dynamic datagrid dropdown valuechanged repopulate 2nd list editmo

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I have a dynamic datagrid. I have custom classes for the controls

public class CreateEditItemTemplateDDL : ITemplate {
DataTable dtBind;
string strddlName;
string strSelectedID;
string strDataValueField;
string strDataTextField;

public CreateEditItemTemplateDDL(string DDLName,string DataValueField,string
DataTextField,string SelectedValueField,DataTable DDLSource){
dtBind=DDLSource;
strDataValueField=DataValueField;
strDataTextField=DataTextField;
strSelectedID=SelectedValueField;
this.strddlName=DDLName;}
public void InstantiateIn(Control objContainer){
DropDownList ddl = new DropDownList();
ddl.DataBinding+=new EventHandler(ddl_DataBinding);
objContainer.Controls.Add(ddl);}

private void ddl_DataBinding(object sender, EventArgs e){
DropDownList ddl= (DropDownList)sender;
ddl.ID=strddlName;
ddl.DataSource=dtBind;
ddl.DataValueField=strDataValueField;
ddl.DataTextField=strDataTextField;
for (int i=0;i<dtBind.Rows.Count;i++){
if(strSelectedID == dtBind.Rows[strDataValueField].ToString()) {
ddl.SelectedIndex=i; }
}}}

I do a creategrid and display the grid from dataset after load
I can get the selected values from hidden fields.

MY Problem

How can the user select a new value in the first drop down list and change
the range of possible values in the second drop down list, select the new
values and then update the row back to the database. (I know how to do the
update it is the display of a new range of values in the second list have the
first list has an index changed event.) Example the first list from database
has 2 values fruit and vegetable, vegetable is selected for the database row
in edit mode and in the second list where tomato was selected (choices might
be lettuce or tomato.) the choice violates database integrity data needs to
have been repopulated to show the fruit range of choices. User selects value
fruit first list, I need the second list to repopulate with range of fruits
example orange and grape, new row at top inserted value is 0 says pick a
fruit. I started with creating an ascx control which is a dropdown list. I
can get this control to drop on an aspx form with a label, choose a new value
and based on event handler for index changed in the ascx return the new value
based on the index selected to the form label showing the value for the
selected list array. I next put this ascx in the datagrid with

ITemplate temp= Page.LoadTemplate("Controls/StoreSelector.ascx");
tcl5.EditItemTemplate = temp;
//the control id is storeList

I had to first change to ascx code to get this to work, in the databind on
page load in the ascx I can not do a check for (!(Page.IsPostBack)) because
the list is displayed only when the row is clicked for edit so if I did that
the list does not populate with any values.

Next how do I get the dropdown list selected value to be the value from the
datagrid underlying datatable for the row. What am I doing inside the ascx
(what public exposure) that I can access the property in the event
itemdatabound of the datagrid to set the selected value? I always get the
first item in the dropdownlist.
I tried putting in the ascx code behind

public string SelectedValue{
get{return storeList.SelectedValue;}
set{storeList.SelectedValue = value;}
}

In the aspx code behind I try to pass a value from datagrid datatable for
the row

private void dgRt2_ItemDataBound(object sender, DataGridItemEventArgs e){
DataGrid dg = (DataGrid)sender;
int cnt = e.Item.ItemIndex + (dg.PageSize * dg.CurrentPageIndex);
DataTable dtBound = (DataTable)dg.DataSource;

if(e.Item.ItemType==ListItemType.EditItem) {
string strZone = dtBound.Rows[cnt]["zonelu"].ToString();
SISWebAdmin.Controls.StoreSelector ddlz =
(SISWebAdmin.Controls.StoreSelector)e.Item.Cells[0].FindControl("storeList");
ddlz.SelectedValue = strZone;}


When I debug I pass the line where I do a cast of the control to the ascx
and do a find control in the datagrid
I have the correct string in the strZone variable from the datatable that I
need to set to but I error on the line ddlz .SelectedValue = strZone

I get the follow error message and stack trace

e1 at SISWebAdmin.WebForm1.dgRt2_ItemDataBound(Object sender,
DataGridItemEventArgs e) in c:\program files\common files\microsoft
shared\web server
extensions\60\template\layouts\siswebadmin\webform1.aspx.cs:line 804 at
System.Web.UI.WebControls.DataGrid.OnItemDataBound(DataGridItemEventArgs e)
at System.Web.UI.WebControls.DataGrid.CreateItem(Int32 itemIndex, Int32
dataSourceIndex, ListItemType itemType, Boolean dataBind, Object dataItem,
DataGridColumn[] columns, TableRowCollection rows, PagedDataSource
pagedDataSource) at
System.Web.UI.WebControls.DataGrid.CreateControlHierarchy(Boolean
useDataSource) at
System.Web.UI.WebControls.BaseDataList.OnDataBinding(EventArgs e) at
System.Web.UI.WebControls.BaseDataList.DataBind() at
SISWebAdmin.WebForm1.fillgrid(String sFullname, String strZone, String
strArea, String strTitle) in c:\program files\common files\microsoft
shared\web server
extensions\60\template\layouts\siswebadmin\webform1.aspx.cs:line 368Object
reference not set to an instance of an object.SISWebAdmin


For weeks I could not get into news groups, account issues I don't know what
and have posted everywhere, no replies. If there is a better way tell me, I
have 3 dropdowns in dynamic grid that basically depend on each other for
their range of values. The user in edit mode would theorectically changed
the first range and need new value selection for second then third before the
row can be changed in the database. Please help me. Thank you cindy