instance

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace CalculatorTest
{
    public interface ICalculator
    {
        int writeNumber();
        string writeText { get; set; }
        int operand1 { get; set; }
        int operand2 { get; set; }
    }
    public class Calculator : ICalculator
    {
        public string writeText { get; set; }
        public int operand1 { get; set; }
        public int operand2 { get; set; }

        public int writeNumber()
        {
            return operand1 + operand2;
        }

        public Calculator(/*int operand1, int operand2*/)
        {
            /*this.operand1 = operand1;
            this.operand2 = operand2;*/
        }
    }

    class Program
    {
        static void Main(string[] args)
        {
            ICalculator Clc = new Calculator(/*50, 31*/);
            Clc.writeText = "Hello!";
            Clc.operand1 = 50;
            Clc.operand2 = 31;
            Clc.writeNumber();
            Console.WriteLine(Clc.writeText);
            Console.WriteLine(Clc.writeNumber());
        }
    }
}