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The using statement

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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 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.

Local variables declared in a resource-acquisition are read-only, and must include an initializer.

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.




For example, a using statement of the form

using (R r1 = new R ())

is precisely equivalent to

R r1 = new R();
try
finally

A resource-acquisition may acquire multiple resources of a given type. This is equivalent to nested using statements. For example, a using statement of the form

using (R r1 = new R(), r2 = new R())

is precisely equivalent to:

using (R r1 = new R())
     using (R r2 = new R())

which is, by expansion, precisely equivalent to:

R r1 = new R();
try {
     R r2 = new R();
     try
     finally
}
finally








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