Why is my FileStream object being disposed of when I'm "using" a BinaryReader object?

Consider the following function:

    private int GetSomethingFromFile(FileStream fs) 
    {
        using (BinaryReader br = new BinaryReader(fs))
        {
            fs.Seek(0, SeekOrigin.Begin);
            return br.ReadInt32();
        }
    }

A FileStream object is passed in as a parameter and a BinaryReader is declared with a using statement. 开发者_高级运维When I try to use that FileStream object, after calling this function, it throws a System.ObjectDisposedException. Why is that FileStream object being disposed of along with the BinaryReader object?

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It is a very good question, and I don't know why it was decided that this was how it should be, but alas it is documented to be this way:

BinaryReader class

Close: Closes the current reader and the underlying stream.

If you check out this answer to the question How do I “fork” a Stream in .NET? then you'll see that he refers to a class called NonClosingStreamWrapper in a library called MiscUtil that @Jon Skeet has written that you can use to wrap around the stream to prevent it from being closed.

You would use it like this (for your example):

private int GetSomethingFromFile(FileStream fs) 
{
    using (var wrapper = new NonClosingStreamWrapper(fs))
    using (BinaryReader br = new BinaryReader(wrapper))
    {
        fs.Seek(0, SeekOrigin.Begin);
        return br.ReadInt32();
    }
}

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Because disposing the binary reader disposes its underlying stream.

Use "using" in the caller method instead.

The reason is arbitrary: .NET class library is implemented this way.