(need help) Simple problem in C#

[Paul]

Try this.

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows.Forms;

namespace WindowsFormsApplication1
{
    public partial class Form1 : Form
    {

        private readonly int firstbox;

        public Form1()
        {
            InitializeComponent();
        }

        private void Form1_Load(object sender, EventArgs e)
        {

        }

        public void button1_Click(object sender, EventArgs e)
        {
            int firstbox = Int32.Parse( textBox1.Text ); //here you input first value
            int firstvalue = Int32.Parse( textBox1.Text ); // this is to store the first value you write in textbox
            textBox1.Text = ""; //this is so that the textbox clears after the click
        }


        public void button2_Click(object sender, EventArgs e)
        {
            int secondbox = Int32.Parse( textBox1.Text );  //second input!
            int secondvalue = Int32.Parse( textBox1.Text ); //this is to store the second value that you input after buttonclick
            int result = firstbox + secondbox; //the sum
            int limitbox = Int32.Parse( textBox2.Text ); //limit!

            if (result <= limitbox)
            { //if you only write < and the MAX is 20, it will give you Invalid because 20 is not less than 20
                textBox1.Text = result.ToString();
            }
            else
            {
                MessageBox.Show("Invalid input");
            }


        }
    }
}

I made firstbox an instance variable otherwise it would have been out of context.

Remember to edit the namespace. You may also need to remove readonly.