How to Mock HttpContext.User.Identity.Name in Asp.Net MVC 4

you can initialize your controller with fake context with fake principal as shown below

var fakeHttpContext = new Mock<HttpContextBase>();
var fakeIdentity = new GenericIdentity("User");
var principal = new GenericPrincipal(fakeIdentity, null);

fakeHttpContext.Setup(t => t.User).Returns(principal);
var controllerContext = new Mock<ControllerContext>();
controllerContext.Setup(t => t.HttpContext).Returns(fakeHttpContext.Object);

_requestController = new RequestController();

//Set your controller ControllerContext with fake context
_requestController.ControllerContext = controllerContext.Object;