Question

On an ASP.NET web page, I have an UpdatePanel. Inside this UpdatePanel, I have an “Add” button which dynamically adds any number of a User Control that I have created to a PlaceHolder. The User Control contains a label, a button, and a 3rd-party control. The button inside the User Control is for removing that particular User Control from the parent panel. Both the “Add” and “Remove” functionality work correctly. However, the event handler on the Add button sometimes stops working with no apparent pattern or cause. If I add 2 UCs and remove 1, sometimes the Add button continues to work and sometimes it doesn’t. Any step toward finding a cause would be helpful.

For context, the UCs are called “Whereas” because they are WHEREAS clauses in resolutions. Here is the code, cut down to relevant content only:

Edit.aspx

...
<asp:ScriptManager ID="sm1" runat="server"></asp:ScriptManager>
<asp:Panel ID="pnlMain" runat="server">
    <div class="form-horizontal">
        <asp:UpdatePanel ID="upnlWhereas" runat="server">
            <ContentTemplate>
                <asp:PlaceHolder ID="phWhereas" runat="server"></asp:PlaceHolder>
                <div class="control-group">
                    <asp:Label ID="lblAddWhereas" AssociatedControlID="btnAdd1Whereas" CssClass="control-label" runat="server"></asp:Label>
                    <div class="controls">
                        <asp:Button ID="btnAdd1Whereas" CssClass="btn" UseSubmitBehavior="false" Text="Add Whereas" runat="server" />
                    </div>
                </div>
            </ContentTemplate>
        </asp:UpdatePanel>
    </div>
</asp:Panel>
...

Edit.aspx.vb (class is Edit2)

...
Private Property ControlsList As List(Of String)
    Get
        If ViewState("ControlsList") Is Nothing Then
            ViewState("ControlsList") = New List(Of String)
        End If

        Return CType(ViewState("ControlsList"), List(Of String))
    End Get
    Set(ByVal Value As List(Of String))
        ViewState("ControlsList") = Value
    End Set
End Property

Private ReadOnly Property NextID As Integer
    Get
        Return ControlsList.Count + 1
    End Get
End Property

Protected Overrides Sub LoadViewState(savedState As Object)
    MyBase.LoadViewState(savedState)

    For Each waID As String In ControlsList
        Dim NewWhereas As Whereas
        NewWhereas = Me.LoadControl("~/UserControls/Whereas.ascx")

        NewWhereas.ID = waID
        phWhereas.Controls.Add(NewWhereas)

        AddHandler NewWhereas.OnRemoveWhereasClick, AddressOf Whereas_OnRemoveWhereasClick
    Next
End Sub

...

Private Sub btnAdd1Whereas_Click(sender As Object, e As System.EventArgs) Handles btnAdd1Whereas.Click
    Dim NewWhereas As Whereas
    NewWhereas = Me.LoadControl("~/UserControls/Whereas.ascx")

    NewWhereas.ID = "wa" + NextID.ToString
    phWhereas.Controls.Add(NewWhereas)

    ControlsList.Add(NewWhereas.ID)
End Sub

Private Sub Whereas_OnRemoveWhereasClick(ByVal sender As Object)
    Dim thisButton As Button = sender
    Dim thisWhereas As Whereas = thisButton.Parent.Parent

    phWhereas.Controls.Remove(thisWhereas)
    ControlsList.Remove(thisWhereas.ID)
End Sub
...

Whereas.ascx.vb

Public Delegate Sub ActionClick(ByVal sender As Object)

Public Class Whereas
    Inherits System.Web.UI.UserControl

    ...

    Public Event OnRemoveWhereasClick As ActionClick

    Public Sub btnRemoveWhereas_Click(ByVal sender As Object, ByVal e As EventArgs)
        RaiseEvent OnRemvoeWhereasClick(sender)
    End Sub
End Class

Answer 1

After disabling the UpdatePanel (thanks to Yellowfog for the suggestion), I found that the following was happening:

1. Clicking “Add” would add a user control with ID “wa1.”
2. Clicking “Add” again would count the number of existing user controls in the list and add a user control with ID “wa2.”
3. Clicking “Remove” on control “wa1” would remove “wa1.”
4. Clicking “Add” would count the number of existing user controls in the list (1) and attempt to add a user control with ID “wa2” which would cause an error because that control already exists on the page. This error wasn’t being displayed because the call was asynchronous.

The solution was to generate the IDs of the dynamically-added user controls using GUIDs instead of integers based on the number of controls in the list. This way they are essentially guaranteed to be unique.