It depends on what kind of id you need. E.g. you could assign a Guid to each new node by setting node.Id = Guid.NewGuid(). If you need consecutive integer numbers as identifiers, you could keep and update an integer counter as value of diagram.Tag and increment it for each new node:

If diagram.Tag Is Nothing Then diagram.Tag = 0
node.Id = Diagram.Tag
Diagram.Tag = CInt(Diagram.Tag) + 1 

Whatever method you use, you might want to keep it in a separate sub and call from event handlers such as NodeCreated, NodePasted, and also after you create items programmatically.

I hope that helps,
Stoyan

The Id property was first added in version 5.3.5 of the control. If you have an older version, use Tag instead.

I hope that helps,
Stoyan