C# ONP Interpreter

using System;
using System.Collections.Generic;

namespace onpInterpreter
{
    class Program
    {
        static void Main(string[] args)
        {
            while (true)
            {
                Console.Clear();
                Console.WriteLine("Enter ONP expression: ");
                string expression = Console.ReadLine();
                try
                {
                    double result = ONP(expression);
                    Console.WriteLine($"Expression value equals {result}.");
                }catch(Exception ex)
                {
                    Console.WriteLine($"Error occured: \"{ex.Message}\"");
                }
                Console.WriteLine("Press ESC to close program or any other key to continue...");
                if (Console.ReadKey().Key == ConsoleKey.Escape)
                    break;
            }
        }
        static double ONP(string expression)
        {
            if (!string.IsNullOrEmpty(expression))
            {
                List<double> _memoryTower = new List<double>();
                string[] _syntax = expression.Split(' ');
                foreach (var c in _syntax)
                {
                    int number;
                    if (int.TryParse(c, out number))
                    {
                        _memoryTower.Add(number);
                    }
                    else
                    {
                        switch (c)
                        {
                            case "+":
                                if (_memoryTower.Count > 1)
                                {
                                    _memoryTower.Add(_memoryTower[_memoryTower.Count - 2] + _memoryTower[_memoryTower.Count - 1]);
                                    _memoryTower.RemoveRange(_memoryTower.Count - 3, 2);
                                }
                                else
                                    throw new Exception($"Unexpected {c} in expression!");
                                break;
                            case "-":
                                if (_memoryTower.Count > 1)
                                {
                                    _memoryTower.Add(_memoryTower[_memoryTower.Count - 2] - _memoryTower[_memoryTower.Count - 1]);
                                    _memoryTower.RemoveRange(_memoryTower.Count - 3, 2);
                                }
                                else
                                    throw new Exception($"Unexpected {c} in expression!");
                                break;
                            case "*":
                                if (_memoryTower.Count > 1)
                                {
                                    _memoryTower.Add(_memoryTower[_memoryTower.Count - 2] * _memoryTower[_memoryTower.Count - 1]);
                                    _memoryTower.RemoveRange(_memoryTower.Count - 3, 2);
                                }
                                else
                                    throw new Exception($"Unexpected {c} in expression!");
                                break;
                            case "/":
                                if (_memoryTower.Count > 1)
                                {
                                    _memoryTower.Add(_memoryTower[_memoryTower.Count - 2] / _memoryTower[_memoryTower.Count - 1]);
                                    _memoryTower.RemoveRange(_memoryTower.Count - 3, 2);
                                }
                                else
                                    throw new Exception($"Unexpected {c} in expression!");
                                break;
                            case "^":
                                if (_memoryTower.Count > 1)
                                {
                                    _memoryTower.Add(System.Math.Pow(_memoryTower[_memoryTower.Count - 2], _memoryTower[_memoryTower.Count - 1]));
                                    _memoryTower.RemoveRange(_memoryTower.Count - 3, 2);
                                }
                                else
                                    throw new Exception($"Unexpected {c} in expression!");
                                break;
                            default:
                                throw new Exception($"Unexpected {c} in expression!");
                        }
                    }
                }
                if (_memoryTower.Count == 1)
                    return _memoryTower[0];
            }
            throw new Exception("ONP expression in not valid!");
        }
    }
}