At work we write a lot of ASP.NET REST controllers. Some of the controllers have their API documented in the Azure REST API specs in the form of a Swagger specification, and some are consumed internally by a system that requires us to expose a Swagger specification. This is our problem: say we want to write a controller for Widgets that supports CRUD operations. A developer has to:

  1. Write the C# code.
  2. Write the Swagger JSON necessary.
  3. Ensure (erm, try really hard) to make sure the swagger and the C# stay consistent with each other.
  4. Ship the Swagger to the clients.

There are existing tools like NSwag and Swashbuckle that are almost tailor made for this but they assume you’re going to generate perfectly legal OpenAPIv2/3 documents, which I’m not because this internal swagger has some… customizations. We can easily adapt a proper Swagger document into our custom format, ultimately we are trying to generate a swagger document at build time from our source code which we will then transform some more (again, at build time!).


Thankfully NSwag has a component that does almost exactly what we need: NSwag.MSBuild. With this package, we can generate a Swagger document for some or all of the controllers in an assembly, and then we can modify the document to fit our custom Swagger format. Let’s see how it works:

First I generate a small project with:

> dotnet new webapi

At this time, this template contains one controller with a GET method on /WeatherForecast that returns an object with weather forecast data.

Then I add a dependency on NSwag with

> dotnet add package NSwag.AspNetCore
> dotnet add package NSwag.MSBuild 

This is my Startup.cs with the call to AddSwaggerDocument, this is necessary for the generator to discover controllers in the assembly:

public class Startup
    public Startup(IConfiguration configuration)
        Configuration = configuration;

    public IConfiguration Configuration { get; }

    public void ConfigureServices(IServiceCollection services)

    public void Configure(IApplicationBuilder app, IWebHostEnvironment env)
        if (env.IsDevelopment())
        app.UseEndpoints(endpoints =>

And finally I add a target in the project file to call the Swagger generator executable. Note that I am on a Mac so I have to call the generator with the property for .NET 5.

<Target Name="NSwag" AfterTargets="Build">
    Command="$(NSwagExe_Net50) aspnetcore2openapi /assembly:$(TargetDir)$(MSBuildProjectName).dll /output:swagger.json" />

Some nice build output:

Microsoft (R) Build Engine version 16.8.0+126527ff1 for .NET
Copyright (C) Microsoft Corporation. All rights reserved.

  Determining projects to restore...
  All projects are up-to-date for restore.
  swaggergen -> /Users/gustavo/code/swaggergen/bin/Debug/net5.0/swaggergen.dll
  NSwag command line tool for .NET Core Net50, toolchain v13.11.3.0 (NJsonSchema v10.4.4.0 (Newtonsoft.Json v12.0.0.0))
  Visit for more information.
  NSwag bin directory: /Users/gustavo/.nuget/packages/nswag.msbuild/13.11.3/tools/Net50
  Code has been successfully written to file.
  Duration: 00:00:00.8377024

Build succeeded.
    0 Warning(s)
    0 Error(s)

Time Elapsed 00:00:03.55

And finally, a swagger.json file is produced next to our project file. Let’s take a look at it:

  "x-generator": "NSwag v13.11.3.0 (NJsonSchema v10.4.4.0 (Newtonsoft.Json v12.0.0.0))",
  "swagger": "2.0",
  "info": {
    "title": "My Title",
    "version": "1.0.0"
  "produces": [
  "paths": {
    "/WeatherForecast": {
      "get": {
        "tags": [
        "operationId": "WeatherForecast_Get",
        "responses": {
          "200": {
            "x-nullable": false,
            "description": "",
            "schema": {
              "type": "array",
              "items": {
                "$ref": "#/definitions/WeatherForecast"
  "definitions": {
    "WeatherForecast": {
      "type": "object",
      "required": [
      "properties": {
        "date": {
          "type": "string",
          "format": "date-time"
        "temperatureC": {
          "type": "integer",
          "format": "int32"
        "temperatureF": {
          "type": "integer",
          "format": "int32"
        "summary": {
          "type": "string"

Pretty good! The generator:

  1. Found the controller in the assembly.
  2. Generated an object in paths for each controller method.
  3. Generated definitions for the objects the controller returns.

For a very simple controller, that Swagger document is a whopping 141 lines long! I will grant that it is a full Swagger document with all of the ceremony of Swagger, but still even if it was only 70 lines that would be a huge change to review. If the author tells me “The swagger is automatically generated from the code”, then I’ll spend exactly 0 seconds looking at it and instead focus on reviewing the code that produced it.


Generating Swagger files from C# code is extremely easy with NSwag. Armed with these tools, you can reduce the amount of time you spend toiling at the Swagger mines.