Question

I would like to puplate a list dynamically, and bind it to an asp repeater which use a custom control.

Here is my custom control (.acsx file) :

<%@ Control Language="VB" ClassName="ControlCar" %>
<script runat="server">
    Private m_car As Car = Nothing
    Public Property Car() As Car
        Get
            Car= m_car
        End Get
        Set(ByVal value As Car)
            m_car = value
        End Set
    End Property
    
    Protected Sub Panel_OnLoad(ByVal sender As Object, ByVal e As System.EventArgs)
        If Me.m_car Is Nothing Then
            lit_color.Text = "(m_car Is Nothing)"
        Else
            lit_color.Text = "color of Me.m_car is (" & Me.m_car.Color & ")"
        End If
    End Sub
    
</script>

<asp:Panel ID="panel" OnLoad="Panel_OnLoad" runat="server">
    this is a car<br />
    color = <asp:Literal ID="lit_color" runat="server"></asp:Literal><br />
    <br />
</asp:Panel>

So I create a repeater in my aspx file :

<%@ Register TagPrefix="uc" TagName="ControlCar" Src="myfile.ascx" %>
<asp:Repeater ID="id_repeater" runat="server">
    <headertemplate>
    </headertemplate>
    <itemtemplate>  
    <uc:ControlCar id="ControlCarTemplate" Car=<%# CType(Container.DataItem, Car)%> runat="server" />
    </itemtemplate>
</asp:Repeater>    

List to populate repeater is declared :

Private Shared list_cars As ArrayList

On LoadPage event, I add one item in list :

If IsPostBack Then
    Return
End If

list_cars = New ArrayList()
Dim car As Car = New Car
car.Color = "red"
list_cars .Add(car)
id_repeater.DataSource = list_cars 
id_repeater.DataBind()

When I display page, I correctly see this :

this is a car

color = color of Me.m_car is (red)

OK, it works fine on load page event

But now, I want add item on button clic event defined here :

Sub OnClickAdd(ByVal sender As Object, ByVal e As System.EventArgs)
    Dim car = New Car
    car.Color = "blue"
    list_cars.Add(car)
    id_repeater_products.DataSource = list_cars
    id_repeater_products.DataBind()
End Sub

On click event, I see now that :

this is a car

color = (m_car Is Nothing)

this is a car

color = (m_car Is Nothing)

It doesn’t work, I think Container.DataItem is Nothing value, it is good instance only when loading page

I don’t know how to fix it, any help ?

Answer 1

On your Page_Load, use Not Page.IsPostBack

If Not Page.IsPostBack Then

    list_cars = New ArrayList()
    Dim car As Car = New Car
    car.Color = "red"
    list_cars .Add(car)
    id_repeater.DataSource = list_cars 
    id_repeater.DataBind()

End If

Whenever you click the button, a postback occurs and the list is overwritten with list_cars = New ArrayList() in Page_Load.

And also, asp.net is not windows application, you cannot add item to existing list because of the postback unless you hold the list in Session or ViewState

In your codebehind of your page add this property

Public Property list_cars As ArrayList

    Get
       If ViewState("list_cars") IsNot Nothing Then
          Dim list As ArrayList = CType(ViewState("list_cars"), ArrayList)
          return list
       End If

       return Nothing 'or return an empty list
    End Get

    Set
         ViewState("list_cars") = value
    End Set

End Property

To use this in your page:

Dim updatedList As ArrayList = list_cars
updatedList.Add(car)

list_cars = updatedList