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POImplementing cascading DropDownList binding in a templated control
 

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    <p>I have 2 <code>DropDownList</code> controls on my form, the second of which uses the <code>SelectedValue</code> of the first as one of its binding parameters.</p> <p>Both <code>DropDownList</code> controls are in a <code>FormView.InsertItemTemplate</code> with <code>SelectedValue</code> properties bound to the <code>FormView</code>'s datasource using a Binding Expression. </p> <p>The first time the <code>FormView</code> renders in Insert mode, everything works fine. The problem is after an <code>AutoPostBack</code> from the first <code>DropDownList</code>, the <code>FormView</code> doesn't (re-)bind, however since the <code>ControlParameter</code> on the second <code>DropDownList</code> has changed, it DOES bind (as intended), but an exception occurs on the Binding Expression of the second DDL, I assume since the <code>FormView</code> is not binding on that pass:</p> <blockquote> <p>System.InvalidOperationException: Databinding methods such as Eval(), XPath(), and Bind() can only be used in the context of a databound control.</p> </blockquote> <p>Here is the markup:</p> <pre><code>&lt;InsertItemTemplate&gt; . . . &lt;tr class="GridViewRowB"&gt; &lt;td class="GridViewCell"&gt; Offense Type &lt;/td&gt; &lt;td class="GridViewCell"&gt; &lt;asp:DropDownList ID="ddlOffenseType" runat="server" DataSourceID="dsOffenseType" AutoPostBack="true" DataValueField="OffenseTypeID" DataTextField="Description" SelectedValue='&lt;%# Bind("OffenseTypeID") %&gt;'&gt; &lt;/asp:DropDownList&gt; &lt;asp:ObjectDataSource ID="dsOffenseType" runat="server" TypeName="OffenseType" SelectMethod="GetAll"&gt; &lt;SelectParameters&gt; &lt;asp:Parameter Name="ActiveOnly" DefaultValue="True" Type="Boolean" /&gt; &lt;/SelectParameters&gt; &lt;/asp:ObjectDataSource&gt; &lt;/td&gt; &lt;/tr&gt; &lt;tr class="GridViewRowA"&gt; &lt;td class="GridViewCell"&gt; Attorney &lt;/td&gt; &lt;td class="GridViewCell"&gt; &lt;asp:DropDownList ID="ddlAttorney" runat="server" DataSourceID="dsAttorney" DataValueField="AttorneyID" DataTextField="AttorneyNameWithCount" SelectedValue='&lt;%# Bind("AttorneyID") %&gt;'&gt; &lt;/asp:DropDownList&gt; &lt;asp:ObjectDataSource ID="dsAttorney" runat="server" TypeName="Attorney" SelectMethod="GetAttorneyWithCaseCount"&gt; &lt;SelectParameters&gt; &lt;asp:Parameter Name="ActiveOnly" DefaultValue="True" Type="Boolean" /&gt; &lt;asp:ControlParameter Name="OffenseTypeID" Type="Int32" ControlID="ddlOffenseType" PropertyName="SelectedValue" /&gt; &lt;/SelectParameters&gt; &lt;/asp:ObjectDataSource&gt; &lt;/td&gt; &lt;/tr&gt; . . . &lt;/InsertItemTemplate&gt; </code></pre> <p>My question is: What is the best way to make this functionality work? Is it possible to keep both DDL's inside the template? I would prefer to avoid using the AJAX toolkit or other client-side solutions.</p>
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