My goal is to having a infragistics dropdownlist enabling multi selection and at each selection I want an event fired server side without refreshing the whole page.

This is my aspx page :

<%@ Register assembly="Infragistics45.Web.v16.1, Version=16.1.20161.1000, Culture=neutral, PublicKeyToken=7dd5c3163f2cd0cb" namespace="Infragistics.Web.UI.ListControls" tagprefix="ig" %>

<asp:Content ID="BodyContent" ContentPlaceHolderID="MainContent" runat="server">
<ig:WebDropDown ID="WebDropDown1" runat="server" Width="200px" OnSelectionChanged="WebDropDown1_SelectionChanged" EnableMultipleSelection="true" EnableClosingDropDownOnSelect="false" AutoPostBack="true">
</ig:WebDropDown>
<asp:UpdatePanel runat="server" UpdateMode="Conditional">
<Triggers>
<asp:AsyncPostBackTrigger ControlId="WebDropDown1" EventName="SelectionChanged"/>
</Triggers>
</asp:UpdatePanel>
</asp:Content>

This is my code-behind page :

private List<string> allPossiblechoices = new List<string>() { "a", "b", "c","d","e" };

private List<string> defaultChoices = new List<string>() { "a", "b", "c" };

protected void Page_Load(object sender, EventArgs e)
{
if (!this.IsPostBack)
{
foreach(var choice in allPossiblechoices)
{
WebDropDown1.Items.Add(
new DropDownItem()
{
Text = choice,
Value = choice,
Selected = defaultChoices.Contains(choice)
}
);
}
}
}

protected void WebDropDown1_SelectionChanged(object sender, DropDownSelectionChangedEventArgs e)
{
// I put a breakpoint here to see what e.NewSelection and e.OldSelection are
}

By default, when the page is requested for the first time, the dropdown is composed of a,b,c,d,e and only a,b,c are selected.

When I select d, a request is indeed send to the server (I put a breakpoint in my event handler) and the results are correct :

EventArgs e.OldSelection contains a,b,c.<br>
EventArgs e.NewSelection contains a,b,c,d.

Then, I deselect d and the results are the following :

EventArgs e.OldSelection contains a,b,c.d.<br>
EventArgs e.NewSelection contains a,b,c,d.

I don't understand why EventArgs e.NewSelection contains d even if I deselected it.

The fact that it's even more strange, is that I have done the same thing without the updatePanel and everything works fine, the selection (new and old) are correct.

Thanks in advance for your help.