System.Xml.XmlBufferReader.GetHexCharEntity C# (CSharp) Méthode

        private int GetHexCharEntity(int offset, int length)
        {
            byte[] buffer = _buffer;
            DiagnosticUtility.DebugAssert(buffer[offset + 0] == '&', "");
            DiagnosticUtility.DebugAssert(buffer[offset + 1] == '#', "");
            DiagnosticUtility.DebugAssert(buffer[offset + 2] == 'x', "");
            DiagnosticUtility.DebugAssert(buffer[offset + length - 1] == ';', "");
            int value = 0;
            for (int i = 3; i < length - 1; i++)
            {
                byte ch = buffer[offset + i];
                int digit = 0;
                if (ch >= '0' && ch <= '9')
                    digit = (ch - '0');
                else if (ch >= 'a' && ch <= 'f')
                    digit = 10 + (ch - 'a');
                else if (ch >= 'A' && ch <= 'F')
                    digit = 10 + (ch - 'A');
                else
                    XmlExceptionHelper.ThrowInvalidCharRef(_reader);
                DiagnosticUtility.DebugAssert(digit >= 0 && digit < 16, "");
                value = value * 16 + digit;
                if (value > SurrogateChar.MaxValue)
                    XmlExceptionHelper.ThrowInvalidCharRef(_reader);
            }
            return value;
        }