| title | description | ms.topic | ms.date | helpviewer_keywords | |||
|---|---|---|---|---|---|---|---|
How to execute cleanup code using finally |
Learn how to execute cleanup code using a 'finally' statement. Finally statements ensure that any necessary cleanup of objects occurs immediately. |
how-to |
05/14/2021 |
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The purpose of a finally statement is to ensure that the necessary cleanup of objects, usually objects that are holding external resources, occurs immediately, even if an exception is thrown. One example of such cleanup is calling xref:System.IO.Stream.Close%2A on a xref:System.IO.FileStream immediately after use instead of waiting for the object to be garbage collected by the common language runtime, as follows:
:::code language="csharp" source="snippets/exceptions/Program.cs" ID="NoCleanup":::
To turn the previous code into a try-catch-finally statement, the cleanup code is separated from the working code, as follows.
:::code language="csharp" source="snippets/exceptions/Program.cs" ID="WithCleanup":::
Because an exception can occur at any time within the try block before the OpenWrite() call, or the OpenWrite() call itself could fail, we aren't guaranteed that the file is open when we try to close it. The finally block adds a check to make sure that the xref:System.IO.FileStream object isn't null before you call the xref:System.IO.Stream.Close%2A method. Without the null check, the finally block could throw its own xref:System.NullReferenceException, but throwing exceptions in finally blocks should be avoided if it's possible.
A database connection is another good candidate for being closed in a finally block. Because the number of connections allowed to a database server is sometimes limited, you should close database connections as quickly as possible. If an exception is thrown before you can close your connection, using the finally block is better than waiting for garbage collection.