StringExtension

ToValue<T>

var x="100+100".ToValue<int>(-1);//200

ToMap

var dic =new Dictionary<string,string>(){ $a...}
var rep="$a".ToMap(dic); //ex)$a replace dic["$a"]

using System;
using System.Collections.Generic;
using static System.Console;
using System.Text.RegularExpressions;
using System.Linq;

    class Program
    {
        static void Main(string[] args)
        {
            var dic =new Dictionary<string,string>(){
                {"$a","100"},
                {"$aa","200"},
                {"$aaa","-300"},
            };

            var x="$a+1";
            var a=x.ToMap(dic);//100+1 
            var b=a.ToValue<int>(-1);//101
            var c="$a+1>100".ToMap(dic).ToValue<bool>(false);//true
            var d="360.1%90".ToValue<int>(-1);//0
            var e="$a+$aa==300".ToMap(dic).ToValue<bool>(false);//true;
            var f="100/5>100/6".ToValue<bool>(false);//true;


            Console.WriteLine("Hello World!");
        }
    }

StringExtensions

    public static class CmdlineEx
    {
        static bool isMathable(this string me)
        {
            var rule = Regex.Escape("0123456789.+-/*%!<>=");
            var pattern = "^[" + rule + "]+$";
            return Regex.IsMatch(me, pattern);
        }

        static bool isNumable(this string me)
        {
            var pattern = @"^[+\-]?[0-9]+\.[0-9]+$|^[+\-]?[0-9]+$";
            return Regex.IsMatch(me, pattern);
        }
        static bool isCompareble(this string me)
        {
            var rule = Regex.Escape("!<>=");
            var pattern = "[" + rule + "]";
            return Regex.IsMatch(me, pattern);
        }

        static bool ToCompare(this string code)
        {
            //a > b
            var map = new Dictionary<string, Func<decimal, decimal, bool>>();
            map["=="] = (a, b) => a == b;
            map["!="] = (a, b) => a != b;
            map[">="] = (a, b) => a >= b;
            map["<="] = (a, b) => a <= b;
            map[">"] = (a, b) => a > b;
            map["<"] = (a, b) => a < b;

            bool f(string d) => Regex.IsMatch(code, Regex.Escape(d));
            if (!map.Keys.Any(f)) return false;
            var ch = map.Keys.Where(f).First();
            var ary = code.Split(ch, 2)
             .Select(d => d.Trim())
             .Select(d => d.ToMath())
             .ToArray()
             ;
            return map[ch](ary[0], ary[1]);
        }

        static decimal ToMath(this string code)
        {
            var map = new Dictionary<string, Func<decimal, decimal, decimal>>();
            map["+"] = (a, b) => a + b;
            map["-"] = (a, b) => a - b;
            map["*"] = (a, b) => a * b;
            map["/"] = (a, b) => a / b;
            map["%"] = (a, b) => (int)a % (int)b;

            bool f(string d) => Regex.IsMatch(code, Regex.Escape(d));
            if (!map.Keys.Any(f)) return f2(code);
            var ch = map.Keys.Where(f).First();
            var ary = code.Split(ch, 2)
             .Select(d => d.Trim())
             .Select(d => f2(d))
             .ToArray()
             ;
            return map[ch](ary[0], ary[1]);
            //
            decimal f2(string s)
            {
                if (decimal.TryParse(s, out var x)) return x;
                else return s.GetHashCode();
            }

        }
                
        public static T ToValue<T>(this string me, T falseValue) where T : IComparable
        {
            if (typeof(T) == typeof(string)) return (T)(object)me;
            //
            if (typeof(T) == typeof(bool))
            {
                decimal vv;
                if (me.isCompareble()) return (T)(object)me.ToCompare();
                else vv = me.ToValue<decimal>(-1);
                //
                var wk = (int)vv;
                if (wk == 1) return (T)(object)true;
                else if (wk == 0) return (T)(object)false;
                return falseValue;
            }

            ///
            decimal v;//数値は一度decimalへ
            if (me.isMathable())
            {
                v = me.ToMath();
            }
            else if (!decimal.TryParse(me, out v)) return falseValue;

            if (typeof(T) == typeof(decimal)) return (T)(object)v;

            if (typeof(T) == typeof(double)) return (T)(object)(double)v;

            if (typeof(T) == typeof(float)) return (T)(object)(float)v;

            if (typeof(T) == typeof(int)) return (T)(object)(int)v;

            if (typeof(T) == typeof(bool))
            {
                var wk = (int)v;
                if (wk == 1) return (T)(object)true;
                else if (wk == 0) return (T)(object)false;
                return falseValue;
            }

            return falseValue;
        }

        public static string ToMap(this string me,Dictionary<string,string> dic,char ch='$')
        {
            ///辞書を使って値を代入する。シンボルは$
            if (!Regex.IsMatch(me, Regex.Escape("" + ch))) return me;
            var keys = dic.Keys
             .Where(d => d.IndexOf(ch) != -1)
             .OrderByDescending(d => d.Length)
             ;
            foreach (var k in keys) me = me.Replace(k, dic[k]);
            return me;
        }

        ///配列は必ず空白文字で初期化されている。
        public static List<string> ToArgs(this string me, int pnum, char sep = ' ')
        {
            var ret = Enumerable.Repeat("", pnum).ToList();
            var ary = me.Split(sep, pnum);
            for (var i = 0; i < ary.Length; i++) ret[i] = ary[i];
            return ret;
        }
        public static string ToLF(this string code)
        {
            return Regex.Replace(code,"\r\n|\r|\n","\n");
        }


    }

逆ポーランドは無し。