Viewstate null on postback - asp.net

Question

So I have a listbox on my page and some textfields. Through the textfields I can add an item to my listbox (click the button, it adds it to a private List which is then set as a ViewState and the list is databound again). My listbox is also in an updatepanel which gets triggered on the button's Click event. Problem: My Viewstate remains null on a postback so it gets reset each time.
Some code:
private const string VIEW_INGREDIENTS = "IngredientsList";
private const string VIEW_LANGUAGE = "CurrentLanguage";
private List<IngredientData> _ingredientsList;
protected void Page_PreInit(object sender, EventArgs e)
{
if (Page.IsPostBack)
{
if (ViewState[VIEW_INGREDIENTS] != null)
{
_ingredientsList = (List<IngredientData>) ViewState[VIEW_INGREDIENTS];
}
}
else
{
// prepare ingredient lists
_ingredientsList = new List<IngredientData>();
}
}
protected void Page_Load(object sender, EventArgs e)
{
lstIngredients.DataSource = _ingredientsList;
lstIngredients.DataTextField = "Text";
lstIngredients.DataValueField = "Name";
lstIngredients.DataBind();
}
protected void btnAddIngredient_Click(object sender, EventArgs e)
{
_ingredientsList.Add(new IngredientData { Name = txtIngredientName.Text, Quantity = txtUnitQuantity.Text, Unit = lstUnits.SelectedValue });
ViewState[VIEW_INGREDIENTS] = _ingredientsList;
lstIngredients.DataSource = _ingredientsList;
lstIngredients.DataBind();
}

Answer 1

You're using vewstate during PreInit ? Try to check that a bit later during PreLoad.

Answer 2

Check if the page has EnableViewState="true":
<%# Page Language="C#" EnableViewState="true" ...
And verify the site-wide setting in web.config:
<pages enableViewState="true" enableViewStateMac="true" ... />
Now ASP.NET has built-in viewstate for list controls, so I wonder why you're writing custom code for it. The default viewstate should work well for what you're trying to accomplish.