C#-Code

using static System.Console;

namespace CS_Aufgabe_249_Taschenrechner
{
    class Program
    {
        static void Main(string[] args)
        {
            PrintInfo();
            Write("Bitte Operation eingeben: "); // 144° (Wurzel aus 144)
            var calc = new Calculator(ReadLine());
            WriteLine(calc); // 144° = 12
        }
        static void PrintInfo()
        {
            WriteLine("Mögliche Operationen:");
            WriteLine("(+) Addition\n(-) Subtraktion\n(*) Multiplikation\n(/) Division");
            WriteLine("(^) Hoch\n(°) Wurzel\n(%) Prozent\n(#) Quersumme");
            WriteLine("(&) Bitweises UND (nur INT)\n");
            WriteLine("Nur 2 Operanden und ein Operator möglich!\n");
            WriteLine("Beispiele: 2+2; -2 - -3; 4^2; 27°3; 20,4 % 84,5; 2&2; 123456789#\n");
            WriteLine("Leerzeichen zwischen Zahlen und Operatoren möglich!\n");
        }
    }
}

C#-Code

using System;
using System.Text.RegularExpressions;
using System.Linq;

namespace CS_Aufgabe_249_Taschenrechner
{
    class Calculator
    {
        private readonly string _mathProblem;

        public Calculator(string mathProblem) => _mathProblem = mathProblem;

        private double GetResult(char op, (double a, double b) v)
        {
            var (s, d, p, q) = MyMath.Operation(v.a, v.b);
            return op switch
            {
                '+' => s,
                '&' => (int)v.a & (int)v.b,
                '-' => d,
                '*' => p,
                '/' => q,
                '°' => Math.Pow(v.a, 1 / Math.Abs(v.b)),
                '^' => Math.Pow(v.a, v.b),
                '%' => v.b * v.a / 100,
                '#' => MyMath.DigitSum((int)v.a),
                _ => 0
            };
        }

        private char GetOperator()
        {
            var r = new Regex(@"[+°*^/%#&\-]{1}");
            var m = r.Matches(_mathProblem).Cast<Match>().Select(x => x.Value[0]).ToArray();
            return m.Length switch
            {
                0 => '0',
                1 => m[0],
                _ => m[0] == '^' ? m[0] : m[1]
            };
        }

        private (double v1, double v2) GetValues()
        {
            var v1 = 0.0;
            var v2 = 2.0;
            var r = new Regex(@"[\-]?[0-9,]+").Matches(_mathProblem);
            if (r.Count > 0)
                _ = double.TryParse(r[0].ToString(), out v1);
            if (r.Count > 1)
                _ = double.TryParse(r[1].ToString(), out v2);
            return (v1, v2);
        }

        public override string ToString() => $"{_mathProblem} = {GetResult(GetOperator(), GetValues())}";
    }
}

C#-Code

namespace CS_Aufgabe_249_Taschenrechner
{
    static class MyMath
    {
        public static (double s, double d, double p, double q) Operation(double a, double b) =>
            (a + b, a - b, a * b, a / (b == 0 ? 1 : b));

        public static int DigitSum(int n) =>
            n != 0 ? n % 10 + DigitSum(n / 10) : 0;
    }
}