System.Net.WebUtility.UrlDecodeInternal C# (CSharp) Method

        private static string UrlDecodeInternal(string value, Encoding encoding)
        {
            if (string.IsNullOrEmpty(value))
            {
                return value;
            }

            int count = value.Length;
            UrlDecoder helper = new UrlDecoder(count, encoding);

            // go through the string's chars collapsing %XX and
            // appending each char as char, with exception of %XX constructs
            // that are appended as bytes
            bool needsDecodingUnsafe = false;
            bool needsDecodingSpaces = false;
            for (int pos = 0; pos < count; pos++)
            {
                char ch = value[pos];

                if (ch == '+')
                {
                    needsDecodingSpaces = true;
                    ch = ' ';
                }
                else if (ch == '%' && pos < count - 2)
                {
                    int h1 = HexToInt(value[pos + 1]);
                    int h2 = HexToInt(value[pos + 2]);

                    if (h1 >= 0 && h2 >= 0)
                    {     // valid 2 hex chars
                        byte b = (byte)((h1 << 4) | h2);
                        pos += 2;

                        // don't add as char
                        helper.AddByte(b);
                        needsDecodingUnsafe = true;
                        continue;
                    }
                }

                if ((ch & 0xFF80) == 0)
                    helper.AddByte((byte)ch); // 7 bit have to go as bytes because of Unicode
                else
                    helper.AddChar(ch);
            }
            
            if (!needsDecodingUnsafe)
            {
                if (needsDecodingSpaces)
                {
                    // Only spaces to decode
                    return value.Replace('+', ' ');
                }

                // Nothing to decode
                return value;
            }

            return helper.GetString();
        }