Question

Using reflector I found the following code in the System.Web.UI.WebControls.Parameter class:

internal void UpdateValue(HttpContext context, Control control)
{
    object obj2 = this.ViewState["ParameterValue"];
    object obj3 = this.Evaluate(context, control);
    this.ViewState["ParameterValue"] = obj3;
    if (((obj3 == null) && (obj2 != null)) ||
            ((obj3 != null) && !obj3.Equals(obj2)))
    {
        this.OnParameterChanged();
    }
}

The line

this.OnParameterChanged();

is what triggers the ParametersChanged-event in the end, as I understand (through this._owner.CallOnParametersChanged() in OnParameterChanged).

Now my question is: what happens if EnableViewState is set to false (e.g. on the whole page)? And as an addendum: I am using ControlParameters pointing at the SelectedValue of a DropDownList.

I assume this would mean:

1. obj2 would either be the value that has been set earlier in the same request (i.e. UpdateValue has been called before) or null if no other value has been set yet.
2. As a DropDownLists SelectedValue cannot be null obj3 will never be null.
3. OnParametersChanged will always be invoked the first time UpdateValues is called in any request.

But if the same entries are loaded into the DropDownList during a PostBack ASP.NET retains the selection in the list across the PostBack even if ViewState is disabled (I can only guess how that works but it does…).

This comes down to the following: if ViewState is disabled and the same values are being loaded into the DropDownList on every request and the user had chosen entry X “long ago” and triggered a PostBack by other means than changing the selection in the DropDownList ASP.NET would fire a ParametersChanged-event on the ParameterCollecion (because of #3 in the list above) although in my understanding the parameter did not change its value.

Am I wrong or does this mean I cannot (should not) disable ViewState if this behaviour is causing trouble?

---

UPDATE (not directly related to the question)

Just to get rid of the question how the DropDownList preserves the selected value through a post back if viewstate is disabled: by its implementation of IPostBackDataHandler…

Reflector reveals (in DropDownList):

protected virtual bool LoadPostData(string postDataKey,
        NameValueCollection postCollection)
{
    string[] values = postCollection.GetValues(postDataKey);
    this.EnsureDataBound();
    if (values != null)
    {
        base.ValidateEvent(postDataKey, values[0]);
        int selectedIndex = this.Items.FindByValueInternal(values[0], false);
        if (this.SelectedIndex != selectedIndex)
        {
            base.SetPostDataSelection(selectedIndex);
            return true;
        }
    }
    return false;
}

So it essentially retrieves the value of the selected item in the POSTed data and if it can find an item with this key in the list of items the DropDownList contains it sets it as selected.

Answer 1

You are correct: you cannot disable viewstate if you want the ASP.NET web control ‘changed’ events and behaviors to work.

The basic mechanism they use to determine whether there has been a change is to compare the value serialized in viewstate to the equivalent posted value (if any). TextBox.TextChanged (for example) spells this out clearly in its class documentation:

A TextBox control must persist some
values between posts to the server for
this event to work correctly. Be sure
that view state is enabled for this
control.