Unit test a .NET middleware that uses Response.OnStarting

I need to find a way to unit test this middleware.

I’m trying to unit test a .NET Core middleware that adds a header to a response. The middleware uses the OnStarting callback and I’m having difficulty mocking or forcing its execution. Here is a sample of the middleware:

public async Task InvokeAsync(HttpContext context, RequestDelegate next)
    context.Response.OnStarting(() =>
        context.Response.Headers.Add("Name", "Value");

        return Task.CompletedTask;

    await next(context);

I need to find a way to unit test this middleware, as it expects a concrete instance of HttpContext which makes it difficult to mock with a library like FakeItEasy.

One approach to unit test this middleware is to create a HttpContext instance manually and pass it to the middleware. Here is an example test method using xUnit and Moq:

public async Task InvokeAsync_AddsHeaderToResponse()
    // Arrange
    var httpContext = new DefaultHttpContext();
    var response = new Mock<HttpResponse>();
    httpContext.Response = response.Object;

    var middleware = new SampleMiddleware();

    // Act
    await middleware.InvokeAsync(httpContext, _ => Task.CompletedTask);

    // Assert
    response.VerifySet(r => r.Headers.Add("Name", "Value"), Times.Once);

In this test method, we create a new DefaultHttpContext instance and set its Response property to a Mock<HttpResponse> instance. We then create an instance of the middleware and call its InvokeAsync method with the HttpContext instance we just created.

Finally, we use Moq’s VerifySet method to assert that the Headers.Add method was called with the expected arguments.