System.Security.Util.URLString.UnescapeURL C# (CSharp) Method

        private String UnescapeURL(String url)
        {
            StringBuilder intermediate = new StringBuilder(url.Length);
            int Rindex = 0; // index into temp that gives the rest of the string to be processed
            int index;
            int braIndex = -1;
            int ketIndex = -1;
            braIndex = url.IndexOf('[',Rindex);
            if (braIndex != -1)
                ketIndex = url.IndexOf(']', braIndex);
            
            do
                {
                    index = url.IndexOf( '%', Rindex);

                    if (index == -1)
                    {
                        intermediate = intermediate.Append(url, Rindex, (url.Length - Rindex));
                        break;
                    }
                    // if we hit a '%' in the middle of an IPv6 address, dont process that
                    if (index > braIndex && index < ketIndex)
                    {
                        intermediate = intermediate.Append(url, Rindex, (ketIndex - Rindex+1));
                        Rindex = ketIndex+1;
                        continue;
                    }

                    if (url.Length - index < 2) // Check that there is at least 1 char after the '%'
                        throw new ArgumentException( Environment.GetResourceString( "Argument_InvalidUrl" ) );

                    if (url[index+1] == 'u' || url[index+1] == 'U')
                    {
                        if (url.Length - index < 6) // example: "%u004d" is 6 chars long
                            throw new ArgumentException( Environment.GetResourceString( "Argument_InvalidUrl" ) );

                        // We have a unicode character specified in hex

                        try
                        {
                            char c = (char)(Hex.ConvertHexDigit( url[index+2] ) << 12 |
                                            Hex.ConvertHexDigit( url[index+3] ) << 8  |
                                            Hex.ConvertHexDigit( url[index+4] ) << 4  |
                                            Hex.ConvertHexDigit( url[index+5] ));
                            intermediate = intermediate.Append(url, Rindex, index - Rindex);
                            intermediate = intermediate.Append(c);
                        }
                        catch(ArgumentException) // Hex.ConvertHexDigit can throw an "out of range" ArgumentException
                        {
                            throw new ArgumentException( Environment.GetResourceString( "Argument_InvalidUrl" ) );
                        }
                                        
                        Rindex = index + 6 ; //update the 'seen' length
                    }
                    else
                    {
                        // we have a hex character.
                                         
                        if (url.Length - index < 3) // example: "%4d" is 3 chars long
                             throw new ArgumentException( Environment.GetResourceString( "Argument_InvalidUrl" ) );

                        try 
                        {
                            char c = (char)(Hex.ConvertHexDigit( url[index+1] ) << 4 | Hex.ConvertHexDigit( url[index+2] ));

                            intermediate = intermediate.Append(url, Rindex, index - Rindex);
                            intermediate = intermediate.Append(c);
                        }
                        catch(ArgumentException) // Hex.ConvertHexDigit can throw an "out of range" ArgumentException
                        {
                            throw new ArgumentException( Environment.GetResourceString( "Argument_InvalidUrl" ) );
                        }
                        
                        Rindex = index + 3; // update the 'seen' length
                    }  

                } 
            while (true);
            return intermediate.ToString();
        }