怎么把switch-case/if-else转换成委托

求助 如何把switch-case/if-else转换成委托啊
代码如下：
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 killIfElse
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }

        private void Form1_Load(object sender, EventArgs e)
        {
            createListBox();
        }
        public void createListBox()
        {
            listBox1.Items.Add("1");
            listBox1.Items.Add("2");
            listBox1.Items.Add("3");
            listBox1.Items.Add("4");
        }
        private void listBox1_MouseClick(object sender, MouseEventArgs e)
        {
            if(listBox1.SelectedIndex==0)
            {
                textBox1.Text = listBox1.SelectedItems[0].ToString();
            }
            else if (listBox1.SelectedIndex == 1)
            {
                textBox1.Text = listBox1.SelectedItems[0].ToString();
            }
            else if (listBox1.SelectedIndex == 2)
            {
                textBox1.Text = listBox1.SelectedItems[0].ToString();
            }
            else if (listBox1.SelectedIndex == 3)
            {
                textBox1.Text = listBox1.SelectedItems[0].ToString();
            }
        }
    }
}
这里是鼠标点击ListBox1后会将ListBox1的内容显示到TextBox上，我参考了张子阳的委托实例，结果还是不对。
（因为我这里要写个项目，listbox1里面起码得有三百行，如果全是ifelse或swithcase会造成代码冗余等等问题还是有数）
哪位大神帮我写一份参考参考啊 万分感激！
------解决思路----------------------
           Dictionary<string, int> dir = new Dictionary<string, int>();
            dir.Add("TreeNode:A", 1);
            dir.Add("TreeNode:B", 2);
            dir.Add("TreeNode:C", 3);
            dir.Add("TreeNode:D", 4);
                listBox1.SelectedIndex = dir[treeView1.SelectedNode.ToString()];

            Dictionary<string, string> dir2 = new Dictionary<string, string>();
            dir2 .Add("TreeNode:A", "50");
            dir2 .Add("TreeNode:B", "30");
            dir2 .Add("TreeNode:C", "20");
            dir2 .Add("TreeNode:D", "10");
      
            DataText.Text = dir2[treeView1.SelectedNode.ToString()];

要学会变通，不要傻傻的，只会拿别人的代码COPY ，