combobox 绑定数据后再添加一项

DataTable dt = EmployerSet.Tables["EmployeesTable"];

DataRow dr = dt.NewRow();
   dr["FirstName"] = "--选择所有--";
   dt.Rows.InsertAt(dr, 0);

     this.comboBox1.DataSource = dt;
     this.comboBox1.DisplayMember = "FirstName";