wpf - C# DragDrop object with children results in System.InvalidOperationException -

I have dragged and applied droppings to the previewviews in WPF, which works as intended. However, it is only adding the basic element of the data object (surrounded by one) to the value of the tree, but adding the visual tree to all children (and their children and so on).

Here is my first attempt to add children to the data object (this should be the first sub-level added):

  Public Zero DotNonputation Organization (OrlandoWoilModal Organizations) {_projOrgan Unauthorized); ObservableCollection & LT; OrganLocationViewModel & gt; Sub-organs = organization. Body organization; Foreign objects (Organizational view model node in sub-organ) {Org Lection. Shab organization.ed (node); Node Parent Operations Location = Orionation; }}   
 Now when I can drag and drop the item in which there is no child, it works as a purpose (and in the visual tree along with the object tree Is added). However, when I try to drag and drop the children in any number, then it goes through the code  foreach  the code block, then when it ends and try to complete the dragon This error passes through this error message:  
  An unrestricted exception of type 'System.InvalidOperationException' has occurred in PresentationCore.dll   Additional information: Archive Was modified; The calculation function can not be executed   
 and I am using the  DoDragDrop  method with the  system.Windows.DragDrop  method, here The reason that is the line is error:  System.Windows.DragDrop.DoDragDrop (m_DragInfo.VisualSource, data, m_DragInfo.Effects);   
 Does anybody know how can I rejoin children's nodes in the object tree?   
 
 
 The exception message states that, the problem is that you are calculating and modifying the same archive At the same time, which is not allowed:    ObservableCollection & lt; OrganLocationViewModel & gt; Sub-organs = organization. Body organization; Foreign objects (Organizational view model node in sub-organ) {Org Lection. Shab organization.ed (node); Node Parent Operations Location = Orionation; }   
 Here you add items inside the  organLocation.SubOrganLocations  and  foreach  operator. And this behavior is probably not what you want.