How to mock the Request on Controller in ASP.Net MVC?

Using Moq:

var request = new Mock<HttpRequestBase>();
// Not working - IsAjaxRequest() is static extension method and cannot be mocked
// request.Setup(x => x.IsAjaxRequest()).Returns(true /* or false */);
// use this
request.SetupGet(x => x.Headers).Returns(
    new System.Net.WebHeaderCollection {
        {"X-Requested-With", "XMLHttpRequest"}
    });

var context = new Mock<HttpContextBase>();
context.SetupGet(x => x.Request).Returns(request.Object);

var controller = new YourController();
controller.ControllerContext = new ControllerContext(context.Object, new RouteData(), controller);

UPDATED:

Mock Request.Headers["X-Requested-With"] or Request["X-Requested-With"] instead of Request.IsAjaxRequest().

For anyone using NSubstitute I was able to modify the above answers and do something like this... (where Details is the Action method name on the controller)

 var fakeRequest = Substitute.For<HttpRequestBase>();
 var fakeContext = Substitute.For<HttpContextBase>();
 fakeRequest.Headers.Returns(new WebHeaderCollection { {"X-Requested-With", "XMLHttpRequest"}});
 fakeContext.Request.Returns(fakeRequest);
 controller.ControllerContext = new ControllerContext(fakeContext, new RouteData(), controller);
 var model = new EntityTypeMaintenanceModel();
        
 var result = controller.Details(model) as PartialViewResult;
        
 Assert.IsNotNull(result);
 Assert.AreEqual("EntityType", result.ViewName);

Here is a working solution using RhinoMocks. I've based it on a Moq solution I found at http://thegrayzone.co.uk/blog/2010/03/mocking-request-isajaxrequest/

public static void MakeAjaxRequest(this Controller controller)
{
        MockRepository mocks = new MockRepository();

        // Create mocks
        var mockedhttpContext = mocks.DynamicMock<HttpContextBase>();
        var mockedHttpRequest = mocks.DynamicMock<HttpRequestBase>();

        // Set headers to pretend it's an Ajax request
        SetupResult.For(mockedHttpRequest.Headers)
            .Return(new WebHeaderCollection() {
                {"X-Requested-With", "XMLHttpRequest"}
            });

        // Tell the mocked context to return the mocked request
        SetupResult.For(mockedhttpContext.Request).Return(mockedHttpRequest);

        mocks.ReplayAll();

        // Set controllerContext
        controller.ControllerContext = new ControllerContext(mockedhttpContext, new RouteData(), controller);
}

Is AjaxRequest is an extension method. So you can do it the following way using Rhino:

    protected HttpContextBase BuildHttpContextStub(bool isAjaxRequest)
    {
        var httpRequestBase = MockRepository.GenerateStub<HttpRequestBase>();   
        if (isAjaxRequest)
        {
            httpRequestBase.Stub(r => r["X-Requested-With"]).Return("XMLHttpRequest");
        }

        var httpContextBase = MockRepository.GenerateStub<HttpContextBase>();
        httpContextBase.Stub(c => c.Request).Return(httpRequestBase);

        return httpContextBase;
    }

    // Build controller
    ....
    controller.ControllerContext = new ControllerContext(BuildHttpContextStub(true), new RouteData(), controller);