Modifying item in viewstate - is it safe to do?

Question

I'm still a relative noob, however I was surprised at the results of a small test I did.

I want to store a list of string in viewstate. To date when I want to modify the list I retrieved it from viewstate, performed a List.Add and saved it back to viewstate.

However, I then decided to do a simple test, here it is below:

    protected void Page_Load(object sender, EventArgs e)
    {
        if (!IsPostBack)
        {
            List<string> s = new List<string>();
            s.Add("abc");
            s.Add("def");
            ViewState.Add("test", s);

            s.Add("one");
            s.Add("two");
        }
        var t = (List<string>)ViewState["test"];

        foreach (var str in t)
        {
            Response.Write(str + "<br>");
        }
    }

As you can see, I create a list, add it to viewstate, then modify the list. To my suprise the list is modified in viewstate , even after postback.

The question is, is it safe to do this, and if so is it considered bad practice.

Thanks in advance.

Answer 1

Kind of subjective, but I don't see anything wrong with this by practice. This is something that happens all the time within different form controls on the page. The controls will edit viewstate information pertaining to themselves and persist that over postback. The reason the list changes in the viewstate when you modify it after adding it to the viewstate is that it doesn't get immediately serialized. The list is passed by reference to the viewstate collection where it sits until the page serializes the viewstate shortly before completing the request.