C# 0 The Complete Reference

PART I

C h a p t e r   2 0 :  

U n s a f e   C o d e ,   P o i n t e r s ,   N u l l a b l e   T y p e s ,   a n d   M i s c e l l a n e o u s   T o p i c s  

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PART IPART I

    }

  }

}

Notice that both 

test1.cs

 and 

test2.cs

 define a namespace called 

MyNS

, and that within 

that namespace, both files define a class called 

MyClass

. Thus, without an 

extern

 alias, no 

program could have access to both versions of 

MyClass

.

The third file, 

test3.cs

, which is shown next, uses 

MyClass

 from both 

test1.cs

 and 

test2.cs

. It is able to do this because of the 

extern

 alias statements.

// extern alias statements must be at the top of the file.

extern alias Asm1;

extern alias Asm2;

using System;

class Demo {

  static void Main() {

    Asm1::MyNS.MyClass t = new Asm1::MyNS.MyClass();

    Asm2::MyNS.MyClass t2 = new Asm2::MyNS.MyClass();

  }

}

Start by compiling 

test1.cs

 and 

test2.cs

 into DLLs. This can be easily done from the 

command line by using these commands:

csc /t:library test1.cs

csc /t:library test2.cs

Next, compile 

test3.cs

 by using this command line:

csc /r:Asm1=test1.dll /r:Asm2=test2.dll test3.cs

Notice the use of the 

/r

 option, which tells the compiler to reference the metadata found in 

the associated file. In this case, the alias