Basic GPD Check

// Place If statement on an executable control (Button, label, image, etc.)
 if (openFileDialog1.ShowDialog() == DialogResult.OK)
            {
                FileName = openFileDialog1.FileName;
                Reader = new BinaryReader(File.Open(openFileDialog1.FileName, FileMode.Open));
                ReadFile();
        }
void ReadFile()
        {
            String Magic = Encoding.ASCII.GetString(Reader.ReadBytes(4));
        if (Magic != "XDBF")
        {
        MessageBox.Show("File isn't a valid GPD.");
        }
    }