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POmanually adding ajax control toolkit cascading drop down
 

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    <p>Visual Studio 2010, .NET Framework 4.0 Trying to manually add an AjaxControlToolkit CascadingDropDown extender to a webpage dynamically. The web page has a placeholder. When it posts back the controls are built dynamically and added to the placeholder. I have regular ASP.NET drop down list boxes that I want to add CascadingDropDown extender to.</p> <pre><code>myCascadingDropDown = New AjaxControlToolkit.CascadingDropDown myCascadingDropDown.ID = "cdd_" &amp; myDisplayedField.ControlID myCascadingDropDown.BehaviorID = "cdd_" &amp; myDisplayedField.ControlID myCascadingDropDown.TargetControlID = myDisplayedField.ControlID myCascadingDropDown.Category = myDisplayedField.ControlID myCascadingDropDown.PromptText = "Select " &amp; aField.Caption myCascadingDropDown.ServicePath = "CascadingDropDown.asmx" myCascadingDropDown.ServiceMethod = "DropDownListBoxChanged" myPanelForFields.Controls.Add(myCascadingDropDown) </code></pre> <p>The error is the last line where it is adding the CascadingDropDown. The error is:</p> <blockquote> <p>Value cannot be null. Parameter <code>name</code>: No target control is set for the <code>CascadingDropDown</code> extender.</p> </blockquote> <p>And if you look at the property TargetControl on myCascadingDropDown it is null (nothing). But you can not set it (it is read only). I assumed that since I set TargetControlID that ASP.NET would set TargetControl from the TargetControlID.</p> <p>Any ideas or suggestions?</p> <p>More complete exception:<br> System.ArgumentNullException was unhandled by user code Message=Value cannot be null.<br> Parameter name: No target control is set for the CascadingDropDown extender. ParamName=No target control is set for the CascadingDropDown extender.<br> Source=AjaxControlToolkit<br> StackTrace:<br> at AjaxControlToolkit.CascadingDropDown.CascadingDropDown_ClientStateValuesLoaded(Object sender, EventArgs e)<br> at AjaxControlToolkit.ExtenderControlBase.LoadClientStateValues()<br> at AjaxControlToolkit.ExtenderControlBase.OnLoad(EventArgs e)<br> at System.Web.UI.Control.LoadRecursive()<br> at System.Web.UI.Control.AddedControl(Control control, Int32 index)<br> at System.Web.UI.ControlCollection.Add(Control child) </p> <p><strong>ANSWER in VB.NET:</strong></p> <p>Yuriy Rozhovetskiy nailed it. I am programming this application in vb.net so first I created a global variable (my functions in this class are shared/static): </p> <pre><code>Shared ControlToResolve As System.Web.UI.Control </code></pre> <p>Then the code I created was: </p> <pre><code>myCascadingDropDown = New AjaxControlToolkit.CascadingDropDown myCascadingDropDown.ID = "cdd_" &amp; myDisplayedField.ControlID myCascadingDropDown.BehaviorID = "cdd_" &amp; myDisplayedField.ControlID myCascadingDropDown.TargetControlID = myDisplayedField.ControlID myCascadingDropDown.Category = myDisplayedField.ControlID myCascadingDropDown.PromptText = "Select " &amp; aField.Caption myCascadingDropDown.ServicePath = "CascadingDropDown.asmx" myCascadingDropDown.ServiceMethod = "DropDownListBoxChanged" ControlToResolve = myBSCComboBox AddHandler myCascadingDropDown.ResolveControlID, AddressOf myCascadingDropDown_ResolveControlID myPanelForFields.Controls.Add(myCascadingDropDown) </code></pre> <p>then the handler is: </p> <pre><code>Shared Sub myCascadingDropDown_ResolveControlID(sender As Object, e As ResolveControlEventArgs) e.Control = ControlToResolve End Sub </code></pre> <p>I defined the TargetControl of the CascadingDropDown as "ControlToResolve". I set ControlToResolve to the target Drop Down List Box before I add the CascadingDropDown to the panel holding the controls. When I add the CascadingDropDown, it calls the handler myCascadingDropDown_ResolveControlID which assigns the drop down list box to the CascadingDropDown.Target</p> <p>My web service looks like: </p> <pre><code>&lt;System.Web.Services.WebService(Namespace:="http://tempuri.org/")&gt; _ &lt;System.Web.Services.WebServiceBinding(ConformsTo:=WsiProfiles.BasicProfile1_1)&gt; _ &lt;System.Web.Script.Services.ScriptService()&gt; _ &lt;ToolboxItem(False)&gt; _ Public Class CascadingDropDown Inherits System.Web.Services.WebService &lt;WebMethod(EnableSession:=True)&gt; _ Public Function DropDownListBoxChanged(ByVal knownCategoryValues As String, ByVal category As String) As CascadingDropDownNameValue() End Function </code></pre> <p>and it calls this web service with my ControlID every time I need values in one of the drop down list boxes. BTW - using the ControlID I can tell which drop down list box wants data, if it is a parent or child drop down list box and can go get the data. </p>
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