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<string> 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 List<IngredientData> _ingredientsList;
protected void Page_Load(object sender, EventArgs e)
{
// prepare ingredient lists
_ingredientsList = new List<IngredientData>();
if (Page.IsPostBack)
{
if (ViewState["IngredientsList"] != null)
{
_ingredientsList = (List<IngredientData>) ViewState["IngredientsList"];
}
}
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["IngredienstList"] = _ingredientsList;
lstIngredients.DataSource = _ingredientsList;
lstIngredients.DataBind();
}
Any idea how I can fix this? Am I doing something wrong?

Answer 1

btnAddIngredient_Click is adding to "IngredienstList" not "IngredientsList" (note the spelling).
You can avoid this kind of typo by using a constant:
private const string IngredientsListViewStateKey = "IngredientsList";
then referring to it like this:
ViewState[IngredientsListViewStateKey] = _ingredientsList;