TreeView

Question

there's something strange i don't understand:
i tried to add to a TreeNode a NodeTreeCollection, and when i did it with another node, the result was one list with the both Collections together - this is not what i need, of course. i used AddRange method to add the collections.
when i did it with Add - it worked just fine.

what did i do wrong?

this.treeView.Nodes[i].Nodes.AddRange(DayArray);

10x