Problem

All elements of an subelement should be catched

Approach – Solution

Recursively iterate through all subelements and AddRange (already a list) to the List.
The interuption of the recursion is, when an empty List is used as parameter for AddRange, because the loop will not be called where the AddRange-Method is in on next iteration.

Solution – Approach

private List<EA.Element> getElementsOfElement(EA.Element actualElement)
{
    List<EA.Element> elementsOfElement = new List<EA.Element>();
    EA.Collection elements;
    elements = actualElement.Elements;

    foreach (EA.Element element in elements)
    {
        elementsOfElement.Add(element);
        elementsOfElement.AddRange(getElementsOfElement(element));
    }
    return elementsOfElement;
}