problem dynamically removing items from a droplist..?

21 Feb 2006 10:24 AM

Milsnips

hi there,

i have the following HTML code:
--------------------------
<asp:DropDownList id="hddList" runat="server">
      <asp:ListItem Value="1">Item 1</asp:ListItem>
      <asp:ListItem Value="2">Item 2</asp:ListItem>
      <asp:ListItem Value="3">Item 3</asp:ListItem>
      <asp:ListItem Value="4">Item 4</asp:ListItem>
      <asp:ListItem Value="5">Item 5</asp:ListItem>
      <asp:ListItem Value="6">Item </asp:ListItem>
</asp:DropDownList>
--------------------------

and the following VB.NET code behind:
----------------------------------
Private Sub hddList_PreRender(ByVal sender As Object, ByVal e As
System.EventArgs) Handles hddList.PreRender
     If hddList.Items.Count > 0 Then
          If Request.QueryString("type") <> "" Then
          For Each i As ListItem In hddList.Items
               If i.Value <> Request.QueryString("type") Then
               'remove item not to be shown
                    hddList.Items.Remove(i.Text)
               End If
          Next
          End If
     End If
End Sub
----------------------------------

What i am doing is if querystring value = , say 3, i want to programatically
remove all items and only leave Value=3 in there but i am getting an error:

Exception Details: System.InvalidOperationException: Collection was
modified; enumeration operation may not execute.

please help,
Paul

21 Feb 2006 11:50 AM

Kevin Spencer

You're using an enumerator, which iterates from item number 1 to item number
last, but you're removing each item as you move forward through the
Collection. This makes the enumerator unable to correctly iterate through
the list, as the size of it has changed. For future reference, if you ever
need to loop through a Collection to find something, start at the end. But
in this case, you don't need any such thing. The DropDownList Items property
is a List ItemCollection, which has a FindByText and FindByValue method,
which you can use without any looping. Example:

          If Request.QueryString("type") <> "" Then
               Dim li As ListItem =
hddList.Items.FindByValue(Request.QueryString("type"))
               If Not IsNothing(li) Then
                              hddList.Items.Remove(li)
              End If
          End If

--
HTH,

Kevin Spencer
Microsoft MVP
..Net Developer
To a tea you esteem
a hurting back as a wallet.

Show quoteHide quote

"Milsnips" <milsn***@hotmail.com> wrote in message
news:%23hKD3DtNGHA.812@TK2MSFTNGP10.phx.gbl...
> hi there,
>
> i have the following HTML code:
> --------------------------
> <asp:DropDownList id="hddList" runat="server">
>      <asp:ListItem Value="1">Item 1</asp:ListItem>
>      <asp:ListItem Value="2">Item 2</asp:ListItem>
>      <asp:ListItem Value="3">Item 3</asp:ListItem>
>      <asp:ListItem Value="4">Item 4</asp:ListItem>
>      <asp:ListItem Value="5">Item 5</asp:ListItem>
>      <asp:ListItem Value="6">Item </asp:ListItem>
> </asp:DropDownList>
> --------------------------
>
> and the following VB.NET code behind:
> ----------------------------------
> Private Sub hddList_PreRender(ByVal sender As Object, ByVal e As
> System.EventArgs) Handles hddList.PreRender
>     If hddList.Items.Count > 0 Then
>          If Request.QueryString("type") <> "" Then
>          For Each i As ListItem In hddList.Items
>               If i.Value <> Request.QueryString("type") Then
>               'remove item not to be shown
>                    hddList.Items.Remove(i.Text)
>               End If
>          Next
>          End If
>     End If
> End Sub
> ----------------------------------
>
> What i am doing is if querystring value = , say 3, i want to
> programatically remove all items and only leave Value=3 in there but i am
> getting an error:
>
> Exception Details: System.InvalidOperationException: Collection was
> modified; enumeration operation may not execute.
>
> please help,
> Paul
>
>
>
>

21 Feb 2006 6:32 PM

Milsnips

thanks Kevin,

i'll try that.. although i want kinda the opposite of what u showed me.. i
want to remove all items that dont equal the querystring("type") - is it
possible this way, or would i need to do something else?

thanks,
Paul

Show quoteHide quote

"Kevin Spencer" <kevin@DIESPAMMERSDIEtakempis.com> wrote in message
news:OQs0U0tNGHA.420@tk2msftngp13.phx.gbl...
> You're using an enumerator, which iterates from item number 1 to item
> number last, but you're removing each item as you move forward through the
> Collection. This makes the enumerator unable to correctly iterate through
> the list, as the size of it has changed. For future reference, if you ever
> need to loop through a Collection to find something, start at the end. But
> in this case, you don't need any such thing. The DropDownList Items
> property is a List ItemCollection, which has a FindByText and FindByValue
> method, which you can use without any looping. Example:
>
>          If Request.QueryString("type") <> "" Then
>               Dim li As ListItem =
> hddList.Items.FindByValue(Request.QueryString("type"))
>               If Not IsNothing(li) Then
>                              hddList.Items.Remove(li)
>              End If
>          End If
>
> --
> HTH,
>
> Kevin Spencer
> Microsoft MVP
> .Net Developer
> To a tea you esteem
> a hurting back as a wallet.
>
>
> "Milsnips" <milsn***@hotmail.com> wrote in message
> news:%23hKD3DtNGHA.812@TK2MSFTNGP10.phx.gbl...
>> hi there,
>>
>> i have the following HTML code:
>> --------------------------
>> <asp:DropDownList id="hddList" runat="server">
>>      <asp:ListItem Value="1">Item 1</asp:ListItem>
>>      <asp:ListItem Value="2">Item 2</asp:ListItem>
>>      <asp:ListItem Value="3">Item 3</asp:ListItem>
>>      <asp:ListItem Value="4">Item 4</asp:ListItem>
>>      <asp:ListItem Value="5">Item 5</asp:ListItem>
>>      <asp:ListItem Value="6">Item </asp:ListItem>
>> </asp:DropDownList>
>> --------------------------
>>
>> and the following VB.NET code behind:
>> ----------------------------------
>> Private Sub hddList_PreRender(ByVal sender As Object, ByVal e As
>> System.EventArgs) Handles hddList.PreRender
>>     If hddList.Items.Count > 0 Then
>>          If Request.QueryString("type") <> "" Then
>>          For Each i As ListItem In hddList.Items
>>               If i.Value <> Request.QueryString("type") Then
>>               'remove item not to be shown
>>                    hddList.Items.Remove(i.Text)
>>               End If
>>          Next
>>          End If
>>     End If
>> End Sub
>> ----------------------------------
>>
>> What i am doing is if querystring value = , say 3, i want to
>> programatically remove all items and only leave Value=3 in there but i am
>> getting an error:
>>
>> Exception Details: System.InvalidOperationException: Collection was
>> modified; enumeration operation may not execute.
>>
>> please help,
>> Paul
>>
>>
>>
>>
>
>