Fun Parser

Program.cs

using System;
using System.Collections.Generic;

namespace FunParser
{
    class ParseResult
    {
        public string Method { get; set; }

        public string Path { get; set; }

        public IDictionary<string, string> Queries { get; set; } = new Dictionary<string, string>();
    }
    class Program
    {
        static void Main(string[] args)
        {
            // 'METHOD="AES-128",URI="path?key1=value1&key2=value2"'

            ParseResult parseResult = Parse("METHOD=\"AES-128\",URI=\"path?key1=value1&key2=value2\"");

            Console.WriteLine($"Method: {parseResult.Method}");
            
            foreach ((string key, string value) in parseResult.Queries)
            {
                Console.WriteLine($"Key: {key}, Value: {value}");
            }
        }

        public static ParseResult Parse(ReadOnlySpan<char> input)
        {
            Span<int> quotePositions = stackalloc int[4];

            for (int i = 0, j = 0; i < input.Length; i++)
            {
                if (input[i] == '"')
                    quotePositions[j++] = i;
            }

            ParseResult result = new ParseResult
            {
                Method = input[(quotePositions[0] + 1)..quotePositions[1]].ToString(),
            };

            ReadOnlySpan<char> uriString = input[(quotePositions[2] + 1)..(quotePositions[3])];

            int questionMarkPosition = 0;

            for (int i = 0; i < uriString.Length; i++)
            {
                if (uriString[i] == '?')
                    questionMarkPosition = i;
            }

            ReadOnlySpan<char> queryString = uriString[(questionMarkPosition + 1)..^0];

            int queryStringLength = queryString.Length;

            string key = null;
            string value = null;

            for (int i = 0, j = 0; i < queryStringLength; i++)
            {
                char c = queryString[i];

                if (c == '=')
                {
                    key = queryString[j..i].ToString();
                    j = i;
                }

                if (c == '&')
                {
                    value = queryString[(j + 1)..i].ToString();
                    j = i + 1;
                }

                if (i == queryStringLength - 1)
                {
                    value = queryString[(j + 1)..(i + 1)].ToString();
                }

                if (key != null && value != null)
                {
                    result.Queries.Add(key, value);

                    key = null;
                    value = null;
                }
            }

            return result;
        }
    }
}

Console Output:

Method: AES-128
Key: key1, Value: value1
Key: key2, Value: value2