Hi. Shouldn't you also replace the diagram in the usercontrol's root panel? I.e. remove the old one from panel.Children and add the one returned by GetDiagramFromHashTable.

I have another problem. I have an MDI interface with 2 user controls in 2 MDI windows. Each user control keeps the Diagram object. I select one derived from ShapeNode object and copy it into clipboard.

public void Copy_Executed(object sender, ExecutedRoutedEventArgs e)
	  {
		List<DiagramLink> deselectedLinks = ExcludeUnconnectedLinksFromSelection();
		m_Diagram.CopyToClipboard(false);
		foreach (DiagramLink link in deselectedLinks)
		    m_Diagram.Selection.Links.Add(link);
	  }
 

After that I try to paste them into the another user control with another Diagram object. The code is the following:

CompositeCmd undoCommand = new CompositeCmd(m_Diagram, UndoCommands.AddNode);
		m_Diagram.PasteFromClipboard(new Vector(50, 50), false);
		List<ReassignedHids> hids = new List<ReassignedHids>();
		foreach (DiagramNode node in m_Diagram.Selection.Nodes)
		{
		    AddItemCmd cmd = new AddItemCmd(node);
		    undoCommand.AddSubCmd(cmd);
		    if (node is TPDENode)
		    {
			  TPDENode tpdeNode = node as TPDENode;
			  if (tpdeNode.ShadowProps.ShowShadows)
			  {
				DropShadowEffect effect = new DropShadowEffect();
				effect.Color = tpdeNode.ShadowProps.ShadowColor;
				effect.ShadowDepth = tpdeNode.ShadowProps.ShadowDepth;
				tpdeNode.Effect = effect;
				tpdeNode.Repaint(false);
			  }
			  if (node is TechnologicalOperation)
			  {
				TechnologicalOperation operation = node as TechnologicalOperation;
				ReassignedHids hid = new ReassignedHids(operation.Id, Id.GetDescendant(LastAssignedId, SqlHierarchyId.Null));
				hids.Add(hid);
				operation.Id = Id.GetDescendant(LastAssignedId, SqlHierarchyId.Null);
				LastAssignedId = operation.Id;
				BreadCrumbProcessDescription desc = new BreadCrumbProcessDescription(operation.Text, operation.Id,
				    NodeType.SimpleProcess);
				AddProcessToBreadcrumb(desc);
			  }
			  else if (node is OperationGroupNode)
			  {
				OperationGroupNode operationGrp = node as OperationGroupNode;
				ReassignedHids hid = new ReassignedHids(operationGrp.Id, Id.GetDescendant(LastAssignedId, SqlHierarchyId.Null));
				hids.Add(hid);
				operationGrp.Id = Id.GetDescendant(LastAssignedId, SqlHierarchyId.Null);
				LastAssignedId = operationGrp.Id;
				BreadCrumbProcessDescription desc = new BreadCrumbProcessDescription(operationGrp.Text,
				    operationGrp.Id, NodeType.ComplexProcess);
				AddProcessToBreadcrumb(desc);
			  }
		    }
		}
		e.Source = hids;
		try
		{
		    undoCommand.Execute();
		}
		catch (Exception ex)
		{
		}
 

After that I catch the exception "Must disconnect specified child from current parent Visual before attaching to new parent Visual." When I do the same within one Diagram object all works ok

I have used th your source codes and seen that the exception take place after AddVisualChild(v); in the method internal void AddVisual(Visual v) of the class TranslateLayer: FrameworkElement. In the output console it looks like the following

Step into: Stepping over method without symbols 'System.Collections.CollectionBase.Count.get'
Step into: Stepping over method without symbols 'System.Collections.CollectionBase.Count.get'
Step into: Stepping over property 'MindFusion.Diagramming.Wpf.DiagramItem.UIElement.get'.  To step into properties, go to Tools->Options->Debugging and uncheck 'Step over properties and operators (Managed only)'.
Step into: Stepping over property 'MindFusion.Diagramming.Wpf.DiagramItem.Adorner.get'.  To step into properties, go to Tools->Options->Debugging and uncheck 'Step over properties and operators (Managed only)'.
Step into: Stepping over method without symbols 'System.Windows.Media.Visual.AddVisualChild'
A first chance exception of type 'System.ArgumentException' occurred in PresentationCore.dll
 

For doing the same with standard ShapeNode object I need to modify my code. So it will take some time

No. I only one time call m_Diagram.PasteFromClipboard(new Vector(50, 50), false);

No. I only one time call m_Diagram.PasteFromClipboard(new Vector(50, 50), false);  

However the foreach loop after that adds the pasted nodes again, if you are still using the code posted above. I suppose you will get such exceptions in that case.

What do you think the AddItemCmd commands created in that loop do? I have added the command to my test and it throws that exception indeed:

diagram.PasteFromClipboard(new Vector(10, 10));
diagram.ExecuteCommand(new AddItemCmd(diagram.Selection.Nodes[0]));

Otherwise it works fine if you remove the AddItemCmd line.

PasteFromClipboard already does that as an atomic operation. It creates a composite command internally with AddItemCmds for each item, so synchronizing with your other controls should not be any different from what you are doing now (in response to ActionUndone I guess?).