C# Simple Mobile Keypad Windows Form Application

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C#

Hello every one, I'm a newbie in programming yet I know some logic in running a windows application. Can anybody help me in coding my Mobile keypad alike in Windows Form Application?

Here is sample of the code snippet:

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

namespace mobileKeypad
{
    public partial class Form1 : Form
    {
       

        int abc;
        int def;

        public Form1()
        {
            InitializeComponent();
        }

        private void btn2abc_Click(object sender, EventArgs e)
        {

            abc++;

            if (abc == 5)
                abc = 1;


            switch (abc)
            {
                case 1: txtDisplayTxt.Text = "a";
                    break;
                case 2: txtDisplayTxt.Text = "b";
                    break;
                case 3: txtDisplayTxt.Text = "c";
                    break;
                case 4: txtDisplayTxt.Text = "2";
                    break;
            }
        }

        private void btn3def_Click(object sender, EventArgs e)
        {

            def++;

            if (def == 5)
                def = 1;


            switch (def)
            {
                case 1: txtDisplayTxt.Text = "d";
                    break;
                case 2: txtDisplayTxt.Text = "e";
                    break;
                case 3: txtDisplayTxt.Text = "f";
                    break;
                case 4: txtDisplayTxt.Text = "3";
                    break;
            }
        }

    }
}

The problem is, whenever I press button 2, it happens to show only the letters but when I press other buttons, it the text don't append with the current value of text instead it overwrites the current value of the textbox. Can anyone help me to repair it? What I want to happen is make the buttons append with the current values of textbox, can it work without the help of timer? Or timer should be there?