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Editorial Team
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Asked: 2026-06-18T04:22:15+00:00

I ran into something interesting when using a StreamWriter with a FileStream to append

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I ran into something interesting when using a StreamWriter with a FileStream to append text to an existing file in .NET 4.5 (haven’t tried any older frameworks). I tried two ways, one worked and one didn’t. I’m wondering what the difference between the two is.

Both methods contained the following code at the top

if (!File.Exists(filepath))
    using (File.Create(filepath));

I have the creation in a using statement because I’ve found through personal experience that it’s the best way to ensure that the application fully closes the file.

Non-Working Method:

using (FileStream f = new FileStream(filepath, FileMode.Append,FileAccess.Write))
    (new StreamWriter(f)).WriteLine("somestring");

With this method nothing ends up being appended to the file.

Working Method:

using (FileStream f = new FileStream(filepath, FileMode.Append,FileAccess.Write))
    using (StreamWriter s = new StreamWriter(f))
        s.WriteLine("somestring");

I’ve done a bit of Googling, without quite knowing what to search for, and haven’t found anything informative. So, why is it that the anonymous StreamWriter fails where the (non-anonymous? named?) StreamWriter works?

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1 Answer

  1. Editorial Team

    It sounds like you did not flush the stream.

    http://msdn.microsoft.com/en-us/library/system.io.stream.flush.aspx

    It looks like StreamWriter writes to a buffer before writing to the final destination, in this case, the file. You may also be able to set the AutoFlush property and not have to explicitly flush it.

    http://msdn.microsoft.com/en-us/library/system.io.streamwriter.autoflush.aspx

    To answer your question, when you use the “using” block, it calls dispose on the StreamWriter, which must in turn call Flush.

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