System.IO.FileStream.Init C# (CSharp) Method

        private void Init(FileMode mode, FileShare share)
        {
            // Disallow access to all non-file devices from the Win32FileStream
            // constructors that take a String.  Everyone else can call 
            // CreateFile themselves then use the constructor that takes an 
            // IntPtr.  Disallows "con:", "com1:", "lpt1:", etc.
            int fileType = Interop.Kernel32.GetFileType(_fileHandle);
            if (fileType != Interop.Kernel32.FileTypes.FILE_TYPE_DISK)
            {
                _fileHandle.Dispose();
                throw new NotSupportedException(SR.NotSupported_FileStreamOnNonFiles);
            }

            // This is necessary for async IO using IO Completion ports via our 
            // managed Threadpool API's.  This (theoretically) calls the OS's 
            // BindIoCompletionCallback method, and passes in a stub for the 
            // LPOVERLAPPED_COMPLETION_ROUTINE.  This stub looks at the Overlapped
            // struct for this request and gets a delegate to a managed callback 
            // from there, which it then calls on a threadpool thread.  (We allocate
            // our native OVERLAPPED structs 2 pointers too large and store EE state
            // & GC handles there, one to an IAsyncResult, the other to a delegate.)
            if (_useAsyncIO)
            {
                try
                {
                    _fileHandle.ThreadPoolBinding = ThreadPoolBoundHandle.BindHandle(_fileHandle);
                }
                catch (ArgumentException ex)
                {
                    throw new IOException(SR.IO_BindHandleFailed, ex);
                }
                finally
                {
                    if (_fileHandle.ThreadPoolBinding == null)
                    {
                        // We should close the handle so that the handle is not open until SafeFileHandle GC
                        Debug.Assert(!_exposedHandle, "Are we closing handle that we exposed/not own, how?");
                        _fileHandle.Dispose();
                    }
                }
            }

            _canSeek = true;

            // For Append mode...
            if (mode == FileMode.Append)
            {
                _appendStart = SeekCore(0, SeekOrigin.End);
            }
            else
            {
                _appendStart = -1;
            }
        }