C# 0 The Complete Reference

GetOb( )

 is called for an object of type 

Gen2

, but the base class version is called for an object of type 

Gen

.

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Notice one other thing: This line

iOb = new Gen2(99);

is valid because 

iOb

 is a variable of type 

Gen

. Thus, it can refer to any object of type 

Gen

 or any object of a class derived from 

Gen

, including 

Gen2

. Of course, 

iOb

 couldn’t be used to refer to an object of type 

Gen2

, for example, because of 

the type mismatch.

Overloading Methods That Use Type Parameters

Methods that use type parameters to declare method parameters can be overloaded. However, 

the rules are a bit more stringent than they are for methods that don’t use type parameters. 

In general, a method that uses a type parameter as the data type of a parameter can be 

overloaded as long as the signatures of the two versions differ. This means the type and/or 

number of their parameters must differ. However, the determination of type difference is 

not based on the generic type parameter, but on the type argument substituted for the type 

parameter when a constructed type is created. Therefore, it is possible to overload a method 

that uses type parameters in such a way that it “looks right,” but won’t work in all specific 

cases.

For example, consider this generic class:

// Ambiguity can result when overloading methods that

// use type parameters.

//

// This program will not compile.

using System;

// A generic class that contains a potentially ambiguous

// overload of the Set() method.

class Gen {

  T ob1;

  V ob2;

  // ...

  // In some cases, these two methods

  // will not differ in their parameter types.

  public void Set(T o) {

    ob1 = o;

  }

  public void Set(V o) {

    ob2 = o;

  }

}

class AmbiguityDemo {

  static void Main() {

    Gen ok = new Gen();

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