calculator.cs

public class Calculator
{
    public static int Add(IEnumerable<int> operands)
    {
        return operands.Sum();
    }
}

calculatorApp.cs

public class CalculatorApp
{
    public void Start(string prompt)
    {
        do {
          
            try { 

                Console.WriteLine(prompt);

                var operands = GetInput();

                Console.WriteLine(Calculator.Add(operands));

            }
            catch(Exception ex) { Console.WriteLine(ex); }

        } while(true);
    }

    private IEnumerable<int> GetInput()
    {
        var input = Console.ReadLine();

        return input.Split(' ').Cast<int>();
    }
}

main.cs

public static void Main()
{
   var calculator = new CalculatorApp();
   calculator.Start("Type in some numbers for me to add!");
}