Folder Tree WPF Control: Part 2 - Recursive Checkbox Behavior

FolderTreeSelection.cs

public void Add(FolderTreeItem folderTreeItem, bool include = true)
{
    if (folderTreeItem.SelectionItem == null && include)
    {
        // node is checked for the first time, add a selection item
        folderTreeItem.SelectionItem = new Item() { TreeItem = folderTreeItem, Include = include };
        items.Add(folderTreeItem.SelectionItem);
    }
    else if (folderTreeItem.SelectionItem != null && folderTreeItem.SelectionItem.Include != include)
    {
        // node with a current state is checked/unchecked
        if (!include && (folderTreeItem.Parent == null || folderTreeItem.Parent.SelectionItem == null))
        {
            // special behavior if root node is unchecked
            items.RemoveAll(x => x.TreeItem.Equals(folderTreeItem));
            folderTreeItem.SelectionItem = null;
        }
        else
        {
            // determine if this node itself was specifically checked/unchecked
            var match = items.Where(x => x.TreeItem.Equals(folderTreeItem)).SingleOrDefault();
            if (match != null)
            {
                // if so, adjust it's state
                match.Include = include;
            }
            else
            {
                // if not, add a new selection item
                folderTreeItem.SelectionItem = new Item() { TreeItem = folderTreeItem, Include = include };
                items.Add(folderTreeItem.SelectionItem);
            }
        }
    }

    // process children nodes if any are loaded
    if (folderTreeItem.FoldersLoaded)
    {
        foreach (var childItem in folderTreeItem.Folders)
        {
            // all children node states should be wiped out and given a reference to this nodes state
            items.RemoveAll(x => x.TreeItem.Equals(childItem));
            childItem.SelectionItem = folderTreeItem.SelectionItem;
            childItem.IsChecked = include;

            // recurse to go as many levels deep as we can
            // recursive calls will skip the code above because the selection item is already set here
            Add(childItem, include);
        }
    }

    // give the parent nodes either checked or indeterminate states
    ProcessParents(folderTreeItem);
}

private void ProcessParents(FolderTreeItem folderTreeItem)
{
    if (folderTreeItem.Parent == null)
        return;
    bool indeterminate = false;
    bool? current = folderTreeItem.IsChecked;

    // walk up the tree
    do
    {
        folderTreeItem = folderTreeItem.Parent;

        // if any of the other children have a different checkbox state, set to indeterminate
        if (indeterminate || folderTreeItem.Folders.Any(x => x.IsChecked != current))
        {
            folderTreeItem.IsChecked = null;
            indeterminate = true;
        }
        else
        {
            folderTreeItem.IsChecked = current;
        }
    } while (folderTreeItem.Parent != null);
}