How to return a Stream from a method, knowing it should be disposed?

c# stream using idisposable
Frank Martin picture Frank Martin · Aug 17, 2015 · Viewed 18.6k times · Source

I have a method that takes FileStream as input. This method is running inside a for loop.

private void UploadFile(FileStream fileStream)
{
    var stream = GetFileStream();
    // do things with stream
}

I have another method which creates and returns the FileStream:

private FileStream GetFileStream()
{
    using(FileStream fileStream = File.Open(myFile, FileMode.Open))
    {
        //Do something
        return fileStream;
    }
}

Now the first method throws an ObjectDisposedException when I try to access the returned FileStream, probably because it is already closed since I am using "using" to properly dispose the stream.

If I don't use "using" and instead use it as follows, then the FileStream remains open and the next iteration of the loop (operating on the same file) throws an exception telling the file is already in use:

private FileStream GetFileStream()
{
    FileStream fileStream = File.Open(myFile, FileMode.Open);
    //Do something
    return fileStream;
}

If I use a try-finally block, where I close the stream in the finally then it also throws the ObjectDisposedException.

How to effectively return file stream and close it?

Answer

Douglas picture Douglas · Aug 17, 2015

When you return an IDisposable from a method, you are relegating the responsibility of disposing it to your caller. Thus, you need to declare your using block around the entire usage of the stream, which in your case presumably spans the UploadFile call.

using (var s = GetFileStream())
    UploadFile(s);

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