The .NET type system won't allow type declarations of the form you're attempting. One reason why this is disallowed should be intuitive: how would MyGenericClass act when T is a sealed class (e.g. System.String)?
If you absolutely need this functionality (and you know that the type T you'll be using isn't sealed), you can generate proxies at runtime using the classes in the Reflection.Emit namespace. It may also be possible to achieve this effect using AOP tools like PostSharp.
