Issue related to Virtualizing stack panel( WPF) in dotnetframework 4.0

HI 

I am facing an issue related to Virtualizing stack panel in dotnet framework 4.0 and above. THere is no issue if i am choosing target framework as 3.5

Please check the line of code given below where the problem lies.

 PropertyInfo pi = objPanel.GetType().GetProperty("CacheEnd", BindingFlags.NonPublic | BindingFlags.Instance);

when executing the above line of code and choosing target framework as 3.5 pi variable is populating with value. so no problem in 3.5

But when i am choosing the target framework as 4.0 pi becomes null

what is the reason for this? Is there any alternate way to resolve this issue ?Please provide solution for this