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

        private unsafe int WriteFileNative(SafeFileHandle handle, byte[] bytes, int offset, int count, NativeOverlapped* overlapped, out int errorCode)
        {
            Debug.Assert(handle != null, "handle != null");
            Debug.Assert(offset >= 0, "offset >= 0");
            Debug.Assert(count >= 0, "count >= 0");
            Debug.Assert(bytes != null, "bytes != null");
            // Don't corrupt memory when multiple threads are erroneously writing
            // to this stream simultaneously.  (the OS is reading from
            // the array we pass to WriteFile, but if we read beyond the end and
            // that memory isn't allocated, we could get an AV.)
            if (bytes.Length - offset < count)
                throw new IndexOutOfRangeException(SR.IndexOutOfRange_IORaceCondition);

            Debug.Assert((_useAsyncIO && overlapped != null) || (!_useAsyncIO && overlapped == null), "Async IO and overlapped parameters inconsistent in call to WriteFileNative.");

            // You can't use the fixed statement on an array of length 0.
            if (bytes.Length == 0)
            {
                errorCode = 0;
                return 0;
            }

            int numBytesWritten = 0;
            int r = 0;

            fixed (byte* p = bytes)
            {
                if (_useAsyncIO)
                    r = Interop.Kernel32.WriteFile(handle, p + offset, count, IntPtr.Zero, overlapped);
                else
                    r = Interop.Kernel32.WriteFile(handle, p + offset, count, out numBytesWritten, IntPtr.Zero);
            }

            if (r == 0)
            {
                errorCode = GetLastWin32ErrorAndDisposeHandleIfInvalid();
                return -1;
            }
            else
            {
                errorCode = 0;
                return numBytesWritten;
            }
        }