System.Text.InternalEncoderBestFitFallbackBuffer.Fallback C# (CSharp) Method

        public override bool Fallback(char charUnknownHigh, char charUnknownLow, int index)
        {
            // Double check input surrogate pair
            if (!Char.IsHighSurrogate(charUnknownHigh))
                throw new ArgumentOutOfRangeException(nameof(charUnknownHigh), SR.Format(SR.ArgumentOutOfRange_Range, 0xD800, 0xDBFF));

            if (!Char.IsLowSurrogate(charUnknownLow))
                throw new ArgumentOutOfRangeException(nameof(charUnknownLow), SR.Format(SR.ArgumentOutOfRange_Range, 0xDC00, 0xDFFF));
            Contract.EndContractBlock();

            // If we had a buffer already we're being recursive, throw, it's probably at the suspect
            // character in our array.  0 is processing last character, < 0 is not falling back
            // Shouldn't be able to get here, table would have to be messed up.
            Debug.Assert(_iCount < 1, "[InternalEncoderBestFitFallbackBuffer.Fallback(surrogate)] Fallback char " + ((int)_cBestFit).ToString("X4", CultureInfo.InvariantCulture) + " caused recursive fallback");

            // Go ahead and get our fallback, surrogates don't have best fit
            _cBestFit = '?';
            _iCount = _iSize = 2;

            return true;
        }