C# 0 The Complete Reference

PART I

C h a p t e r   1 6 :  

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449

PART IPART I

    // The following line is inherently ambiguous!

    // Does it refer to CountDown in Counter or

    // to CountDown in AnotherCounter?

    CountDown cd1 = new CountDown(10); // Error! ! !

    // ...

  }

}

If you try to compile this program, you will receive an error message stating that this 

line in 

Main( )

 is ambiguous:

CountDown cd1 = new CountDown(10); // Error! ! !

The trouble is that both namespaces, 

Counter

 and 

AnotherCounter

, declare a class called 

CountDown

, and both namespaces have been brought into view. Thus, to which version of 

CountDown

 does the preceding declaration refer? The 

::

 qualifier was designed to handle 

these types of problems.

To use the 

::

, you must first define an alias for the namespace you want to qualify. Then, 

simply qualify the ambiguous element with the alias. For example, here is one way to fix the 

preceding program:

// Demonstrate the :: qualifier.

using System;

using Counter;

using AnotherCounter;

// Give Counter an alias called Ctr.

using Ctr = Counter;

// Declare a namespace for counters.

namespace Counter {

  // A simple countdown counter.

  class CountDown {

    int val;

    public CountDown(int n) {

      val = n;

    }

    // ...

  }

}

// Another counter namespace.

namespace AnotherCounter {

  // Declare another class called CountDown, which

  // is in the AnotherCounter namespace.

  class CountDown {

    int val;

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    public CountDown(int n) {

      val = n;

    }

    // ...

  }

}

class AliasQualifierDemo {

  static void Main() {

    // Here, the :: operator to resolve

    // tells the compiler to use the CountDown

    // that is in the Counter namespace.

    Ctr::CountDown cd1 = new Ctr::CountDown(10);

    // ...

  }

}

In this version, the alias