C# 0 The Complete Reference

class

 or 

struct

must be the first constraint in the list.

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Here is an example that demonstrates the reference type constraint:

// Demonstrate a reference constraint.

using System;

class MyClass {

  //...

}

// Use a reference constraint.

class Test where T : class {

  T obj;

  public Test() {

    // The following statement is legal only

    // because T is guaranteed to be a reference

    // type, which can be assigned the value null.

    obj = null;

  }

  // ...

}

class ClassConstraintDemo {

  static void Main() {

    // The following is OK because MyClass is a class.

    Test x = new Test();

    // The next line is in error because int is a value type.

//    Test y = new Test();

  }

}

First, notice how 

Test

 is declared:

class Test where T : class {

The

class

 constraint requires that any type argument for 

T

 be a reference type. In this 

program, this is necessary because of what occurs inside the 

Test

 constructor:

public Test() {

  // The following statement is legal only

  // because T is guaranteed to be a reference

  // type, which can be assigned the value null.

  obj = null;

}

Here,