In c# what does 'where T : class' mean?

Question

In C# what does where T : class mean?

Ie.

public IList DoThis() where T : class

Answer 1

It is called a type parameter constraint. Effectively it constraints what type T can be.

The type argument must be a reference type; this applies also to any class, interface, delegate, or array type.

Constraints on Type Parameters (C# Programming Guide)