Visual Basic 1002005 [A Developers Notebook] [Electronic resources] نسخه متنی

2.11. Dispose of Objects Automatically

In .NET, it's
keenly important to make sure objects that use unmanaged resources
(e.g., file handles, database connections, and graphics contexts)
release these resources as soon as possible. Toward this end, such
objects should always implement the IDisposable
interface, and provide a Dispose( ) method that
you can call to release their resources immediately.

Note: Worried that you'll have objects floating
around in memory, tying up resources until the garbage collector
tracks them down? With the Using statement, you can make sure
disposable objects meet with a timely demise.

The only problem with this technique is that you must always remember
to call the Dispose( ) method (or another method
that calls Dispose( ), such as a Close(
) method). VB 2005 provides a new safeguard you can apply
to make sure Dispose( ) is always called: the
Using statement.

2.11.1. How do I do that?

You use the Using statement in a block structure.
In the first line, when you declare the Using
block, you specify the disposable object you are using. Often,
you'll also create the object at the same time using
the New keyword. Then, you write the code that
uses the disposable object inside the Using block.
Here's an example with a snippet of code that
creates a new file and writes some data to the file:

Using NewFile As New System.IO.StreamWriter("c:\MyFile.txt")
NewFile.WriteLine("This is line 1")
NewFile.WriteLine("This is line 2")
End Using
' The file is closed automatically. 
' The NewFile object is no longer available here.

In this example, as soon as the execution leaves the
Using block, the Dispose( )
method is called on the NewFile object, releasing
the file handle.

2.11.2. What about...

...errors that occur inside a Using block?
Thankfully, .NET makes sure it disposes of the resource no matter how
you exit the Using block, even if an unhandled
exception occurs.

The Using statement makes sense with all kinds of
disposable objects, such as:

Files (including
FileStream, StreamReader, and
StreamWriter)

Database connections (including
SqlConnection,
OracleConnection, and
OleDbConnection)

Network connections (including
TcpClient, UdpClient,
NetworkStream, FtpWebResponse,
HttpWebResponse)

Graphics
(including Image, Bitmap,
Metafile, Graphics)

2.11.3. Where can I learn more?

For the language lowdown, refer to the index entry
"Using block" in the MSDN Help.