C# - Nerede Hata Yapıyorum?

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;
using System.Collections;
using System.IO;

namespace WindowsFormsApp4
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }
        ArrayList a10 = new ArrayList(30);
        ArrayList b10 = new ArrayList(30);
        ArrayList c10 = new ArrayList(30);



        private void treeView1_AfterSelect(object sender, TreeViewEventArgs e)
        {

        }

        private void listBox1_SelectedIndexChanged(object sender, EventArgs e)
        {
          
        }

        private void button1_Click(object sender, EventArgs e)
        {
            switch (comboBox1.SelectedText)
            {

              

                case "10-A":
                    {
                        a10.Add(textBox1.Text);
                      

                        break;
                    }
                case "10-B":
                    {
                        b10.Add(textBox1.Text);
                    

                        break;
                    }
                case "10-C":
                    {
                        c10.Add(textBox1.Text);
                      
                  
                        break;
                    }

                  
            }
        }

        private void comboBox1_SelectedIndexChanged(object sender, EventArgs e)
        {

        }

        private void comboBox2_SelectedIndexChanged(object sender, EventArgs e)
        {
          
            switch (comboBox2.SelectedText)
            {
              
                case "10-A":
                    {
                        foreach (var item in a10)
                        {
                            listBox1.Items.Add(item);
                        }

                        break;
                    }
                case "10-B":
                    {
                        foreach (var item in b10)
                        {
                            listBox1.Items.Add(item);
                        }

                        break;
                    }
                case "10-C":
                    {
                        foreach (var item in c10)
                        {
                            listBox1.Items.Add(item);
                        }

                        break;
                    }
            }
        }

        private void Form1_Load(object sender, EventArgs e)
        {

        }

        private void diğerForumToolStripMenuItem_Click(object sender, EventArgs e)
        {
          
        }
    }
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