C# How do I run some code for the selected item in a listbox?

private void btnEcho_Click(object sender, EventArgs e)
    {
         listbox1.SelectedItem......
    }

string selectedText = listbox1.SelectedItem.ToString();

String s = listbox1.SelectedItem.Value.ToString();

switch (listbox1.SelectedItem)
{
  case "A":
       // Code when select A
       break;
  case "B":
       // Code when select B
       break;
  ... (and so on).
}