What does "where" mean in a C# class declaration?

Question

I tried to google this, but all I could find was documents on ordinary class declarations.

public class DataContextWrapper<T> : IDataContextWrapper where T : DataContext, new()
{

}

I see that the class implements IDataContextWrapper, inherits from DataContext and varies with type T depending on how it is instantiated.

I don't know what "where T" or the ", new()" might mean.

Answer 1

It's a generic constraint and restricts what types can be passed into the generic parameter.

In your case it requires that T is indentical to or derived from DataContext and has a default(argumentless) constructor(the new() constraint).

You need generic constraints to actually do something non trivial with a generic type.

• The new() constraint allows you to create an instance with new T().
• The DataContext constraint allows you to call the methods of DataContext on an instance of T

MSDN wrote:

where T : <base class name> The type argument must be or derive from the specified base class.

where T : new() The type argument must have a public parameterless constructor. When used together with other constraints, the new() constraint must be specified last.

Answer 2

Only allow types T that are derived from or implement DataContext, and have a public constructor that takes no arguments.

Answer 3

It's a generic type constraint and specifies constraint on the generic types (for example, only classes, or must implement a specific interface).

In this case, T must be a class that is either DataContext or inherits from it and must have a parameterless public constructor (the new() constraint).

Answer 4

It's a generic type restriction. In this case, T must inherit from DataContext and be a type with a constructor that takes no arguments.