Using OpenAPI with .NET Core
fetched at September 16, 2020

In this article, we’ll look at using OpenAPI with .NET Core. OpenAPI is a specification for describing RESTful APIs. First, I’ll show you how to use OpenAPI to describe the APIs provided by an ASP.NET Core service. Then, we’ll use the API description to generate a strongly-typed client to use the web service with C#.

Writing OpenAPI descriptions

Developers use the OpenAPI specification to describe RESTful APIs. We can then use OpenAPI descriptions to generate a strongly-typed client library that is capable of accessing the APIs.

Note: Swagger is sometimes used synonymously with OpenAPI. It refers to a widely used toolset for working with the OpenAPI specification.

Build the web service

In this section, we’ll use the open source Swashbuckle.AspNetCore package to provide an OpenAPI description of an ASP.NET Core application.

We start by creating a webapi template application:

$ dotnet new webapi -o WebApi1
$ cd WebApi1

The webapi template includes a REST API to get a weather forecast. The API is implemented in the WeatherForecastController.cs file.

Next, we add the Swashbuckle.AspNetCore package:

$ dotnet add package Swashbuckle.AspNetCore

Now, we make a few edits to the Startup.cs file:

public void ConfigureServices(IServiceCollection services)
+   services.AddSwaggerGen();

  public void Configure(IApplicationBuilder app, IWebHostEnvironment env)


+   app.UseSwagger();
+   app.UseSwaggerUI(c =>
+   {
+     c.SwaggerEndpoint("/swagger/v1/swagger.json", "My API V1");
+   });


In the ConfigureServices method, we call AddSwaggerGen. Calling AddSwaggerGen makes the API description available. The API can then be consumed through ASP.NET Core’s dependency injection (DI) system. UseSwagger uses these descriptions to create an HTTP endpoint at /swagger/v1/swagger.json. The UseSwaggerUI then provides a user interface (UI) at /swagger that allows users to easily consume the exposed API from a browser.

Note: The methods called in Startup.cs accept a delegate for configuration. For useful options, see the ASP.NET Core documentation, Get started with Swashbuckle and ASP.NET Core.

Run the app

You can run the application and browse to the Swagger UI, which is shown in Figure 1.

A screenshot of the weather app in the Swagger UI.

Figure 1: The weather app in the Swagger UI.

The Swashbuckle.AspNetCore packages picked up the ASP.NET endpoints. The exposed UI makes it easy to invoke the REST endpoints.

Consuming OpenAPI descriptions

In this section, we’ll look at consuming a RESTful API that has an OpenAPI description. To consume the API, we’ll use the open source package, NSwag.ApiDescription.Client.

First, we create a new console project, and download the OpenAPI description from our ASP.NET application:

$ dotnet new console -o console
$ cd console
$ mkdir openapi
$ wget --no-check-certificate https://localhost:5001/swagger/v1/swagger.json -O openapi/weather.json

Now, we’ll make a few edits to the project file. These edits will be used to generate a strongly-typed client when the .NET project is built:

<Project Sdk="Microsoft.NET.Sdk">
+  <ItemGroup>
+    <PackageReference Include="Newtonsoft.Json" Version="12.0.2" />
+    <PackageReference Include="NSwag.ApiDescription.Client" Version="13.0.5" />
+  </ItemGroup>
+  <ItemGroup>
+    <OpenApiReference Include="openapi/weather.json" Namespace="WeatherService">
+      <ClassName>WeatherClient</ClassName>
+      <OutputPath>WeatherClient.cs</OutputPath>
+    </OpenApiReference>
+  </ItemGroup>

We’ve added references to the NSwag.ApiDescription.Client and Newtonsoft.Json packages. An OpenApiReference element refers to the API description that we downloaded earlier. It adds attributes that are required to generate the code, such as the class name, namespace, and filename.

Build the client

Now, we’ll invoke the build command. Invoking the command generates a WeatherClient.cs file, which lives under the obj directory:

$ dotnet build

We can now edit the Program.cs file and use the strongly-typed WeatherClient class that we’ve just generated:

static async Task Main(string[] args)
  // Configure HttpClientHandler to ignore certificate validation errors.
  using var httpClientHandler = new HttpClientHandler();
  httpClientHandler.ServerCertificateCustomValidationCallback = (message, cert, chain, errors) => { return true; };

  // Create WeatherClient.
  using var httpClient = new HttpClient(httpClientHandler);
  var weatherClient = new WeatherService.WeatherClient("http://localhost:5000", httpClient);

  // Call WeatherForecast API.
  var forecast = await weatherClient.WeatherForecastAsync();
  foreach (var item in forecast)
    Console.WriteLine($"{item.Date} - {item.Summary}");

Run the app

Finally, we run the application:

$ dotnet run
7/1/2020 1:18:18 PM +02:00 - Mild
7/2/2020 1:18:18 PM +02:00 - Bracing
7/3/2020 1:18:18 PM +02:00 - Freezing
7/4/2020 1:18:18 PM +02:00 - Balmy
7/5/2020 1:18:18 PM +02:00 - Bracing

As you can see, the weather report is mixed.


In this article, you learned about the OpenAPI specification, which is sometimes used synonymously with Swagger. Developers use the OpenAPI spec to describe RESTful APIs in preparation for being consumed by a client. I showed you how to use the Swashbuckle.AspNetCore package to provide an OpenAPI description of an API implemented using ASP.NET Core. Then, we used the NSwag.ApiDescription.Client package to generate a strongly-typed client capable of consuming the API.


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