In C# what does where T : class mean?

Ie.

public IList<T> DoThis<T>() where T : class

10 Answers

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.

2

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.

2

It is a generic type constraint. In this case it means that the generic type T has to be a reference type (class, interface, delegate, or array type).

0

That restricts T to reference types. You won't be able to put value types (structs and primitive types except string) there.

1

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 }

2

it means that the type used as T when the generic method is used must be a class - i.e. it cannot be a struct or built in number like int or double

// Valid:
var myStringList = DoThis<string>();
// Invalid - compile error
var myIntList = DoThis<int>();

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)

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.").

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

'T' represents a generic type. It means it can accept any type of class. The following article might help:

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