Question

This is my code :

private string[] MesiSelezionati;

protected void Page_Load(object sender, EventArgs e)
{
    MesiSelezionati = new string[] { "2", "4" };
    UpdateMesi();
}

override protected void OnInit(EventArgs e)
{
    for (int i = 1; i <= 12; i++)
    {
        HtmlGenericControl meseItem = new HtmlGenericControl("a") { InnerHtml = "mese" };
        meseItem.Attributes.Add("href", "javascript:void(0);");

        HiddenField hf = new HiddenField();
        hf.Value = "0";
        hf.ID = "idMese_" + i.ToString();

        meseItem.Controls.Add(hf);

        panelMesi.Controls.Add(meseItem);
    }

    base.OnInit(e);
}

private void UpdateMesi()
{
    foreach (HtmlGenericControl a in panelMesi.Controls.OfType<HtmlGenericControl>())
    {
        HiddenField hf = a.Controls.OfType<HiddenField>().LastOrDefault();
        if (MesiSelezionati.Contains(hf.ID.Split('_').LastOrDefault()))
        {
            hf.Value = "1";
            a.Attributes.Add("class", "box-ricerca-avanzata-item link-box selected");
        }
    }
}

When I call the page, all is ok! The problem is when I call the same page (so, postback) thanks to a asp:LinkButton. I get a System.NullReferenceException on if (MesiSelezionati.Contains(hf.ID.Split('_').LastOrDefault())).

Seems that the HiddenField of the 2° and 4° link (which corrispond to the position at MesiSelezionati = new string[] { "2", "4" };) are null. Why? And how can I fix it?

EDIT : code for Mark M

HtmlGenericControl optionBox = new HtmlGenericControl("div");
optionBox.Attributes["class"] = "option-box";

HtmlGenericControl optionBoxItem = new HtmlGenericControl("a") { InnerHtml = " " };
optionBoxItem.Attributes.Add("href", "javascript:void(0);");
optionBoxItem.Attributes.Add("class", "option-box-item");

HtmlGenericControl optionBoxTesto = new HtmlGenericControl("a") { InnerText = Categoria.Categoria };
optionBoxTesto.Attributes.Add("href", "javascript:void(0);");
optionBoxTesto.Attributes.Add("class", "option-box-testo");

HiddenField hf = new HiddenField();
hf.Value = "0";
hf.ID = "categoria_" + Categoria.UniqueID;

optionBox.Controls.Add(optionBoxItem);
optionBox.Controls.Add(optionBoxTesto);
optionBox.Controls.Add(hf);

panelCategorieGuida.Controls.Add(optionBox);

Answer 1

You can update hidden fields on postback, just not before Load. When executing OnInit, the controls have not been populated using the request & view state values. Your updates are being overwritten.

EDIT: I found the root cause of your issue & learned something in the process.

1. You set the InnerHtml property of the anchor tag (InnerHtml = "mese") in the OnInit method. Under the covers this assignment is ViewState["innerhtml"] = "mese".
2. You assign a css class to the anchor after ViewState tracking has begun, so the ViewState restoration machinery will apply to this control on postback.

When you postback the anchor tags with added css classes will be subject to HtmlContainerControl.LoadViewState (which occurs between InitComplete and PreLoad). If the LoadViewState method detects that ViewState[“innerhtml”] has a value it wipes out all of the control’s child controls (calling Controls.Clear()) and creates a LiteralControl to contain the innerhtml value, adding it as the sole child control.

Basically this means that you cannot set both the InnerHtml property and add any controls to a descendant of HtmlContainerControl if that control will ever be subject to ViewState tracking.

To fix your example; instead of setting InnerHtml to add the link text, create a LiteralControl with the desired text and add it to the anchor’s child control collection.