A simple calculator.

Calculator.cs

using System;

namespace CalculatorDelegate
{
    class Program
        
    {
        private static double Item1 { get; set; }
        private static double Item2 { get; set; }
        private static string Action { get; set; }
        private delegate double Del(double a, double b);

        private static void Main(string[] args)
        {
            while (true)
            {
                Console.WriteLine(ParseInput());
            }
        }

        private static double ParseInput()
        {
            Console.WriteLine("Enter a mathematical expression, please");
            var userInput = Console.ReadLine();
            var userInputLength = userInput.Length;
            Del handler;
            
            for (var i = 1; i < userInputLength; i++)
            {
                
                if ( userInput[i].Equals('+') || userInput[i].Equals('-') ||
                    userInput[i].Equals('*') | userInput[i].Equals('/'))
                {
                        Action = Convert.ToString(userInput[i]);
                        double.TryParse(userInput.Substring(0, i), out var tempItem1);
                        double.TryParse(userInput.Substring(i + 1, userInputLength - i - 1), out var tempItem2);
                   
                        Item1 = tempItem1;
                        Item2 = tempItem2;
                    
                        if (Action.Equals("/") && Item2 == 0)
                        {
                            Console.WriteLine("You can't divide by zero");
                            return 0;
                        }

                        break;
                }
            }
            
            switch (Action)
            {
                case "+":
                    handler = GetSum;
                    break;
                
                case "-":
                    handler = GetDiff;
                    break;
       
                case "*":
                    handler = GetMult;
                    break;
          
                case "/":
                    handler = GetDiv;
                    break;
                
                default:
                    return 0;
            }

            return handler(Item1, Item2);
        }

        private static double GetSum(double a, double b)
        {
            return a + b;
        }

        private static double GetDiff(double a, double b)
        {
            return a - b;
        }

        private static double GetMult(double a, double b)
        {
            return a * b;
        }

        private static double GetDiv(double a, double b)
        {
            return a / b;
        }

    }
}