Losing value when clicking on button in ascx control that is rendered in asp:Repeater

Question

I created a asp:Repeater that I fill with .ascx controls:

protected void Page_Load(object sender, EventArgs e)
{
    Repeater1.DataSource = listOfData;
    Repeater1.DataBind();            
}

On page I have:

<uc:Product runat="server"                                             
    ImportantData='<% #Eval("ImportantData") %>'                                               
    id="DetailPanel1" />

Inside Product.ascx.cs I have:

public int ImportantData{ get; set; }
protected void Page_Load(object sender, EventArgs e)
{

}

On Product.ascx I have:

<asp:ImageButton ID="btn_Ok" runat="server" 
     onclick="btn_Ok_Click"  
     ImageUrl="~/image.png" />

Problem is when I click on image button I get error:

A critical error has occurred. Invalid postback or callback argument. Event validation is enabled using <pages enableEventValidation="true"/> in configuration or <%@ Page EnableEventValidation="true" %> in a page...

I have tried to change first code to this:

protected void Page_Load(object sender, EventArgs e)
{
    if(!Page.IsPostBack)
    {
        Repeater1.DataSource = listOfData;
        Repeater1.DataBind();            
    }
    // Repeater1.DataBind(); // and tried to put DataBind() here
}

But then when I click on image button ImportantData is empty.
What I did wrong here?

Answer 1

PageLoad is happening before your postback event, change your event to PreRender:

protected void Page_PreRender(object sender, EventArgs e)
{
    Repeater1.DataSource = listOfData;
    Repeater1.DataBind();            
}

This will bind your repeater after the postback and maintain your postback value.

Edit:

Here's a good picture showing the entire WebForms page lifecycle:

As you can see

ProcessPostData event is called AFTER Load event.

So you want to bind your Repeater after the PostData has been processed.

This can be done on PreRender