Question

I’m a bit confused here. I’m copying all the controls from one form to a panel on the main form and for some reason only about half of them copy.

 Private Sub switchComponent()
         Dim selection As String = TreeView1.SelectedNode.Text

         Panel1.Controls.Clear()

         Dim query = From cont In serverDic(selection).Controls
                     Select cont

         For Each copier As Control In query
             Panel1.Controls.Add(copier)
         Next

 End Sub

serverDic is defined as:

 Dim serverDic As New Dictionary(Of String, frmServer)

When stepping through the code, serverDic(selection).Controls has 12 elements, but only 6 of them get copied. Next time this gets called, only 3 get copied. Does Panel1.Controls.clear() somehow kill the references?

EDIT: Just to show that there are infact 12 elements in the collection:

Answer 1

The problem here is that you are iterating over a collection that you are changing. When you add a Control to an container it is implicitly removed from it’s previous parent and hence query. This is why you see exactly half of the items get removed.

With most collections this would be more apparent because they would throw if modified during an enumeration. The primary source of query here though is ControlCollection which does allow for modifications while enumerating.

To fix this problem just add the following line before the For Each loop.

query = query.ToList()