how-to-execute-cleanup-code-using-finally.md

title	How to: Execute Cleanup Code Using finally (C# Programming Guide)
ms.date	07/20/2015
ms.prod	.net
ms.technology	devlang-csharp
ms.topic	article
helpviewer_keywords	try/finally blocks [C#] exceptions [C#], try/finally block exception handling [C#], try/finally block
ms.assetid	1b1e5aef-3f32-4a88-9d39-b5fffb33bdaf
caps.latest.revision	21
author	BillWagner
ms.author	wiwagn

How to: Execute Cleanup Code Using finally (C# Programming Guide)

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-csharpcsProgGuideExceptions#16]

Example

To turn the previous code into a try-catch-finally statement, the cleanup code is separated from the working code, as follows.

[!code-csharpcsProgGuideExceptions#17]

Because an exception can occur at any time within the try block before the OpenWrite() call, or the OpenWrite() call itself could fail, we are not 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 is not 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 is 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, this is another case where using the finally block is better than waiting for garbage collection.

See Also

C# Programming Guide
Exceptions and Exception Handling
Exception Handling
using Statement
try-catch
try-finally
try-catch-finally