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How to execute cleanup code using finally

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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 Close on a FileStream immediately after use instead of waiting for the object to be garbage collected by the common language runtime, as follows:

static void CodeWithoutCleanup()
{
    FileStream? file = null;
    FileInfo fileInfo = new FileInfo("./file.txt");

    file = fileInfo.OpenWrite();
    file.WriteByte(0xF);

    file.Close();
}

Example

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

static void CodeWithCleanup()
{
    FileStream? file = null;
    FileInfo? fileInfo = null;

    try
    {
        fileInfo = new FileInfo("./file.txt");

        file = fileInfo.OpenWrite();
        file.WriteByte(0xF);
    }
    catch (UnauthorizedAccessException e)
    {
        Console.WriteLine(e.Message);
    }
    finally
    {
        file?.Close();
    }
}

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 FileStream object isn't null before you call the Close method. Without the null check, the finally block could throw its own 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.

See also