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PODoes the ASP.NET ParameterCollection (or its ParametersChanged event) depend on viewstate?
 

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  1. PODoes the ASP.NET ParameterCollection (or its ParametersChanged event) depend on viewstate?
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    <p>Using reflector I found the following code in the <code>System.Web.UI.WebControls.Parameter</code> class:</p> <pre><code>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) &amp;&amp; (obj2 != null)) || ((obj3 != null) &amp;&amp; !obj3.Equals(obj2))) { this.OnParameterChanged(); } } </code></pre> <p>The line</p> <pre><code>this.OnParameterChanged(); </code></pre> <p>is what triggers the <code>ParametersChanged</code>-event in the end, as I understand (through <code>this._owner.CallOnParametersChanged()</code> in <code>OnParameterChanged</code>).</p> <p>Now my question is: what happens if <code>EnableViewState</code> is set to <code>false</code> (e.g. on the whole page)? And as an addendum: I am using <code>ControlParameter</code>s pointing at the <code>SelectedValue</code> of a <code>DropDownList</code>.</p> <p>I assume this would mean:</p> <ol> <li><code>obj2</code> would either be the value that has been set earlier in the same request (i.e. <code>UpdateValue</code> has been called before) or <code>null</code> if no other value has been set yet.</li> <li>As a <code>DropDownList</code>s <code>SelectedValue</code> cannot be <code>null</code> <code>obj3</code> will never be null.</li> <li><code>OnParametersChanged</code> will always be invoked <em>the first time <code>UpdateValues</code> is called in any request</em>.</li> </ol> <p>But if <em>the same entries are loaded into the <code>DropDownList</code> during a PostBack</em> 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...).</p> <p>This comes down to the following: if ViewState is disabled and the same values are being loaded into the <code>DropDownList</code> 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 <code>ParametersChanged</code>-event on the <code>ParameterCollecion</code> (because of #3 in the list above) although in my understanding the parameter did not change its value.</p> <p>Am I wrong or does this mean I cannot (should not) disable ViewState if this behaviour is causing trouble?</p> <hr> <p><strong>UPDATE</strong> (not directly related to the question)</p> <p>Just to get rid of the question how the <code>DropDownList</code> preserves the selected value through a post back if viewstate is disabled: by its implementation of <code>IPostBackDataHandler</code>...</p> <p>Reflector reveals (in <code>DropDownList</code>):</p> <pre><code>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; } </code></pre> <p>So it essentially retrieves the value of the selected item in the <code>POST</code>ed data and if it can find an item with this key in the list of items the <code>DropDownList</code> contains it sets it as selected.</p>
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