On the C# FAQ blog, there was a post on why reference parameter types aren't polymorphic. In the feedback section, there was some further confusion. Part of the problem is that the terminology is a bit confusing. You have variables that are reference or value types. And, you can pass either type by reference or value. There are really 4 independent combinations here. Jon Skeet pointed to an excelent article of his on parameter passing in C#. In fact, he's got a bunch of interesting articles.