8.12 Namespaces and assemblies

王朝other·作者佚名  2006-01-10
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8.12 Namespaces and assemblies

The programs presented so far have stood on their own except for dependence

on a few system-provided

classes such as System.Console. It is far more common, however, for

real-world applications to consist of

several different pieces, each compiled separately. For example, a

corporate application might depend on

several different components, including some developed internally and some

purchased from independent

software vendors.

Namespaces and assemblies enable this component-based system. Namespaces

provide a logical

organizational system. Namespaces are used both as an .internal.

organization system for a program, and as

an .external. organization system.a way of presenting program elements that

are exposed to other

programs.

Assemblies are used for physical packaging and deployment. An assembly may

contain types, the executable

code used to implement these types, and references to other assemblies.

To demonstrate the use of namespaces and assemblies, this section revisits

the .hello, world. program

presented earlier, and splits it into two pieces: a class library that

provides messages and a console

application that displays them.

The class library will contain a single class named HelloMessage. The

example

// HelloLibrary.cs

namespace CSharp.Introduction

{

public class HelloMessage

{

public string Message {

get {

return "hello, world";

}

}

}

}

shows the HelloMessage class in a namespace named CSharp.Introduction. The

HelloMessage

class provides a read-only property named Message. Namespaces can nest, and

the declaration

namespace CSharp.Introduction

{.}

is shorthand for two levels of namespace nesting:

namespace CSharp

{

namespace Introduction

{.}

}

C# LANGUAGE SPECIFICATION

46

The next step in the componentization of .hello, world. is to write a

console application that uses the

HelloMessage class. The fully qualified name for the class.

CSharp.Introduction.HelloMessage.could be used, but this name is quite long

and unwieldy. An

easier way is to use a using namespace directive, which makes it possible

to use all of the types in a

namespace without qualification. The example

// HelloApp.cs

using CSharp.Introduction;

class HelloApp

{

static void Main() {

HelloMessage m = new HelloMessage();

System.Console.WriteLine(m.Message);

}

}

shows a using namespace directive that refers to the CSharp.Introduction

namespace. The occurrences

of HelloMessage are shorthand for CSharp.Introduction.HelloMessage.

C# also enables the definition and use of aliases. A using alias directive

defines an alias for a type. Such

aliases can be useful in situation in which name collisions occur between

two class libraries, or when a small

number of types from a much larger namespace are being used. The example

using MessageSource = CSharp.Introduction.HelloMessage;

shows a using alias directive that defines MessageSource as an alias for

the HelloMessage class.

The code we have written can be compiled into a class library containing

the class HelloMessage and an

application containing the class HelloApp. The details of this compilation

step might differ based on the

compiler or tool being used. A command-line compiler might enable

compilation of a class library and an

application that uses that library with the following command-line

invocations:

csc /target:library HelloLibrary.cs

csc /reference:HelloLibrary.dll HelloApp.cs

which produce a class library named HelloLibrary.dll and an application

named HelloApp.exe.

 
 
 
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