Retrieve data Grideview when SelectedIndexChanged

Question

Hi everybody 

I have dropdown list contain all the University inserted in my database and I want when the event SelectedIndexChanged clicked  to show  a grideview contain data related to the selected item  I created the gridview and I wrote this code when SelectedIndexChanged 

  protected void DropDownList3_SelectedIndexChanged(object sender, EventArgs e)

        {

            string Grdvw = GridView1.Visible.ToString();

                  

                

        }