更新

WPFDark/Internals/TreeViewItemExtensions.cs

@@ -0,0 +1,38 @@
+using System;
+using System.Windows.Controls;
+using System.Windows.Media;
+
+namespace WPFDark.Internals
+{
+    internal static class TreeViewItemExtensions
+    {
+        internal static int GetDepth(this TreeViewItem item)
+        {
+            try
+            {
+                TreeViewItem? parent;
+                while ((parent = GetParent(item)) != null)
+                {
+                    return GetDepth(parent) + 1;
+                }
+            }
+            catch
+            {
+                // ignored
+            }
+
+            return 0;
+        }
+
+        private static TreeViewItem? GetParent(TreeViewItem item)
+        {
+            var parent = VisualTreeHelper.GetParent(item);
+            while (!(parent is TreeViewItem || parent is TreeView))
+            {
+                parent = VisualTreeHelper.GetParent(parent ?? throw new InvalidOperationException());
+            }
+
+            return parent as TreeViewItem;
+        }
+    }
+}