How I can access a control inside an ItemTemplate and connect it to a datasource by the time it is initialized? - asp.net

Question

I know that there are similar questions to this one out there, but I can't seem to find any that address my issue.
I have a GridView that is populated with data from my database. The user has the option to delete or edit this data, a pretty standard operation. To insert data into the GridView, the user fills out a form and presses the submit button. In the form, there are three textboxes and a DropDownList. As you may expect, this means that, inside the GridView in edit mode, there are three textboxes and a DropDownList. My issue is this: When the user presses the Edit button, the application breaks. It says something to the effect of "myDropDownList cannot be loaded because the selected value is not in the list of items." Basically, the DropDownList inside the EditItemTemplate in the GridView is empty. I have tried to populate it the same way that I populate the other DropDownList (the one in the form), but it is not initialized until the GridView is put in edit mode. Here is the markup for the EditItemTemplate:
I have omitted some code for brevity.
<asp:TemplateField HeaderText="LocationCode" SortExpression="LocationCode">
<EditItemTemplate>
<asp:DropDownList ID="Ddl_EditLocCode" SelectedValue='<%# Bind("LocationCode") %>' runat="server"></asp:DropDownList>
<asp:RequiredFieldValidator ID="rfvLocCode" runat="server" ErrorMessage="Location Code is a required field."
ControlToValidate="Ddl_EditLocCode" ForeColor="Red"></asp:RequiredFieldValidator>
</EditItemTemplate>
<ItemTemplate>
<asp:Label runat="server" Text='<%# Bind("LocationCode") %>' ID="Label5"></asp:Label>
</ItemTemplate>
</asp:TemplateField>
Here is the code I have written to populate the DropDownList inside the EditItemTemplate:
Note that this is the RowDataBound event handler.
Private Sub Gv_Assignments_RowDataBound(sender As Object, e As GridViewRowEventArgs) Handles Gv_Assignments.RowDataBound
If e.Row.RowType = DataControlRowType.DataRow Then
If DataControlRowState.Edit Then
Dim editLocCodeDdl As DropDownList = DirectCast(e.Row.FindControl("Ddl_EditLocCode"), DropDownList)
CreateDataSourceForEditDdl(editLocCodeDdl)
End If
End If
End Sub
Protected Sub CreateDataSourceForEditDdl(editLocCodeDdl As DropDownList)
Try
Dim conn As New Connection
Dim DataReader As SqlDataReader
Dim listItem As New ListItem
Using conn.Conn
conn.Conn.Open()
Using cmd As New SqlCommand
With cmd
.Connection = conn.Conn
.CommandType = CommandType.Text
.CommandText = "SELECT [location_code], [loc_name] FROM [myTable] ORDER BY [location_code]"
End With
DataReader = cmd.ExecuteReader()
Dim locationCode As String
Dim loc_name As String
While DataReader.Read()
locationCode = DataReader.Item("location_code").ToString()
loc_name = DataReader.Item("loc_name").ToString()
editLocCodeDdl.Items.Add(New ListItem(locationCode & "-" & loc_name))
End While
End Using
End Using
Catch ex As Exception
End Try
End Sub
Perhaps the RowDataBound event handler is not where I should be attempting to populate the edit mode DropDownList... I'm not sure. Any help is greatly appreciated.
Edit: RowUpdating Code
Here is the RowUpdating code:
Private Sub Gv_Assignments_RowUpdating(sender As Object, e As GridViewUpdateEventArgs) Handles Gv_Assignments.RowUpdating
'change the lessonID to the appropriate ID based on the lesson name
'get the lesson name, then set the label text in the lesson ID column to the correct ID by calling GetLessonID(lessonname)
Dim lessonNameBox As DropDownList = DirectCast(Gv_Assignments.Rows(e.RowIndex).FindControl("ddlEditLessonName"), DropDownList)
'Dim lessonIdLabel As Label = DirectCast(Gv_Assignments.Rows(e.RowIndex).FindControl("LblLessonID"), Label)
'Dim ID As Integer = Integer.Parse(Gv_Assignments.Rows(e.RowIndex).Cells(1).Text)
'lessonIdLabel.Text = GetLessonID(lessonNameBox.SelectedValue)
Gv_Assignments.Rows(e.RowIndex).Cells(3).Text = GetLessonID(lessonNameBox.SelectedValue)
'UpdateLessonID(lessonIdLabel.Text, ID)
End Sub
As you can see, I am doing pretty much nothing in this function. I should add that I used the GridView's smart tag to assign a SqlDataSource to the GridView. Every other editable cell gets successfully updated when I edit it in edit mode. For some reason, when I removed this line: SelectedValue='<%# Bind("LocationCode") %>', the Ddl_EditLocCode successfully populated with the desired values, but it failed to update on the back end and front end when I edited it; in fact, it made the value Null. Any ideas?
Edit 2: The SqlDataSource
<asp:SqlDataSource ID="sds_GvAssignments" runat="server" ConnectionString='<%$ ConnectionStrings:DedicatedTerminal_devConnectionString %>'
DeleteCommand="DELETE FROM [CreatedAssignments] WHERE [ID] = #ID"
InsertCommand="INSERT INTO [CreatedAssignments] ([LessonName], [LessonID], [StartDate], [EndDate], [LocationCode],
[DateAssigned]) VALUES (#LessonName, #LessonID, #StartDate, #EndDate, #LocationCode, #DateAssigned)"
SelectCommand="SELECT [ID], [LessonName], [LessonID], [StartDate], [EndDate], [LocationCode], [DateAssigned]
FROM [CreatedAssignments]" UpdateCommand="UPDATE [CreatedAssignments] SET [LessonName] = #LessonName,
[LessonID] = #LessonID, [StartDate] = #StartDate, [EndDate] = #EndDate, [LocationCode] = #LocationCode,
[DateAssigned] = #DateAssigned WHERE [ID] = #ID">
<DeleteParameters>
<asp:Parameter Name="ID" Type="Int32"></asp:Parameter>
</DeleteParameters>
<InsertParameters>
<asp:Parameter Name="LessonName" Type="String"></asp:Parameter>
<asp:Parameter Name="LessonID" Type="String"></asp:Parameter>
<asp:Parameter DbType="Date" Name="StartDate"></asp:Parameter>
<asp:Parameter DbType="Date" Name="EndDate"></asp:Parameter>
<asp:Parameter Name="LocationCode" Type="String"></asp:Parameter>
<asp:Parameter Name="DateAssigned" Type="DateTime"></asp:Parameter>
</InsertParameters>
<UpdateParameters>
<asp:Parameter Name="LessonName" Type="String"></asp:Parameter>
<asp:Parameter Name="LessonID" Type="String"></asp:Parameter>
<asp:Parameter DbType="Date" Name="StartDate"></asp:Parameter>
<asp:Parameter DbType="Date" Name="EndDate"></asp:Parameter>
<asp:Parameter Name="LocationCode" Type="String"></asp:Parameter>
<asp:Parameter Name="DateAssigned" Type="DateTime"></asp:Parameter>
<asp:Parameter Name="ID" Type="Int32"></asp:Parameter>
</UpdateParameters>
</asp:SqlDataSource>

Answer 1

The code to fill the DropDownList seems ok at first glance. First try removing SelectedValue='<%# Bind("LocationCode") %>'. The error means that the value of LocationCode does not exist in the ListItems of Ddl_EditLocCode.
And that would seem to make sense since you are adding a ListItem with a value of both the locationCode AND loc_name (New ListItem(locationCode & "-" & loc_name)), therefore only the value of LocationCode does not exist and throws the error.
The solution is to create a ListItem with both a Text and Value specified.
New ListItem(locationCode & "-" & loc_name, locationCode, true)