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N a m e s p a c e s , t h e P r e p r o c e s s o r, a n d A s s e m b l i e s
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// The following line is inherently ambiguous! // Does it refer to CountDown in Counter or // to CountDown in AnotherCounter CountDown cd1 = new CountDown(10); // Error! ! ! // } }
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PART I
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If you try to compile this program, you will receive an error message stating that this line in Main( ) is ambiguous:
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CountDown cd1 = new CountDown(10); // Error! ! !
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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:
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// 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 // tells the compiler to use the CountDown // that is in the Counter namespace Ctr::CountDown cd1 = new Ctr::CountDown(10); // } }
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In this version, the alias 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); // } }
PART I
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 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 20
The Preprocessor
C# defines several preprocessor directives, which affect the way that your program s source file is interpreted by the compiler These directives affect the text of the source file in which they occur, prior to the translation of the program into object code The term preprocessor directive comes from the fact that these instructions were traditionally handled by a separate compilation phase called the preprocessor Today s modern compiler technology no longer requires a separate preprocessing stage to handle the directives, but the name has stuck C# defines the following preprocessor directives:
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