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's a type constraint on T, specifying that it must be a class.

The where clause can be used to specify other type constraints, e.g.:

where T : struct // T must be a struct
where T : new()  // T must have a default parameterless constructor
where T : IComparable // T must implement the IComparable interface

For more information, check out MSDN's page on the where clause, or generic parameter constraints.

Answer 2

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)

Answer 3

T represents an object type of, it implies that you can give any type of. IList : if IList s=new IList; Now s.add("Always accept string.").

Answer 4

where T: class literally means that T has to be a class. It can be any reference type. Now whenever any code calls your DoThis<T>() method it must provide a class to replace T. For example if I were to call your DoThis<T>() method then I will have to call it like following:

DoThis<MyClass>();

If your metthod is like like the following:

public IList<T> DoThis<T>() where T : class
{
   T variablename = new T();

   // other uses of T as a type

}

Then where ever T appears in your method, it will be replaced by MyClass. So the final method that the compiler calls , will look like the following:

public IList<MyClass> DoThis<MyClass>() 
{
   MyClass variablename= new MyClass();

  //other uses of MyClass as a type

  // all occurences of T will similarly be replace by MyClass
 }

Answer 5

Here T refers to a Class.It can be a reference type.

Answer 6

Simply put this is constraining the generic parameter to a class (or more specifically a reference type which could be a class, interface, delegate, or array type).

See this MSDN article for further details.