WPF TreeView: Where is the ExpandAll() method

The WPF TreeView class does not have an ExpandAll method. Thus you'd have to loop through the nodes and set their IsExpanded properties to true.

It's 2022-01-14 now, and I'm using VS2022 with .net6. I believe during 2015~now, MS built a new method named ExpandSubTree(), which can expand not the whole tree but a single TreeViewItem. It makes things easier.

CollapseAll is quite different because only the first layer need to be collapsed.

public static class TreeViewHelper
{
    public static void ExpandAll(this TreeView treeView)
    {
        foreach (var item in treeView.Items)
        {
            if (treeView.ItemContainerGenerator.ContainerFromItem(item) is TreeViewItem treeViewItem)
                treeViewItem.ExpandSubtree();
        }
    }

    public static void CollapseAll(this TreeView treeView)
    {
        foreach (var item in treeView.Items)
        {
            if (treeView.ItemContainerGenerator.ContainerFromItem(item) is TreeViewItem treeViewItem)
                treeViewItem.IsExpanded = false;
        }
    }

    public static void CollapseAll(this TreeViewItem treeViewItem)
    {
        foreach (var item in treeViewItem.Items)
        {
            if (treeViewItem.ItemContainerGenerator.ContainerFromItem(item) is TreeViewItem subTreeViewItem)
                subTreeViewItem.IsExpanded = false;
        }
    }
}