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In the general case the RadAutoCompleteBox should be defined in the same class as its TokenValidating event handler, which means that you can access it directly. Also you can use the view element parent which is the default RadAutoCompleteBoxElement that is used by the control and most of its properties can be accessed through it:
void radAutoCompleteBox1_TokenValidating(object sender, Telerik.WinControls.UI.TokenValidatingEventArgs e)
AutoCompleteBoxViewElement viewElement = sender as AutoCompleteBoxViewElement;
RadAutoCompleteBoxElement element = viewElement.Parent as RadAutoCompleteBoxElement;
radAutoCompleteBox1.AccessibleDescription = "Description";
Detailed information about our controls structure can be found in the following help section: Overview
I hope this information helps. Should you have any other questions, I will be glad to assist you.
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