Retain expander state in WPF

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WPF

I have listview with grouping under an expander. The items are dynamically generated. I can Access all the expanders in window control and save their state. Afterwards the item source property of listview is changed. Now if I try to get expanders or any other UI control, the function always returns null. The function is given below

public static IEnumerable<T> FindVisualChildren<T>(DependencyObject depObj) where T : DependencyObject<br />
        {<br />
            if (depObj != null)<br />
            {<br />
                for (int i = 0; i < VisualTreeHelper.GetChildrenCount(depObj); i++)<br />
                {<br />
                    DependencyObject child = VisualTreeHelper.GetChild(depObj, i);<br />
                    if (child != null && child is T)<br />
                    {<br />
                        yield return (T)child;<br />
                    }<br />
<br />
                    foreach (T childOfChild in FindVisualChildren<T>(child))<br />
                    {<br />
                        yield return childOfChild;<br />
                    }<br />
                }<br />
            }<br />
        }

What I have tried:

I have tried using several other functions but to no avail.