如何在c#winforms应用程序中获取列表框项目的"密钥"？

using System.Collections.Generic;
using System.Windows.Forms;

Dictionary<string, string> listBoxItems = new Dictionary<string, string>();
listBoxItems.Add("key1", "项目1");
listBoxItems.Add("key2", "项目2");
listBoxItems.Add("key3", "项目3");

listBox1.DataSource = new BindingSource(listBoxItems, null);
listBox1.DisplayMember = "Value";
listBox1.ValueMember = "Key";

string key = listBox1.SelectedValue.ToString();