对编码内容多次UrlDecode

对编码内容多次UrlDecode，并不会影响最终结果。

尝试阅读了微软的源代码，不过不容易读懂。

网址：https://referencesource.microsoft.com/#System/net/System/Net/WebUtility.cs,73c04b8a4fde5039

以下为从网址上复制下来的一些关键代码，不过没看懂。

public static string UrlDecode(string encodedValue)
        {
            if (encodedValue == null)
                return null;

            return UrlDecodeInternal(encodedValue, Encoding.UTF8);
        }

private static string UrlDecodeInternal(string value, Encoding encoding)
        {
            if (value == null)
            {
                return null;
            }

            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

            for (int pos = ; pos < count; pos++)
            {
                char ch = value[pos];

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

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

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

                if ((ch & 0xFF80) == )
                    helper.AddByte((byte)ch); // 7 bit have to go as bytes because of Unicode
                else
                    helper.AddChar(ch);
            }

            return helper.GetString();
        }

 internal void AddChar(char ch)
            {
                if (_numBytes > )
                    FlushBytes();

                _charBuffer[_numChars++] = ch;
            }

  internal void AddByte(byte b)
            {
                if (_byteBuffer == null)
                    _byteBuffer = new byte[_bufferSize];

                _byteBuffer[_numBytes++] = b;
            }

 internal String GetString()
            {
                if (_numBytes > )
                    FlushBytes();

                if (_numChars > )
                    return new String(_charBuffer, , _numChars);
                else
                    return String.Empty;
            }

String(_charBuffer, 0, _numChars);是看不到源代码的，到这里已经变成一行看不懂的代码

        // Creates a new string from the characters in a subarray.  The new string will
        // be created from the characters in value between startIndex and
        // startIndex + length - 1.
        //
        [System.Security.SecuritySafeCritical]  // auto-generated
        [ResourceExposure(ResourceScope.None)]
        [MethodImplAttribute(MethodImplOptions.InternalCall)]
        public extern String(char [] value, int startIndex, int length);

下一步应该怎么办，只能希望某个大牛指点下了……