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, everything is fine! The problem is that I am invoking the same page (like postback) thanks to asp: LinkButton. I get a System.NullReferenceExceptionon if (MesiSelezionati.Contains(hf.ID.Split('_').LastOrDefault())).

It seems that the HiddenField lines 2 ° and 4 ° (which corresponds to position c MesiSelezionati = new string[] { "2", "4" };) is zero. What for? And how can I fix this?

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);