using System;
using System.Collections.Generic;
using System.Text;
 
namespace ConsoleApplication1
{
    class Program
    {
        static void Main(string[] args)
        {
            bool hv = true;
            int z1, z2;
 
            do {
                hv = true;
                Console.Write("Eingabe Zahl 1: ");
                try {
                   z1 = int.Parse(Console.ReadLine());
 
                }
                catch (Exception exc) {
                    Console.WriteLine(exc.Message);
                    hv = false;
                }
            } while (hv == false);
 
            do {
                hv = true;
                Console.Write("Eingabe Zahl 1: ");
                try {
                   z2 = int.Parse(Console.ReadLine());
 
                }
                catch (Exception exc) {
                    Console.WriteLine(exc.Message);
                    hv = false;
                }
            } while (hv == false);
 
            Console.WriteLine("{0} + {1} = {2}", z1, z2, (z1 + z2));
        }
    }
}

Verwendung der nicht zugewiesenen lokalen Variablen "z1"
Verwendung der nicht zugewiesenen lokalen Variablen "z2"

Console.WriteLine("{0} + {1} = {2}", z1, z2, (z1 + z2));