Question

Is there a way to promoting a Read-Only FileStream to Read-Write? I am looking for functionality similar to the Win32 SDK function ReOpenFile.

Answer 1

Here you go. Uses a bit of pInvoke Interop goodness (badness), but it’ll do it. I’ve skimped and threw in some magic constants for the access and sharemode parameters, so feel free to encapsulate that.

private static void Main()
{
    using (FileStream fs = new FileStream(@"..\..\Program.cs", FileMode.Open, FileAccess.Read, FileShare.ReadWrite))
    {
        using (TextReader tr = new StreamReader(fs))
        {
            Console.WriteLine(tr.ReadToEnd());

            using (FileStream fs1 = new FileStream(ReOpenFile(fs.SafeFileHandle, 3, 3, 0), FileAccess.ReadWrite))
            {
                fs1.Seek(0, SeekOrigin.End);
                using (TextWriter tw = new StreamWriter(fs1))
                {
                    tw.WriteLine("/* this should be all right */");
                }
            }
        }
    }
}

[DllImport("kernel32", SetLastError = true)]
private static extern SafeFileHandle ReOpenFile(SafeFileHandle hOriginalFile, uint dwAccess, uint dwShareMode, uint dwFlags);