C# 0 The Complete Reference

The Reference Type and Value Type Constraints

The next two constraints enable you to indicate that a type argument must be either a 

reference type or a value type. These are useful in the few cases in which the difference 

between reference and value types are important to generic code. Here is the general form 

of the reference type constraint:

where 

T

 : class

In this form of the 

where

 clause, the keyword 

class

 specifies that 

T

 must be a reference type. 

Thus, an attempt to use a value type, such as 

int

 or 

bool

, for 

T

 will result in a compilation 

error.

Here is the general form of the value type constraint:

where 

T

 : struct

In this case, the keyword 

struct

 specifies that 

T

 must be a value type. (Recall that structures 

are value types.) Thus, an attempt to use a reference type, such as 

string

, for 

T

 will result in 

a compilation error. In both cases, when additional constraints are present,