FileStream fileStream = null;

try
{
    fileStream =
        new FileStream(@"c:\file.txt", FileMode.Open, FileAccess.Write);
}
catch (UnauthorizedAccessException e)
{
    // The access requested is not permitted by the operating system
    // for the specified path, such as when access is Write or ReadWrite
    // and the file or directory is set for read-only access.
}
finally
{
    if (fileStream != null)
        fileStream.Close ();
}