如果您使用的是 Typemock ，你可以这样做：

If you are using Typemock, you can do this:

Isolate.WhenCalled(()=>controller.HttpContext.Current.Session["UserID"])
.WillReturn("your id");

测试code将看起来像：

The test code will look like:

[TestMethod]
public void Retrieve_IndexTest()
{
    // Arrange
    const string expectedViewName = "Index";

    IndexController controller = new IndexController();
    Isolate.WhenCalled(()=>controller.HttpContext.Current.Session["UserID"])
    .WillReturn("your id");
    // Act
    var result = controller.Index() as ViewResult;

    // Assert
    Assert.IsNotNull(result, "Should have returned a ViewResult");
    Assert.AreEqual(expectedViewName, result.ViewName, "View name should have been {0}", expectedViewName);
}