The following shows how to find the top item in the displayed tree for a selected item in an extension view. This is applicable to item-view extensions.

public IswItem GetTopItem()
    var currentNodePath = _host.GetCurrentNodePath();

    if (currentNodePath.Count > 0)
        return currentNodePath[0].Owner;

        return _currentItem;

Assuming that _currentItem is the currently selected item in the tree and _host is your IswItemViewHost.