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

        private unsafe void WriteCore(byte[] buffer, int offset, int count)
        {
            Debug.Assert(!_fileHandle.IsClosed, "!_handle.IsClosed");
            Debug.Assert(CanWrite, "_parent.CanWrite");

            Debug.Assert(buffer != null, "buffer != null");
            Debug.Assert(_readPos == _readLength, "_readPos == _readLen");
            Debug.Assert(offset >= 0, "offset is negative");
            Debug.Assert(count >= 0, "count is negative");
            if (_useAsyncIO)
            {
                WriteInternalCoreAsync(buffer, offset, count, CancellationToken.None).GetAwaiter().GetResult();
                return;
            }

            // Make sure we are writing to the position that we think we are
            VerifyOSHandlePosition();

            int errorCode = 0;
            int r = WriteFileNative(_fileHandle, buffer, offset, count, null, out errorCode);

            if (r == -1)
            {
                // For pipes, ERROR_NO_DATA is not an error, but the pipe is closing.
                if (errorCode == ERROR_NO_DATA)
                {
                    r = 0;
                }
                else
                {
                    // ERROR_INVALID_PARAMETER may be returned for writes
                    // where the position is too large (i.e. writing at Int64.MaxValue 
                    // on Win9x) OR for synchronous writes to a handle opened 
                    // asynchronously.
                    if (errorCode == ERROR_INVALID_PARAMETER)
                        throw new IOException(SR.IO_FileTooLongOrHandleNotSync);
                    throw Win32Marshal.GetExceptionForWin32Error(errorCode);
                }
            }
            Debug.Assert(r >= 0, "FileStream's WriteCore is likely broken.");
            _filePosition += r;
            return;
        }