Thread: Cascading Drop Down Lists Invalid Postback or Callback argument Error

I was playing around with the AJAX Control Toolkit CascadingDropDown extender. I set it up to use a webservice method to populate the contents. Everything worked perfectly until I triggered an event that caused a post back. This is when I received the following error:

Invalid postback or callback argument. Event validation is enabled using <pages enableEventValidation="true"/> in configuration or <%@ Page
EnableEventValidation="true" %> in a page. For security purposes, this feature verifies that arguments to postback or callback events originate from
the server control that originally rendered them. If the data is valid and expected, use the ClientScriptManager.RegisterForEventValidation method in
order to register the postback or callback data for validation.

Basically, it is saying that you either have to turn event validation off for the page or register it with event validation. Well, turning off event validation for the page is not an option in my book. That poses a serious security risk.

So I tried registering it with the ClientScriptManager and fed it every possible value for the drop down lists I had on my page. This DID NOT work either.

The solution I found, posted by another person with the same problem, was to override the render method and databind all possible values for the drop down lists.

So, in my code for the page:

Code:

Protected Overrides Sub Render(ByVal writer As System.Web.UI.HtmlTextWriter)
    With Me.DropDownList1
        .DataSource = myDataSource
        .DataTextField = "mydatatextfield"
        .DataValueField = "mydatavaluefield"
        .DataBind()
    End With
 
    MyBase.Render(writer)
End Sub

Above, DropDownList1 is the TargetControlID of the CascadingDropDown. That was it! Any postback event on my page no longer triggered the error.

In my case, I have a Business class that my web method calls to get the data. I use this same business class to get the data in the overrided method:

Code:

Protected Overrides Sub Render(ByVal writer As System.Web.UI.HtmlTextWriter)
    Dim myDataSource As System.Collections.Generic.List(Of MyClass) = MyClass.GetData
    With Me.DropDownList1
        .DataSource = myDataSource
        .DataTextField = "mydatatextfield"
        .DataValueField = "mydatavaluefield"
        .DataBind()
    End With
 
    MyBase.Render(writer)
End Sub

If you don't have something like this, then you should be able to call your web method in the overrided render method or duplicate the code to get the data.

Do you need to just include this in the page that has client populated dropdown?

And do you need to do anything about the other controls on the page? Or just include the client populated control?

I also tried the ClientScript.RegisterForEventValidation method in Page_Render and it didn't help.

OK. I've answered my own questions...
1) Yes.
2) No. Just include the base.render statement

C# version for those that need it

Code:

    protected override void Render(HtmlTextWriter writer)
    {
        this.ddlReceptor.DataSource = Receptor.GetAllReceptorList();
        this.ddlReceptor.DataValueField = "id";
        this.ddlReceptor.DataBind();
        this.ddlReceptor.Items.Add(new ListItem("None", "0"));
        base.Render(writer);
    }

Also need to ensure you add all possible values. For example, I manually add a "None" option to all the returned possibilities, so I also need to add it in the render event.

Hope that helps.