Mocking HttpResponse WriteAsync

Question

I'm trying to get a call to WriteAsync mocked on a mockHttpResponse and I can't figure out the syntax to use.

var responseMock = new Mock<HttpResponse>();
responseMock.Setup(x => x.WriteAsync(It.IsAny<string>(), It.IsAny<CancellationToken>()));

ctx.Setup(x => x.Response).Returns(responseMock.Object);

The test bombs with the following error:

System.NotSupportedException : Invalid setup on an extension method: x => x.WriteAsync(It.IsAny(), It.IsAny())

Ultimately I want to verify the correct string has been written to the response.

How to correctly set this up?

Answer 1

Here's a solution that seems to work in .NET Core 3.1, for completeness:

const string expectedResponseText = "I see your schwartz is as big as mine!";

DefaultHttpContext httpContext = new DefaultHttpContext();
httpContext.Response.Body = new MemoryStream();

// Whatever your test needs to do

httpContext.Response.Body.Position = 0;
using (StreamReader streamReader = new StreamReader(httpContext.Response.Body))
{
    string actualResponseText = await streamReader.ReadToEndAsync();
    Assert.Equal(expectedResponseText, actualResponseText);
}

Answer 2

Moq cannot Setup extension methods. If you know what the extension method accesses then some cases you can mock a safe path through the extension method.

WriteAsync(HttpResponse, String, CancellationToken)

Writes the given text to the response body. UTF-8 encoding will be used.

directly accesses the HttpResponse.Body.WriteAsync where Body is a Stream via the following overload

/// <summary>
/// Writes the given text to the response body using the given encoding.
/// </summary>
/// <param name="response">The <see cref="HttpResponse"/>.</param>
/// <param name="text">The text to write to the response.</param>
/// <param name="encoding">The encoding to use.</param>
/// <param name="cancellationToken">Notifies when request operations should be cancelled.</param>
/// <returns>A task that represents the completion of the write operation.</returns>
public static Task WriteAsync(this HttpResponse response, string text, Encoding encoding, CancellationToken cancellationToken = default(CancellationToken))
{
    if (response == null)
    {
        throw new ArgumentNullException(nameof(response));
    }

    if (text == null)
    {
        throw new ArgumentNullException(nameof(text));
    }

    if (encoding == null)
    {
        throw new ArgumentNullException(nameof(encoding));
    }

    byte[] data = encoding.GetBytes(text);
    return response.Body.WriteAsync(data, 0, data.Length, cancellationToken);
}

This means you would need mock response.Body.WriteAsync

//Arrange
var expected = "Hello World";
string actual = null;
var responseMock = new Mock<HttpResponse>();
responseMock
    .Setup(_ => _.Body.WriteAsync(It.IsAny<byte[]>(),It.IsAny<int>(), It.IsAny<int>(), It.IsAny<CancellationToken>()))
    .Callback((byte[] data, int offset, int length, CancellationToken token)=> {
        if(length > 0)
            actual = Encoding.UTF8.GetString(data);
    })
    .ReturnsAsync();

//...code removed for brevity

//...
Assert.AreEqual(expected, actual);

The callback was used to capture the arguments passed to the mocked member. Its value was stored in a variable to be asserted later in the test.