recommended return type for web API actions is IActionResult, it can't infer what your action is doing and what it returns.

Consider the following example:

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


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

The preceding action returns IActionResult, but inside the action it's returning either CreatedAtRoute or BadRequest. Use data annotations to tell clients which HTTP status codes this action is known to return. Decorate the action with the following attributes:

[ProducesResponseType(typeof(TodoItem), 201)]   // Created
[ProducesResponseType(400)]                     // BadRequest

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:

public ActionResult<TodoItem> Create(TodoItem item)

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

The preceding action returns ActionResult<T>. Inside the action, it's returning CreatedAtRoute. Since the controller is decorated with 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. Decorate the action with the following attributes:

[ProducesResponseType(201)]     // Created
[ProducesResponseType(400)]     // 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, decorate all actions with these attributes.

For guidelines on what HTTP responses your API actions should return, see the RFC 7231 specification.