using System;

public class Program

{

    public static void Main()

    {

        var textbox1 = "5";

        var textbox2 = "5";

        var textbox3 = "";

        if(!string.IsNullOrWhiteSpace(textbox1) || !string.IsNullOrWhiteSpace(textbox2) )

        {

            int.TryParse(textbox1, out var textbox1Int);

            int.TryParse(textbox1, out var textbox2Int);

            textbox3 = (textbox1Int + textbox2Int).ToString();

        }

        Console.WriteLine(textbox3);

    }

}