15.13 The using statement

王朝other·作者佚名  2006-05-06
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The using statement obtains one or more resources, executes a statement, and

then disposes of the resource.

using-statement:

using ( resource-acquisition ) embedded-statement

resource-acquisition:

local-variable-declaration

expression

A resource is a class or struct that implements System.IDisposable, which

includes a single

parameterless method named Dispose. Code that is using a resource can call

Dispose to indicate that the

resource is no longer needed. If Dispose is not called, then automatic

disposal eventually occurs as a

consequence of garbage collection.

If the form of resource-acquisition is local-variable-declaration then the

type of the local-variabledeclaration

must be System.IDisposable or a type that can be implicitly converted to

System.IDisposable. If the form of resource-acquisition is expression then

this expression must be

System.IDisposable or a type that can be implicitly converted to

System.IDisposable.

Chapter 15 Statements

199

Local variables declared in a resource-acquisition are read-only, and must

include an initializer. A compiletime

error occurs if the embedded statement attempts to modify these local

variables (via assignment or the

++ and -- operators) or pass them as ref or out parameters.

A using statement is translated into three parts: acquisition, usage, and

disposal. Usage of the resource is

implicitly enclosed in a try statement that includes a finally clause. This

finally clause disposes of the

resource. If a null resource is acquired, then no call to Dispose is made,

and no exception is thrown.

A using statement of the form

using (ResourceType resource = expression) statement

corresponds to one of two possible expansions. When ResourceType is a value

type, the expansion is

{

ResourceType resource = expression;

try {

statement;

}

finally {

((IDisposable)resource).Dispose();

}

}

Otherwise, when ResourceType is a reference type, the expansion is

{

ResourceType resource = expression;

try {

statement;

}

finally {

if (resource != null) ((IDisposable)resource).Dispose();

}

}

In either expansion, the resource variable is read-only in the embedded

statement.

A using statement of the form

using (expression) statement

has the same two possible expansions, but in this case ResourceType is

implicitly the compile-time type of

the expression, and the resource variable is inaccessible in, and invisible

to, the embedded statement.

When a resource-acquisition takes the form of a local-variable-declaration,

it is possible to acquire multiple

resources of a given type. A using statement of the form

using (ResourceType r1 = e1, r2 = e2, ..., rN = eN) statement

is precisely equivalent to a sequence of nested using statements:

using (ResourceType r1 = e1)

using (ResourceType r2 = e2)

...

using (ResourceType rN = eN)

statement

[Example: The example below creates a file named log.txt and writes two

lines of text to the file. The

example then opens that same file for reading and copies the contained

lines of text to the console.

using System;

using System.IO;

class Test

{

static void Main() {

using (TextWriter w = File.CreateText("log.txt")) {

w.WriteLine("This is line one");

w.WriteLine("This is line two");

}

C# LANGUAGE SPECIFICATION

200

using (TextReader r = File.OpenText("log.txt")) {

string s;

while ((s = r.ReadLine()) != null) {

Console.WriteLine(s);

}

}

}

}

Since the TextWriter and TextReader classes implement the IDisposable

interface, the example can

use using statements to ensure that the underlying file is properly closed

following the write or read

operations. end example]

 
 
 
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