System.IO.BufferedStream.ClearReadBufferBeforeWrite C# (CSharp) Method

        private void ClearReadBufferBeforeWrite()
        {
            Debug.Assert(_readPos <= _readLen, "_readPos <= _readLen [" + _readPos + " <= " + _readLen + "]");

            // No read data in the buffer:
            if (_readPos == _readLen)
            {
                _readPos = _readLen = 0;
                return;
            }

            // Must have read data.
            Debug.Assert(_readPos < _readLen);

            // If the underlying stream cannot seek, FlushRead would end up throwing NotSupported.
            // However, since the user did not call a method that is intuitively expected to seek, a better message is in order.
            // Ideally, we would throw an InvalidOperation here, but for backward compat we have to stick with NotSupported.
            if (!_stream.CanSeek)
                throw new NotSupportedException(SR.NotSupported_CannotWriteToBufferedStreamIfReadBufferCannotBeFlushed);

            FlushRead();
        }