Question

I have following code:

IEnumerable<TreeItem> rootTreeItems = BuildRootTreeItems();

BuildTreeView(rootTreeItems.ElementAt(0));

private static void BuildTreeView(TreeItem treeItem)
    {
        TreeItem subMenuTreeItem = new TreeItem();
        subMenuTreeItem.Header = "1";

        TreeItem subMenuTreeItem2 = new TreeItem();
        subMenuTreeItem.Header = "2";

        treeItem.TreeItems.Add(subMenuTreeItem);
        treeItem.TreeItems.Add(subMenuTreeItem2);
    }

The weird thing is after the BuildTreeView returns, the first element of rootTreeItems doesn’t have any children nodes, while it really has when debugging into the BuildTreeView method.

This problem really confused me for quite a long time, any one has any idea? thanks so much.

Answer 1

You’re most likely hitting a deferred execution issue with IEnumerable<>. The thing to remember is that your IEnumerable<TreeItem> rootTreeItems is not a list, instead it is a promise to get a list each and every time it is asked to do so.

So, if BuildRootTreeItems() creates the IEnumerable<TreeItem> using a LINQ query and it doesn’t force the execution of the query using .ToList() or .ToArray() then each time that you use rootTreeItems you are re-executing the query inside BuildRootTreeItems()

Calling rootTreeItems.ElementAt(0) will cause the query to execute. If later you try to call rootTreeItems.ElementAt(0) again then you are re-executing the query and getting back a different instance of the first TreeItem.

Try changing the first line like so:

IEnumerable<TreeItem> rootTreeItems = BuildRootTreeItems().ToArray();

This forces the execution and will prevent re-execution later. I’ll bet your problem goes away.