The using statement

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-variable-declaration must be either dynamic or a type that can be implicitly converted to System.IDisposable. If the form of resource-acquisition is expression then this expression must be implicitly convertible to System.IDisposable.

Local variables declared in a resource-acquisition are read-only, and must include an initializer. A compile-time error occurs if the embedded statement attempts to modify these local variables (via assignment or the ++ and ‑‑ operators), take the address of them, 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. If the resource is of type dynamic it is dynamically converted through an implicit dynamic conversion (§6.1.8) to IDisposable during acquisition in order to ensure that the conversion is successful before the usage and disposal.

A using statement of the form

using (ResourceType resource = expression) statement

corresponds to one of three possible expansions. When ResourceType is a non-nullable value type, the expansion is

{
ResourceType resource = expression;
try {
statement;
}
finally {
((IDisposable)resource).Dispose();
}
}

Otherwise, when ResourceType is a nullable value type or a reference type other than dynamic, the expansion is

{
ResourceType resource = expression;
try {
statement;
}
finally {
if (resource!= null) ((IDisposable)resource).Dispose();
}
}

Otherwise, when ResourceType is dynamic, the expansion is

{
ResourceType resource = expression;
IDisposable d = (IDisposable)resource;
try {
statement;
}
finally {
if (d!= null) d.Dispose();
}
}

In either expansion, the resource variable is read-only in the embedded statement, and the d variable is inaccessible in, and invisible to, the embedded statement.

An implementation is permitted to implement a given using-statement differently, e.g. for performance reasons, as long as the behavior is consistent with the above expansion.

A using statement of the form

using (expression) statement

has the same three 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

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");
}

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.

Дата добавления: 2015-11-16