Jak zrobić dodawanie w kalkulatorze?

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows;
using System.Windows.Controls;
using System.Windows.Data;
using System.Windows.Documents;
using System.Windows.Input;
using System.Windows.Media;
using System.Windows.Media.Imaging;
using System.Windows.Navigation;
using System.Windows.Shapes;

namespace StanKonta
{
    /// <summary>
    /// Interaction logic for MainWindow.xaml
    /// </summary>
    public delegate void RoutedEventHandler(
    object sender,
    RoutedEventArgs e
);
    public partial class MainWindow : Window
    {

        public MainWindow()
        {
            InitializeComponent();
        }
        int screen;
        double a;
        double result = 0;
        private void Odejmowanie_Click(object sender, RoutedEventArgs e)
        {
            a = double.Parse(TXB_2.Text);
            textbox.Text = (result.ToString());
            result -= a;
            TXB_2.Text = "";
        }

        private void Usuwanie_Click(object sender, RoutedEventArgs e)
        {
            textbox.Text = "";
        }

        private void B_1_Click(object sender, RoutedEventArgs e)
        {
            TXB_2.Text = TXB_2.Text + "1";
        }

        private void B_2_Click(object sender, RoutedEventArgs e)
        {
            TXB_2.Text = TXB_2.Text + "2";
        }

        private void B3_Click(object sender, RoutedEventArgs e)
        {
            TXB_2.Text = TXB_2.Text + "3";
        }

        private void B4_Click(object sender, RoutedEventArgs e)
        {
            TXB_2.Text = TXB_2.Text + "4";
        }
        private void B5_Click(object sender, RoutedEventArgs e)
        {
            TXB_2.Text = TXB_2.Text + "5";
        }

        private void B6_Click(object sender, RoutedEventArgs e)
        {
            TXB_2.Text = TXB_2.Text + "6";
        }

        private void B7_Click(object sender, RoutedEventArgs e)
        {
            TXB_2.Text = TXB_2.Text + "7";
        }

        private void B8_Click(object sender, RoutedEventArgs e)
        {
            TXB_2.Text = TXB_2.Text + "8";
        }

        private void B9_Click(object sender, RoutedEventArgs e)
        {
            TXB_2.Text = TXB_2.Text + "9";
        }

        private void B0_Click(object sender, RoutedEventArgs e)
        {
            TXB_2.Text = TXB_2.Text + "0";
        }

        private void ALEK_Click(object sender, EventArgs e)
        {
            Properties.Settings.Default.textbox = textbox.Text;
            Properties.Settings.Default.Save();
        }

        private void AlEKL_Load(object sender, RoutedEventArgs e)
        {
            textbox.Text = Properties.Settings.Default.textbox;
        }


        private void Przeci_Click(object sender, RoutedEventArgs e)
        {
            TXB_2.Text = TXB_2.Text + ",";
        }

        private void Dodawanie_Click(object sender, RoutedEventArgs e)
        {
            var originalNumber = double.Parse(textbox.Text);
            var numberToAdd = double.Parse(a.ToString()); //a to liczba którą wpiszemy do TXB_2
            var sumOfNumbers = originalNumber + numberToAdd;
            textbox.Text = sumOfNumbers.ToString();
            TXB_2.Text = "";
        }
    }
}
 

a = double.Parse(TXB_2.Text); //<- to się pojawia tylko jeśli chcesz odejmować. przy dodawaniu też by się przydało.

        private void Dodawanie_Click(object sender, RoutedEventArgs e)
        {
            var originalNumber = double.Parse(textbox.Text);
            var numberToAdd = double.Parse(a.ToString()); //a to liczba którą wpiszemy do TXB_2
            var sumOfNumbers = originalNumber + numberToAdd;
            textbox.Text = sumOfNumbers.ToString();
            TXB_2.Text = "";
        }

var originalNumber = double.Parse(textbox.Text); 

var numberToAdd = double.Parse(a.ToString());

TXB_2.Text = TXB_2.Text + ((TextBox)sender).Text;

 
private void B_1_Click(object sender, RoutedEventArgs e)
        {
            TXB_2.Text = TXB_2.Text + "1";
        }

        private void Dodawanie_Click(object sender, RoutedEventArgs e)
        {
            var originalNumber = double.Parse(textbox.Text);
            var numberToAdd = double.Parse(a.ToString()); //a to liczba którą wpiszemy do TXB_2
            var sumOfNumbers = originalNumber + numberToAdd;
            textbox.Text = sumOfNumbers.ToString();
            TXB_2.Text = "";
        }

 

a = double.Parse(TXB_2.Text); 

a = double.Parse(TXB_2.Text);
            var originalNumber = double.Parse( string.IsNullOrEmpty(textbox.Text)?"0":textbox.Text);
            var numberToAdd = double.Parse(a.ToString()); //a to liczba którą wpiszemy do TXB_2
            var sumOfNumbers = originalNumber + numberToAdd;
            textbox.Text = sumOfNumbers.ToString();
            TXB_2.Text = ""; 

 
        int screen;
        double a;
        double result = 0;

 
     double numberToAdd = double.Parse(string.IsNullOrEmpty(TXB_2.Text)?"0": TXB_2.Text);
     double originalNumber = double.Parse(string.IsNullOrEmpty(textbox.Text) ? "0" : textbox.Text);
     double result = originalNumber + numberToAdd;
     textbox.Text = result.ToString();
     TXB_2.Text = "";

 
 private void ALEK_Click(object sender, EventArgs e)
        {
            Properties.Settings.Default.textbox = textbox.Text;
            Properties.Settings.Default.Save();
        }

 private void ALEK_Click(object sender, EventArgs e)
        {
            Properties.Settings.Default.textbox = textbox.Text;
            Properties.Settings.Default.Save();
        } 

 
Properties.Settings.Default.textbox = textbox.Text;
            Properties.Settings.Default.Save();

 
   private void Window_Loaded(object sender, RoutedEventArgs e)
        {
            textbox.Text = Properties.Settings.Default.textbox;
        }

        private void Window_Closed(object sender, EventArgs e)
        {
            Properties.Settings.Default.textbox = textbox.Text;
            Properties.Settings.Default.Save();
        }