Test ASP.NET Core middleware

By Chris Ross

Middleware can be tested in isolation with TestServer. It allows you to:

  • Instantiate an app pipeline containing only the components that you need to test.
  • Send custom requests to verify middleware behavior.

Advantages:

  • Requests are sent in-memory rather than being serialized over the network.
  • This avoids additional concerns, such as port management and HTTPS certificates.
  • Exceptions in the middleware can flow directly back to the calling test.
  • It's possible to customize server data structures, such as HttpContext, directly in the test.

Set up the TestServer

In the test project, create a test:

  • Build and start a host that uses TestServer.

  • Add any required services that the middleware uses.

  • Add the Microsoft.AspNetCore.TestHost NuGet package to the project:

    <ItemGroup>
      <PackageReference Include="Microsoft.AspNetCore.TestHost" Version="3.1.*" />
    </ItemGroup>
    
  • Configure the processing pipeline to use the middleware for the test.

[Fact]
public async Task MiddlewareTest_ReturnsNotFoundForRequest()
{
    using var host = await new HostBuilder()
        .ConfigureWebHost(webBuilder =>
        {
            webBuilder
                .UseTestServer()
                .ConfigureServices(services =>
                {
                    services.AddMyServices();
                })
                .Configure(app =>
                {
                    app.UseMiddleware<MyMiddleware>();
                });
        })
        .StartAsync();

    ...
}

Send requests with HttpClient

Send a request using HttpClient:

[Fact]
public async Task MiddlewareTest_ReturnsNotFoundForRequest()
{
    using var host = await new HostBuilder()
        .ConfigureWebHost(webBuilder =>
        {
            webBuilder
                .UseTestServer()
                .ConfigureServices(services =>
                {
                    services.AddMyServices();
                })
                .Configure(app =>
                {
                    app.UseMiddleware<MyMiddleware>();
                });
        })
        .StartAsync();

    var response = await host.GetTestClient().GetAsync("/");

    ...
}

Assert the result. First, make an assertion the opposite of the result that you expect. An initial run with a false positive assertion confirms that the test fails when the middleware is performing correctly. Run the test and confirm that the test fails.

In the following example, the middleware should return a 404 status code (Not Found) when the root endpoint is requested. Make the first test run with Assert.NotEqual( ... );, which should fail:

[Fact]
public async Task MiddlewareTest_ReturnsNotFoundForRequest()
{
    using var host = await new HostBuilder()
        .ConfigureWebHost(webBuilder =>
        {
            webBuilder
                .UseTestServer()
                .ConfigureServices(services =>
                {
                    services.AddMyServices();
                })
                .Configure(app =>
                {
                    app.UseMiddleware<MyMiddleware>();
                });
        })
        .StartAsync();

    var response = await host.GetTestClient().GetAsync("/");

    Assert.NotEqual(HttpStatusCode.NotFound, response.StatusCode);
}

Change the assertion to test the middleware under normal operating conditions. The final test uses Assert.Equal( ... );. Run the test again to confirm that it passes.

[Fact]
public async Task MiddlewareTest_ReturnsNotFoundForRequest()
{
    using var host = await new HostBuilder()
        .ConfigureWebHost(webBuilder =>
        {
            webBuilder
                .UseTestServer()
                .ConfigureServices(services =>
                {
                    services.AddMyServices();
                })
                .Configure(app =>
                {
                    app.UseMiddleware<MyMiddleware>();
                });
        })
        .StartAsync();

    var response = await host.GetTestClient().GetAsync("/");

    Assert.Equal(HttpStatusCode.NotFound, response.StatusCode);
}

Send requests with HttpContext

A test app can also send a request using SendAsync(Action<HttpContext>, CancellationToken). In the following example, several checks are made when https://example.com/A/Path/?and=query is processed by the middleware:

[Fact]
public async Task TestMiddleware_ExpectedResponse()
{
    using var host = await new HostBuilder()
        .ConfigureWebHost(webBuilder =>
        {
            webBuilder
                .UseTestServer()
                .ConfigureServices(services =>
                {
                    services.AddMyServices();
                })
                .Configure(app =>
                {
                    app.UseMiddleware<MyMiddleware>();
                });
        })
        .StartAsync();

    var server = host.GetTestServer();
    server.BaseAddress = new Uri("https://example.com/A/Path/");

    var context = await server.SendAsync(c =>
    {
        c.Request.Method = HttpMethods.Post;
        c.Request.Path = "/and/file.txt";
        c.Request.QueryString = new QueryString("?and=query");
    });

    Assert.True(context.RequestAborted.CanBeCanceled);
    Assert.Equal(HttpProtocol.Http11, context.Request.Protocol);
    Assert.Equal("POST", context.Request.Method);
    Assert.Equal("https", context.Request.Scheme);
    Assert.Equal("example.com", context.Request.Host.Value);
    Assert.Equal("/A/Path", context.Request.PathBase.Value);
    Assert.Equal("/and/file.txt", context.Request.Path.Value);
    Assert.Equal("?and=query", context.Request.QueryString.Value);
    Assert.NotNull(context.Request.Body);
    Assert.NotNull(context.Request.Headers);
    Assert.NotNull(context.Response.Headers);
    Assert.NotNull(context.Response.Body);
    Assert.Equal(404, context.Response.StatusCode);
    Assert.Null(context.Features.Get<IHttpResponseFeature>().ReasonPhrase);
}

SendAsync permits direct configuration of an HttpContext object rather than using the HttpClient abstractions. Use SendAsync to manipulate structures only available on the server, such as HttpContext.Items or HttpContext.Features.

As with the earlier example that tested for a 404 - Not Found response, check the opposite for each Assert statement in the preceding test. The check confirms that the test fails correctly when the middleware is operating normally. After you've confirmed that the false positive test works, set the final Assert statements for the expected conditions and values of the test. Run it again to confirm that the test passes.

TestServer limitations

TestServer:

  • Was created to replicate server behaviors to test middleware.
  • Does not try to replicate all HttpClient behaviors.
  • Attempts to give the client access to as much control over the server as possible, and with as much visibility into what's happening on the server as possible. For example it may throw exceptions not normally thrown by HttpClient in order to directly communicate server state.
  • Doesn't set some transport specific headers by default as those are not usually relevant to middleware. For more information, see the next section.

Content-Length and Transfer-Encoding headers

TestServer does not set transport related request or response headers such as Content-Length or Transfer-Encoding. Applications should avoid depending on these headers because their usage varies by client, scenario, and protocol. If Content-Length and Transfer-Encoding are necessary to test a specific scenario, they can be specified in the test when composing the HttpRequestMessage or HttpContext. For more information, see the following GitHub issues: