asp.net - GridView doesn't remember state between postbacks

Question

I have a simple ASP page with databound grid (bound to an object source). The grid is within the page of a wizard and has a 'select' checkbox for each row.

In one stage of the wizard, I bind the GridView:

protected void Wizard1_NextButtonClick(object sender, WizardNavigationEventArgs e)
    {
...
        // Bind and display matches
        GridViewMatches.EnableViewState = true;
        GridViewMatches.DataSource = getEmailRecipients();
        GridViewMatches.DataBind();

And when the finish button is clicked, I iterate through the rows and check what's selected:

protected void Wizard1_FinishButtonClick(object sender, WizardNavigationEventArgs e)
{
    // Set the selected values, depending on the checkboxes on the grid.
    foreach (GridViewRow gr in GridViewMatches.Rows)
    {
        Int32 personID = Convert.ToInt32(gr.Cells[0].Text);
        CheckBox selected = (CheckBox) gr.Cells[1].FindControl("CheckBoxSelectedToSend");

But at this stage GridViewMatches.Rows.Count = 0! I don't re-bind the grid, I shouldn't need to, right? I expect the view-state to maintain the state. (Also, if I do rebind the grid, my selection checkboxes will be cleared)

NB: This page also dynamically adds user controls in OnInit method. I have heard that it might mess with the view state, but as far as I can tell, I am doing it correctly and the viewstate for those added controls seems to work (values are persisted between postbacks)

Thanks a lot in advance for any help!

Ryan

UPDATE: Could this be to do with the fact I am setting the datasource programatically? I wondered if the asp engine was databinding the grid during the page lifecycle to a datasource that was not yet defined. (In a test page, the GridView is 'automatically' databound'. I don't want the grid to re-bound I just want the values from the viewstate from the previous post!

Also, I have this in the asp header: ViewStateEncryptionMode="Never" - this was to resolve an occasional 'Invalid Viewstate Validation MAC' message

For reference, my GridView is defined as follows:

<asp:GridView ID="GridViewMatches" runat="server" AllowSorting="True" AutoGenerateColumns="False" 
    BackColor="White" BorderColor="#E7E7FF" BorderStyle="None" BorderWidth="1px" CellPadding="3" 
    OnDataBinding="GridViewMatches_OnBinding">
        <Columns>
            <asp:BoundField DataField="PersonID"><ItemStyle CssClass="hidden"/></asp:BoundField>
            <asp:TemplateField>
                <ItemTemplate>
                    <asp:CheckBox ID="CheckBoxSelectedToSend" runat="server"
                        Checked='<%# DataBinder.Eval(Container.DataItem, "SelectedToSend") %>'/>
                </ItemTemplate>
...

Answer 1

Iterating the controls in the PreInit event (to detect if the 'add another control' or 'remove another control' button was pressed) invalidates the view state!!

Here is the method called from PreInit

public Control GetPostBackControl(Page thePage)
    {
        //return null;

        Control myControl = null;
        string ctrlName = thePage.Request.Params.Get("__EVENTTARGET");
        if (((ctrlName != null) & (ctrlName != string.Empty)))
        {
            myControl = thePage.Master.FindControl(ctrlName);
        }
        else
        {
            foreach (string Item in thePage.Request.Form)
            {
                Control c = thePage.Master.FindControl(Item);
                if (((c) is System.Web.UI.WebControls.Button))
                {
                    myControl = c;
                }
            }

        }

        return myControl;
    }

(I take no credit for this methd, I found it on the web)

If the first line is uncommented, the viewstate is maintained.

Awful!