C# 0 The Complete Reference

Ctr

 is specified for 

Counter

 by the following line:

using Ctr = Counter;

Then, inside 

Main( )

, this alias is used to qualify 

CountDown

, as shown here:

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

The use of the 

::

 qualifier removes the ambiguity because it specifies that the 

CountDown

 in 

Ctr

 (which stands for 

Counter

) is desired, and the program now compiles.

You can use the 

::

 qualifier to refer to the global namespace by using the predefined 

identifier

global

. For example, in the following program, a class called 

CountDown

 is 

declared in both the 

Counter

 namespace and in the global namespace. To access the 

version of 

CountDown

 in the global namespace, the predefined alias 

global

 is used.

// Use the global alias.

using System;

// 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;

    }

    // ...

  }

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}

// Declare another class called CountDown, which

// is in the global namespace.

class CountDown {

  int val;

  public CountDown(int n) {

    val = n;

  }

  // ...

}

class GlobalAliasQualifierDemo {

  static void Main() {

    // Here, the :: qualifier tells the compiler

    // to use the CountDown in the Counter namespace.

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

    // Next, create CountDown object from global namespace.

    global::CountDown cd2 = new global::CountDown(10);

    // ...

  }

}

Notice how the 

global

 identifier is used to access the version of 

CountDown

 in the default 

namespace:

global::CountDown cd2 = new global::CountDown(10);

This same general approach can be generalized to any situation in which you need to 

specify the default namespace.

One final point: You can also use the namespace alias qualifier with 

extern

 aliases, 

which are described in Chapter 20.