ASP.NET dynamic Command Button event not firing

Question

I'm trying to create Command Buttons dynamically, but clicking the button in question doesn't seem to raise the corresponding CommandButton_Click event. I noticed that in the examples on SO a property is set for Button.OnCommand as well as the CommandName and CommandArgument but it isn't an option in intellisense.

So the question is, what am I doing wrong here (code below without the OnCommand), is it accessed in some other way - if so, why do the examples I've found all show it as .OnCommand?

EDIT: Further to help, I have added the handler however the event is still not firing. The buttons reside in an UpdatePanel and are rebuilt on every postback (along with the handler). I have created a simplified example of what I'm doing which is shown below If the button event fires, it writes "EVENT FIRED" to the txtTestFired Textbox - suffice to say I have never seen that. This is really driving me nuts, any help is very gratefully received.

.aspx file

<form id="frmMain" runat="server">
    <asp:ScriptManager ID="scmAddProducts" runat="server">
    </asp:ScriptManager>
    <asp:updatepanel runat="server">
        <ContentTemplate>
            <asp:TextBox ID="txtProduct" runat="server"></asp:TextBox>
            <br />
            <asp:Button ID="btnAddItem" runat="server" Text="Add Line" />&nbsp;
            <asp:TextBox ID="txtTestFired" runat="server"></asp:TextBox>
            <br />
            <br />
            <asp:Panel ID="pnlAddedLines" runat="server"></asp:Panel>
        </ContentTemplate>
    </asp:updatepanel>
</form>

.aspx.vb file

Protected Sub btnAddItem_Click(ByVal sender As Object, ByVal e As System.EventArgs) Handles btnAddItem.Click
    Dim dtItems As New System.Data.DataTable

    If Session("Items") Is Nothing Then
        Dim dcColumn As New System.Data.DataColumn
        dcColumn.DataType = Type.GetType("System.String")
        dcColumn.ColumnName = "Product"
        dtItems.Columns.Add(dcColumn)
        Session("Items") = dtItems
    End If

    dtItems = CType(Session("Items"), System.Data.DataTable)
    Dim drRow As System.Data.DataRow
    drRow = dtItems.NewRow()
    drRow("Product") = txtProduct.Text
    dtItems.Rows.Add(drRow)

    Session("Items") = dtItems
    txtProduct.Text = ""
End Sub
Protected Sub Page_PreRender(ByVal sender As Object, ByVal e As System.EventArgs) Handles Me.PreRender
    If Not Session("Items") Is Nothing Then
        Dim dtItems As System.Data.DataTable = CType(Session("Items"), System.Data.DataTable)
        Dim iItemIndex As Integer = 1
        For Each drRow In dtItems.Rows
            Dim btnClose As New Button
            btnClose.ID = "btnClose" & iItemIndex
            btnClose.CssClass = "formCloseButton"
            btnClose.Text = "X"
            AddHandler btnClose.Click, AddressOf Button_Clicked
            pnlAddedLines.Controls.Add(btnClose)
            btnClose = Nothing

            Dim txtProduct = New TextBox
            txtProduct.ID = "txtProduct" & iItemIndex
            txtProduct.CssClass = "formText"
            txtProduct.Text = drRow("Product")
            txtProduct.Columns = "40"
            pnlAddedLines.Controls.Add(txtProduct)
            iItemIndex += 1

            Dim litHR = New Literal
            litHR.Text = "<hr />"
            pnlAddedLines.Controls.Add(litHR)
            litHR = Nothing
        Next
    End If
End Sub
Private Sub Button_Clicked(ByVal sender As Object, ByVal e As System.EventArgs)
    txtTestFired.Text = "EVENT FIRED"
End Sub

Solution 1

Essentially you need to create the original controls once on page load in order to be able to handle the button click, you then need to clear the controls and re-generate them either in the button click handler or as you have done on pre-render - i think it is cleaner on the click handler, so you need the same code in both (preferable refactored out to a separate method)

The example below would mean minimum code changes for you:

Protected Sub Page_PreRender(ByVal sender As Object, ByVal e As System.EventArgs) Handles Me.PreRender
    If Not Session("Items") Is Nothing Then
        pnlAddedLines.Controls.Clear()
        GenerateControls()
    End If
End Sub
Protected Sub Page_Load(ByVal sender As Object, ByVal e As System.EventArgs) Handles Me.Load
    If Not Session("Items") Is Nothing Then
        GenerateControls()
    End If
End Sub
Private Sub GenerateControls()
    Dim dtItems As System.Data.DataTable = CType(Session("Items"), System.Data.DataTable)
    Dim iItemIndex As Integer = 1
    For Each drRow In dtItems.Rows
        Dim btnClose As New Button
        btnClose.ID = "btnClose" & iItemIndex

Solution 2

You have to wire it up like this (in C#):

myButton.Click += new EventHandler(this.Button_Clicked);

Alternatively you can inject the attribute --> onclick = "Button_Clicked" like this:

myButton.Attributes.Add("onclick", "Button_Clicked");

If you want it in VB.NET go to this page and knock yourself out - I could do it for you but what's the point :)

EDIT: keep in mind you'll have to re-wire the event-handler (as shown above) every page-load.

EDIT: OK - I see the problem now that you added the rest of the code. You are wiring up the event handler on pre_render. Event handlers are executed just before load_complete, which happens before pre_render. This is why you event_handler doesn't execute: you're adding it too late in the page lifecycle. Put that code in the page_load routine and it should work.

Solution 3

I had this same problem, I was loading the controls just fine. the events were firing the first time, but not the second time. The problem was I didn't have a unique identifier tied to the controls. So I added a unique ID field to the control I was adding to the page, and it worked just fine, every click.

Comments

[JohnIdol]

see the edit in my answer in reply to your edit

[Chris]

Moved to page_load and the event fires...BUT...now a different problem (this is why I put it in pre_render originally). The button click event (to add the textbox etc) is happening after the Page Load in the page lifecycle. Therefore when I add this to my test I get the problem that the system is always one postback behind - ie you have to submit twice before the first set of controls are rendered.

[cargo8]

I think your original problem is solved correctly (the event not firing) - your current problem should probably be added as a separate question. sounds a bit like you may be addeing new controls too late, are you re-directing during the click handler?

[Chris]

Thanks, the event is still not firing in the context of what is needed. If it is impossible then sobeit, but I would rather continue the question in context ahead of creating a near duplicate question. The crux of the problem is that a user clicks a button which creates controls. The controls don't know they need to be created in page_load because the button click event is yet to be handled. There is a quick and dirty fix where a fake postback or reload is done after every submit but that is clunky and nasty.

[Admin]

This is C#, its not how you do it in VB.NET

[JohnIdol]

You go to this URL --> developerfusion.com/tools/convert/csharp-to-vb :)

[San Jacinto]

god help your clients if you can't take 13 seconds and figure out the minor syntax differences on your own.

[JohnIdol]

I see you took the time to do that in your answer ... talk about practice what you preach :)

[Chris]

Thanks - I have put this in and it still won't fire! I'm now wondering whether it is because the button is being created within an UpdatePanel - could this be the case in your knowledge?

[Chris]

Thanks - that convertor tool is pretty handy too. Would being in an update panel affect the ability of the button to wire up?

[JohnIdol]

If you're trying to update an update panel by clicking the button then you need to setup the button as a trigger for the update panel. If the button is in the update panel it shouldn't be a problem (the update panel should refresh and your code behind should execute). but maybe I am missing something, updating your question showing the markup (with the update panel) would probably help.

[Thomas]

You need to recreate the button on every Postback. Otherwise the events won't fire.

[JohnIdol]

Correct - I was assuming that (shouldn't have). Are you re-wiring the event-handler every time the page loads?

[San Jacinto]

@Johnldol Actually, I was referring to Chalkey's comment about the answer not being in VB. My point was that "If you can't port something this minor from C# to VB, then there are larger probles you have to solve than getting your event to fire." It was more of a tongue-in-cheek comment. I think you personally went above and beyond posting a converter. I'm actually the only one currently who has voted for your post...

[Chris]

Thanks - I have now added a full code listing as this is still giving me grief - really appreciate your help.

[Chris]

Thanks - I moved it to page load and the event fires...BUT...now a different problem (this is why I put it in prerender originally). The button click event (to add the textbox etc) is happening after the Page Load in the page lifecycle. Therefore when I add this to my test I get the problem that the system is always one postback behind - ie you have to submit twice before the first set of controls are rendered. Any ideas on this!? Thanks again for your help.

[Chris]

Thanks - I'll give this a whirl

[Chris]

Thanks - slight variance on your approach based on my actual needs - what I have is a counter that creates n+1 Command Buttons then clears them for the add/remove that may result from a button click (either add another, or remove one) - that way I will never have more than a +2 surplus of handlers floating around which was a concern. Working like a dream - thank you.

[JohnIdol]

nice timing :) - I would maybe store a counter in the ViewState rather than a DataTable in session?

[Chris]

the problem I found with ViewState is that if the user refreshes the page, the controls are lost (there are about 6 controls per row) - the form may take them a while to build so it would be a bit of a tear-your-hair-out moment for them if the entered data didn't persist on a refresh. Thanks again for all of your help along the way (+1 rep btw on your answer, sorry I can't give more)

[cargo8]

+1 for all the work you put in - apologies for hijacking your effort.

[JohnIdol]

@dice cheers - you spared me the bother of a further edit to my answer :)

[Henrik Andersson]

Could you please elaborate on this answer with some code perhaps?