unit tests for Mvc exception filter detailed here http://www.prideparrot.com/blog/archive/2012/5/exception_handling_in_asp_net_mvc

CustomHandleErrorAttributeTests.cs

[TestFixture]
public class CustomHandleErrorAttributeTests
{
	private CustomHandleErrorAttribute sut;
    private ExceptionContext exceptionContext;
    private readonly Mock<HttpRequestBase> mockRequest= new Mock<HttpRequestBase>();
    private readonly Mock<HttpContextBase> mockContext = new Mock<HttpContextBase>();
    private readonly Mock<HttpResponseBase> mockResponse = new Mock<HttpResponseBase>();

    [SetUp]
    public void Setup()
    {
        sut = new CustomHandleErrorAttribute
        {
            ExceptionType = typeof(HttpException)
        };
        
        mockContext.Setup(htx => htx.Request)
                       .Returns(mockRequest.Object);

        mockContext.Setup(htx => htx.Response)
                   .Returns(mockResponse.Object);

        mockContext.Setup(htx => htx.IsCustomErrorEnabled)
                   .Returns(true);

        exceptionContext = new ExceptionContext()
            {
                HttpContext = mockContext.Object,
                Exception = new HttpException()
            };
    }

    [Test]
    public void WhenAlreadyHandledThenResultPreserved()
    {
        exceptionContext.ExceptionHandled = true;
        sut.OnException(exceptionContext);
        exceptionContext.Result.Should().BeOfType<EmptyResult>();
    }

    [Test]
    public void WhenCustomErrorsNotEnabledThenResultPreserved()
    {
        mockContext.Setup(htx => htx.IsCustomErrorEnabled)
                   .Returns(false);
        sut.OnException(exceptionContext);
        exceptionContext.Result.Should().BeOfType<EmptyResult>();
    }

    [Test]
    public void WhenExceptionTypeDiffersThenResultPreserved()
    {
        exceptionContext.Exception = new Exception();
        sut.OnException(exceptionContext);
        exceptionContext.Result.Should().BeOfType<EmptyResult>();
    }

    [Test]
    public void WhenAjaxRequestThenResultIsJsonResult()
    {         
        mockRequest.Setup(r => r.Headers)
                   .Returns(new NameValueCollection
            {
                {"X-Requested-With", "XMLHttpRequest"}
            });

        sut.OnException(exceptionContext);
        exceptionContext.Result.Should().BeOfType<JsonResult>();
    }
}

Hi,I can't run this test case.
Can you tell me the name of the mock framework?
Thanks.

I know this comment is over a year late, but the mock framework in this gist is Moq.