dropdownlist-how to check the value from in list in the table

[rao6]

error in the below code-connot imlicitly convert type string to 'System.Web.UI.WebControls.ListItem'

protected void DropDownList1_SelectedIndexChanged(object sender, EventArgs e)
    {
        SqlConnection cn=myClass.DBCon();
        SqlCommand cmd=new SqlCommand("select CourseName from Course",cn);
        SqlDataReader dr=cmd.ExecuteReader();
        while (dr.Read())
        {
            if (DropDownList1.SelectedItem = dr["CourseName"].ToString())
            {
//code

            }
        }
    }