The experienced behavior is correct and by design. As you can see IIF() is a Function
and you need to prepare the parameters prior to executing the function.
That's why the reporting engine first evaluates the function parameters and then pass the values to the function.
Our suggestion would be to use an User Function
instead of IIF as shown in the code snippet:
Public Shared Function CustomFormat(ByVal value As String) As String
If IsNumeric(value) Then
Give it a try and let us know how it goes.
All the best,
the Telerik team
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