Why disposing StreamReader makes a stream unreadable?

c# stream idisposable streamreader
Arseni Mourzenko picture Arseni Mourzenko · Oct 10, 2010 · Viewed 7.2k times · Source

I need to read a stream two times, from start to end.

But the following code throws an ObjectDisposedException: Cannot access a closed file exception.

string fileToReadPath = @"<path here>";
using (FileStream fs = new FileStream(fileToReadPath, FileMode.Open))
{
    using (StreamReader reader = new StreamReader(fs))
    {
        string text = reader.ReadToEnd();
        Console.WriteLine(text);
    }

    fs.Seek(0, SeekOrigin.Begin); // ObjectDisposedException thrown.

    using (StreamReader reader = new StreamReader(fs))
    {
        string text = reader.ReadToEnd();
        Console.WriteLine(text);
    }
}

Why is it happening? What is really disposed? And why manipulating StreamReader affects the associated stream in this way? Isn't it logical to expect that a seekable stream can be read several times, including by several StreamReaders?

Answer

Hans Passant picture Hans Passant · Oct 10, 2010

This happens because the StreamReader takes over 'ownership' of the stream. In other words, it makes itself responsible for closing the source stream. As soon as your program calls Dispose or Close (leaving the using statement scope in your case) then it will dispose the source stream as well. Calling fs.Dispose() in your case. So the file stream is dead after leaving the first using block. It is consistent behavior, all stream classes in .NET that wrap another stream behave this way.

There is one constructor for StreamReader that allows saying that it doesn't own the source stream. It is however not accessible from a .NET program, the constructor is internal.

In this particular case, you'd solve the problem by not using the using-statement for the StreamReader. That's however a fairly hairy implementation detail. There's surely a better solution available to you but the code is too synthetic to propose a real one.

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