What does where T : name of type mean? [closed]

Question

I see code like this many times:

 public class BaseList<T> : List<T> where T : BaseBE

My question is what is meaning of this code and why do we write this line like this? I know it is using List<T> but what is the meaning of where T : BaseBE?

Answer 1

This statement where T:BaseBE is a constraint over what T can be. In this specific case, it's telling you that T can be of type BaseBEor from any class inheriting from it, but nothing else.

For more details you could check MSDN, you'll find much more details and examples.

Answer 2

It means that the generic type T must inherit from BaseBE, this is called a type constraint. This allows the type T to be used as a BaseBE within BaseList.

So for example:

class Foo { }

BaseList<Foo> myList; // Wont compile, Foo is not a BaseBE

class Bar : BaseBE { }

BaseList<Bar> myOtherList; // Ok Bar is a BaseBE

You can read about more types of constraint here:

http://msdn.microsoft.com/en-us/library/d5x73970(v=vs.80).aspx

E.g where T : new() means T must have a public parameterless constructor.