对编码内容多次UrlDecode

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 = 0; pos < count; pos++)
            {
                char ch = value[pos];
 
                if (ch == ‘+‘)
                {
                    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);
                        continue;
                    }
                }
 
                if ((ch & 0xFF80) == 0)
                    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 > 0)
                    FlushBytes();
 
                _charBuffer[_numChars++] = ch;
            }

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

 internal String GetString()
            {
                if (_numBytes > 0)
                    FlushBytes();
 
                if (_numChars > 0)
                    return new String(_charBuffer, 0, _numChars);
                else
                    return String.Empty;
            }

        // 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);