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

I have a controller in C# using the ASP.Net MVC framework

public class HomeController:Controller{
  public ActionResult Index()
    {
      if (Request.IsAjaxRequest())
        { 
          //do some ajaxy stuff
        }
      return View("Index");
    }
}

I got some tips on mocking and was hoping to test the code with the following and RhinoMocks

var mocks = new MockRepository();
var mockedhttpContext = mocks.DynamicMock<HttpContextBase>();
var mockedHttpRequest = mocks.DynamicMock<HttpRequestBase>();
SetupResult.For(mockedhttpContext.Request).Return(mockedHttpRequest);

var controller = new HomeController();
controller.ControllerContext = new ControllerContext(mockedhttpContext, new RouteData(), controller);
var result = controller.Index() as ViewResult;
Assert.AreEqual("About", result.ViewName);

However I keep getting this error:

Exception System.ArgumentNullException: System.ArgumentNullException : Value cannot be null. Parameter name: request at System.Web.Mvc.AjaxRequestExtensions.IsAjaxRequest(HttpRequestBase request)

Since the Request object on the controller has no setter. I tried to get this test working properly by using recommended code from an answer below.

This used Moq instead of RhinoMocks, and in using Moq I use the following for the same test:

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["X-Requested-With"]).Returns("XMLHttpRequest");

var context = new Mock<HttpContextBase>();
context.SetupGet(x => x.Request).Returns(request.Object);
var controller = new HomeController(Repository, LoginInfoProvider);
controller.ControllerContext = new ControllerContext(context.Object, new RouteData(), controller);
var result = controller.Index() as ViewResult;
Assert.AreEqual("About", result.ViewName);

but get the following error:

Exception System.ArgumentException: System.ArgumentException : Invalid setup on a non-overridable member: x => x.Headers["X-Requested-With"] at Moq.Mock.ThrowIfCantOverride(Expression setup, MethodInfo methodInfo)

Again, it seems like I cannot set the request header. How do I set this value, in RhinoMocks or Moq?

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);

Looks like you are looking for this,

 var requestMock = new Mock<HttpRequestBase>();
 requestMock.SetupGet(rq => rq["Age"]).Returns("2001");

Usage in Controller :

 public ActionResult Index()
 {
        var age = Request["Age"]; //This will return 2001
 }