LinqDataSource "OnSelecting" event not firing on Post-backs

Question

I have a web page with two dropdownlist controls, each bound to separate LinqDataSource objects. One displays a list of Categories and the other displays a list of Articles. The Category choice drives Article list (at least, that's my intended behaviour). The Article list is also filtered based on the users language preference, stored in the Session and applied in the ArticleLinqDataSource_Selecting event handler.

<asp:Label runat="server" Text="Category Code:" AssociatedControlID="CategoryDropDownList" />
<asp:DropDownList runat="server" ID="CategoryDropDownList" DataSourceID="CategoryLinqDataSource" DataValueField="CategoryID" DataTextField="CategoryCode" AutoPostBack="true" />
...
<asp:Label runat="server" Text="Article Code:" AssociatedControlID="ArticleCodeDropDown" />
<asp:DropDownList runat="server" ID="ArticleCodeDropDown" DataSourceID="ArticleLinqDataSource" DataValueField="ArticleID" DataTextField="ArticleCode" OnDataBound="ArticleCodeDropDown_DataBound" />
 ...
<asp:LinqDataSource runat="server" ID="CategoryLinqDataSource" 
  ContextTypeName="Article.Data.ArticleDataContext"
  TableName="Categories" Select="new (CategoryID, CategoryCode)">
</asp:LinqDataSource>
...
<asp:LinqDataSource runat="server" ID="ArticleLinqDataSource" 
  ContextTypeName="Arcicle.Data.ArticleDataContext"
  TableName="Articles" OrderBy="ArticleCode"
  Select="new (ArticleID, ArticleCode)"
  OnSelecting="ArticleLinqDataSource_Selecting">
</asp:LinqDataSource>

This all works fine when the page first loads. The Category list contains the all the available category values, and the first category in the list is selected. And the corresponding Articles for the first category are displayed appropriately in the Article dropdown control. However, when I change the category (a post-back happens because I have it set to AutoPostBack="true", the Article dropdown does not get refreshed. In other words, the OnSelecting event is not getting fired on subsequent postbacks. Is this the expected bahaviour? If it is, how do I get around this?

Answer 1

You need to call DataBind on the DropDownList in the CategoryDropDownList's SelectedIndexChanged event. It doesn't happen on its own.

e.g.

Protected Sub CategoryDropDownList_SelectedIndexChanged(ByVal sender As Object, ByVal e As System.EventArgs) Handles CategoryDropDownList.SelectedIndexChanged
        ArticleCodeDropDown.DataBind()
End Sub