C# 0 The Complete Reference

IsWorkNumber

 from within 

PhoneList

. As 

you will see, the compiler won’t let you do it. The reason is that 

IsWorkNumber

 is a property 

defined by 

Friend

, not by 

PhoneNumber

. Thus, 

PhoneList

 has no knowledge of it.

Using an Interface Constraint

The interface constraint enables you to specify an interface that a type argument must 

implement. The interface constraint serves the same two important purposes as the base 

class constraint. First, it lets you use the members of the interface within the generic class. 

Second, it ensures that only type arguments that implement the specified interface are used. 

This means that for any given interface constraint, the type argument must be either the 

interface or a type that implements that interface.

The interface constraint uses this form of the 

where

 clause:

where 

T

:

 interface-name

Here, 

T

 is the name of the type parameter, and 

interface-name

 is the name of the interface. 

More than one interface can be specified by using a comma-separated list. If a constraint 

includes both a base class and interface, then the base class must be listed first.

The following program illustrates the interface constraint by reworking the telephone 

list example shown in the previous section. In this version, the 

PhoneNumber

 class has 

been converted into an interface called 

IPhoneNumber

. This interface is then implemented 

by