C# 0 The Complete Reference

PhoneList

 requires 

that a type argument implement the 

IPhoneList

 interface. Because both 

Friend

 and 

Supplier

implement

IPhoneList

, they are valid types to be bound to 

T

. However, 

EmailFriend

 does 

not implement 

IPhoneList

 and cannot be bound to 

T

. To prove this, remove the comment 

symbols from the last two lines in 

Main( )

. As you will see, the program will not compile.

Using the new( ) Constructor Constraint

The

new( )

 constructor constraint enables you to instantiate an object of a generic type. 

Normally, you cannot create an instance of a generic type parameter. However, the 

new( )

constraint changes this because it requires that a type argument supply a parameterless 

constructor. This can be the default constructor provided automatically when no explicit 

constructor is declared or a parameterless constructor explicitly defined by you. With the 

new( )

 constraint in place, you can invoke the parameterless constructor to create an object.

Here is a simple example that illustrates the use of 

new( )

:

// Demonstrate a new() constructor constraint.

using System;

class MyClass {

  public MyClass() {

    // ...

  }

  //...

}

class Test where T : new() {

  T obj;

  public Test() {

    // This works because of the new() constraint.

    obj = new T(); // create a T object

  }

  // ...

}

class ConsConstraintDemo {

  static void Main() {

    Test x = new Test();

  }

}

First, notice the declaration of the 

Test 

class, shown here:

class Test where T : new() {

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