The advantage of an add-in is that it is called from within the context of a running EPLAN version and can access the currently selected objects. This can be achieved by using the SelectionSet class. The class provides a number of methods, to get a list of items currently selected by the user. 

You can either can get the project, which the user currently works on by using the SelectionSet::GetCurrentProject method, or you get the currently selected page(s) by the SelectionSet::GetSelectedPages method. Depending on whether the graphical editor or the page overview dialog has currently the focus, one ore many pages can be selected. 

Most importantly, you can get any set of objects selected from any focused (non-modal) dialog by the SelectionSet.Selection property. The objects are returned by the function as an array of StorableObjects. You can loop over the array and determine the types (and any other information) about the objects. 

The following example shows, how to access the selection. 

SelectionSet selectionSet = new SelectionSet();
StorableObject[] storableObjects = selectionSet.Selection;
if (storableObjects.Length == 0)
{
    Console.WriteLine("No current selection!");
}
else
{
    foreach(StorableObject so in storableObjects)
    {
        if(so is Function)
           Console.WriteLine(" StorableObject is a function: " + ((Function) so).Name);
        else
            Console.WriteLine(" StorableObject: " + so.ToString());
    }
}

Dim selectionSet As New SelectionSet()
Dim storableObjects As StorableObject() = selectionSet.Selection
If storableObjects.Length = 0 Then
   Console.WriteLine("No current selection!")
Else
   Dim so As StorableObject
   For Each so In  storableObjects
      If TypeOf so Is Function Then
         Console.WriteLine((" StorableObject is a function: " + CType(so, Function).Name))
      Else
         Console.WriteLine((" StorableObject: " + so.ToString()))
      End If
   Next so
End If