C# 0 The Complete Reference

v

 will contain 

null

 instead of a reference 

to an object. This means that for the first increment, a new object is required.

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A type parameter for a generic interface can have constraints in the same way as it can 

for a generic class. For example, this version of 

ISeries

 restricts its use to reference types:

public interface ISeries where T : class {

When this version of 

ISeries

 is implemented, the implementing class must also specify the 

same constraint for 

T

, as shown here:

class ByTwos : ISeries where T : class {

Because of the reference constraint, this version of 

ISeries

 cannot be used on value types. 

Thus, in the preceding program, only 

ByTwos

 would be valid.