unable to cast from object to Treeviewitem

Question

I'm trying to Cast from an object to a TreeviewItem and getting the next error:

"unable to cast object of type 'system.string' to type 'system.windows.controls.treeviewitem'."

For 2 days I'm searching for a solution to this, I saw many examples of explicit cast to treeviewitem so I think it legit but its not working.

foreach (Req ObjReq in reqFilter.NewList())
 {
    index = PM_TreeView.Items.Add(ObjReq.Name);
    TreeViewItem ParentNode = new TreeViewItem();

    //this is the Cast I try to do 
    ParentNode = (TreeViewItem)PM_TreeView.Items[index];

    ParentNode.Tag = ObjReq.ID;
    reqFilter["RQ_FATHER_ID"] = (ObjReq.ID.ToString());
    reqFilter.KeepHierarchical = true;
    if (reqFilter.NewList().Count > 0)
        FillReqTreeView(reqFilter, ObjReq);
  }

The main idea is to populate the List in the reqFilter to a Treeview.

Answer 1

The message says you are trying to cast a STRING to a TreeViewItem.

The third line in your code

index = PM_TreeView.Items.Add(ObjReq.Name);

adds a STRING to the Items collection yet the seventh tries to cast this same string to a TreeViewItem

ParentNode = (TreeViewItem)PM_TreeView.Items[index];

Instead of adding a string, add a new TreeViewItem with the Header value you want