How to mock HttpRequestMessage with Moq

Question

I'm working on a web Api. It's an ASP.NET application. I need to create a mock for the method CreateResponse(HttpStatusCode statusCode, T value) of HttpRequestMessage class. But every time I execute my test and get the exception

Test Name:  RHT.Tests.Controllers.AccountControllerTests.Can_login_with_valid_credentials
Test FullName:  RHT.Tests.Controllers.AccountControllerTests.Can_login_with_valid_credentials
Test Source:    Controllers\AccountControllerTests.cs : line 23
Test Outcome:   Failed
Test Duration:  0:00:01.028

Result StackTrace:  
at Moq.Mock.ThrowIfSetupExpressionInvolvesUnsupportedMember(Expression setup, MethodInfo method) in C:\projects\moq4\src\Moq\Mock.cs:line 755
   at Moq.Mock.SetupNonVoidPexProtected(Mock mock, LambdaExpression expression, Condition condition) in C:\projects\moq4\src\Moq\Mock.cs:line 433
   at Moq.PexProtector.Invoke[T1,T2,T3,TResult](Func`4 function, T1 arg1, T2 arg2, T3 arg3) in C:\projects\moq4\src\Moq\PexProtector.cs:line 38
   at Moq.Mock.SetupNonVoid(Mock mock, LambdaExpression expression, Condition condition) in C:\projects\moq4\src\Moq\Mock.cs:line 421
   at Moq.Mock`1.Setup[TResult](Expression`1 expression) in C:\projects\moq4\src\Moq\Mock.Generic.cs:line 317
   at RHT.Tests.Controllers.AccountControllerTests.Can_login_with_valid_credentials() in C:\Users\ttong\source\repos\RedHawkTestPlan\RHT.Tests\Controllers\AccountControllerTests.cs:line 51
Result Message: System.NotSupportedException : Invalid setup on an extension method: m => m.CreateResponse<string>(HttpStatusCode.OK, "value")

The test

        Mock<IAuthProvider> auth = new Mock<IAuthProvider>();
        auth.Setup(m => m.CreateEncryptedCookieValue(
            It.IsAny<bool>(), It.IsAny<string>()))
            .Returns("my_ticket");

        Mock<HttpRequestMessage> req = new Mock<HttpRequestMessage>();
        req.Setup(m => m.CreateResponse(
            HttpStatusCode.OK, "value"))
            .Returns(new HttpResponseMessage(HttpStatusCode.OK));

The test method has 3 mock objects. But only the HttpRequestMessage is fail to initialize. Any idea why it doesn't work?

Answer 1

The answer is in the error message:

System.NotSupportedException: Invalid setup on an extension method

CreateResponse is not a method of the HttpRequestMessage class, but an extension method. Extension methods can't be mocked, since they are actually static methods behind the scenes.