Reading a DAT file with BinaryReader in C#

Question

Im trying to read a DAT file with BinaryReady but I get an exception and don't see why. " Unable to read beyond the end of the stream" is the message I get. My code looks like this:

private void button1_Click(object sender, EventArgs e)

    {
        OpenFileDialog OpenFileDialog = new OpenFileDialog();
        OpenFileDialog.Title = "Open File...";
        OpenFileDialog.Filter = "Binary File (*.dat)|*.dat";
        OpenFileDialog.InitialDirectory = @"C:\";
        if (OpenFileDialog.ShowDialog() == DialogResult.OK)
        {
            FileStream fs = new FileStream(OpenFileDialog.FileName, FileMode.Open);
            BinaryReader br = new BinaryReader(fs);

            label1.Text = br.ReadString();
            label2.Text = br.ReadInt32().ToString();

            fs.Close();
            br.Close();
        }

I hat a certain DAT file with a lot of information and was hoping to be able to read it out and maybe even place it in a table and plot the data. But its been a while I worked with C#. So if anyone could help me I would highly appreciate

which line throws the exception? (P.S. logically you ought to close the reader before you close the stream.) — ADyson
– ADyson, Commented Sep 14, 2020 at 9:05
P.P.S. Since we don't know the file format, or how it was created, it's going to be hard for anyone to say specifically what you ought to do instead, or whether you can get it to work or not. If you give more details then perhaps we could assist better. — ADyson
– ADyson, Commented Sep 14, 2020 at 9:07
Is the file you're reading a binary file or a text file? I'm not sure how that ReadString() is implemented, but if I see a binary reader reading a string I'd assume it reads on until it finds a 00 byte. Can you give the contents of the file you're trying to read, or at least up to the point past that Int32? — Nyerguds
– Nyerguds, Commented Sep 14, 2020 at 11:13

Marc Gravell · Accepted Answer · 2020-09-14 11:10:32Z

BinaryReader is very rarely a good choice for reading an external file; it only really makes sense when used in parallel with code that writes the file using BinaryWriter, since they use the same conventions.

I imagine that what is happening here is that your call to ReadString is trying to use conventions that aren't valid for your file - specifically, it will be reading some bytes (I want to say 4, big endian?) as a length prefix for the string, and then trying to read that many bytes as the string. But if that isn't what the file contents are: that could easily try to read gibberish, interpret it as a huge number, and then fail to read that-many bytes.

If you're processing an arbitrary file (nothing to do with BinaryWriter), then you really need to know a lot about the protocol/format. Given that file extensions are ambiguous, I'm not going to infer anything from ".dat" - what matters is: what is the data and where did it come from?. Only with that information can a sensible comment on reading it be made.

From the comments, here's some (untested) code that should get you started in terms of parsing the contents as a span:

public static YourResultType Process(string path)
{
    byte[] oversized = null;
    try
    {
        int len, offset = 0, read;
        // read the file into a leased buffer, for simplicity
        using (var stream = File.OpenRead(path))
        {
            len = checked((int)stream.Length);
            oversized = ArrayPool<byte>.Shared.Rent(len);
            while (offset < len &&
                (read = stream.Read(oversized, offset, len - offset)) > 0)
            {
                offset += read;
            }
        }
        // now process the payload from the buffered data
        return Process(new ReadOnlySpan<byte>(oversized, 0, len));
    }
    finally
    {
        if (oversized is object)
            ArrayPool<byte>.Shared.Return(oversized);
    }
}
private static YourResultType Process(ReadOnlySpan<byte> payload)
    => throw new NotImplementedException(); // your code here

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