C# 0 The Complete Reference

PART I

C h a p t e r   1 8 :  

G e n e r i c s  

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PART IPART I

The base class constraint uses this form of the 

where

 clause:

where 

T

 : 

base-class-name

Here, 

T

 is the name of the type parameter, and 

base-class-name

 is the name of the base class. 

Only one base class can be specified.

Here is a simple example that demonstrates the base class constraint mechanism:

// A simple demonstration of a base class constraint.

using System;

class A {

  public void Hello() {

    Console.WriteLine("Hello");

  }

}

// Class B inherits A.

class B : A { }

// Class C does not inherit A.

class C { }

// Because of the base class constraint, all type arguments

// passed to Test must have A as a base class.

class Test where T : A {

  T obj;

  public Test(T o) {

    obj = o;

  }

  public void SayHello() {

    // OK to call Hello() because it’s declared

    // by the base class A.

    obj.Hello();

  }

}

class BaseClassConstraintDemo {

  static void Main() {

    A a = new A();

    B b = new B();

    C c = new C();

    // The following is valid because A is the specified base class.

    Test t1 = new Test(a);

    t1.SayHello();

    // The following is valid because B inherits A.

    Test t2 = new Test(b);

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    t2.SayHello();

    // The following is invalid because C does not inherit A.

//    Test t3 = new Test(c); // Error!

//    t3.SayHello(); // Error!

  }

}

In this program, class