DropDownList SelectedIndexChanged event in ASP.Net GridView EditItemTemplate

Unable to genrate dropdown event selectedIndexChange in GridView's Edit Template

 protected void GridViewDepartment_RowUpdating(object sender, GridViewUpdateEventArgs e)  
    {             getConnection = connection.SetConnection();
             GridView GridViewDepartment = (GridView)sender;
             GridViewDepartment.EditIndex = e.RowIndex; // Update Index You Want to edit
             GridViewDepartment.DataSource = departmentTable; // ReBind the GridView
             GridViewDepartment.DataBind();
             int DepartmentID = (int)GridViewDepartment.DataKeys[e.RowIndex].Value;
             DropDownList ddlManager = ((DropDownList)GridViewDepartment.Rows[e.RowIndex].FindControl("ddlDepartManager"));
             string AssignNewManagr = ddlManager.SelectedItem.Text.ToString();
             Response.Write(AssignNewManagr);
             string AssignNewActive = ((DropDownList)GridViewDepartment.Rows[e.RowIndex].FindControl("DropDownListActive")).SelectedItem.ToString();
             Response.Write(AssignNewActive);
             string SPUPDateDepartment = "SPModifyDepartment";
            SqlCommand UPDateCommnad = new SqlCommand(SPUPDateDepartment, getConnection);
            UPDateCommnad.CommandType = CommandType.StoredProcedure;
            UPDateCommnad.Parameters.AddWithValue("@DepartmentId", DepartmentID);
            UPDateCommnad.Parameters.AddWithValue("@DepartmentManager", AssignNewManagr);
            UPDateCommnad.Parameters.AddWithValue("@Active", AssignNewActive);
            UPDateCommnad.ExecuteNonQuery();
            GridViewDepartment.EditIndex = -1;           GetDepartmentList();
    }