dotnet/wpf · error · InvalidOperationException

SR.TreeLoop

Error message

SR.TreeLoop

What it means

During event routing the number of elements processed exceeded MAX_ELEMENTS_IN_ROUTE, which the framework treats as a cyclic or runaway tree. InvalidOperationException(SR.TreeLoop) aborts the route. It protects against infinite loops caused by malformed visual/logical trees.

Solutions

1. Find and fix the cycle in the visual/logical tree (inspect Parent chains of elements involved in the route)
2. Avoid adding/removing or reparenting elements inside handlers while the event is routing
3. Reduce hierarchy depth or split the route
4. Debug with a tree dump (e.g. VisualTreeHelper.GetParent walking) before the event is raised

Example fix

// before
child.Parent = parent; parent.Children.Add(child); // possible cycle when child already ancestor
// after
if (!IsAncestorOf(child)) { parent.Children.Add(child); }

// Verify no parent cycle before raising
bool HasCycle(DependencyObject start) { var seen = new HashSet<DependencyObject>(); var cur = start;
  while (cur != null) { if (!seen.Add(cur)) return true; cur = VisualTreeHelper.GetParent(cur) ?? LogicalTreeHelper.GetParent(cur) as DependencyObject; } return false; }

try { element.RaiseEvent(args); }
catch (InvalidOperationException ex) when (ex.Message.Contains("loop") || ex.Message.Contains("TreeLoop")) { log.Error("Possible visual-tree cycle during routing", ex); }

When it happens

Trigger: Raising a routed event whose visual/logical tree path contains a cycle or an abnormally deep chain; reparenting elements during the route so traversal never terminates.

Common situations: Bugs in custom panels/controls that create parent-child loops (A is visual child of B and vice versa); changing the tree inside an event handler; pathological deep element hierarchies.

Understand the failure class

Background: "This is a bug, please report it": internal invariant violations, unreachable panics, and SNH errors explained — this error's family across 47 libraries.

Related errors

• ArgumentOutOfRangeException(routedEvent)
• InvalidEnumArgumentException: routingStrategy
• SR.ClassTypeIllegal
• SR.DuplicateEventName
• SR.Format(SR.ParserBamlEvent, eventIdName)

AI-assisted analysis of dotnet/wpf@81131a70a4 (2026-09-14). Data as JSON: /api/errors/434467973b1f2719. Report an issue: GitHub.

Appendix: source

Thrown at src/Microsoft.DotNet.Wpf/src/PresentationCore/System/Windows/UIElement.cs:1707

                    UIElement uiElement = e as UIElement;
                    ContentElement contentElement = null;
                    UIElement3D uiElement3D = null;

                    if (uiElement == null)
                    {
                        contentElement = e as ContentElement;

                        if (contentElement == null)
                        {
                            uiElement3D = e as UIElement3D;
                        }
                    }

                    // Protect against infinite loops by limiting the number of elements
                    // that we will process.
                    if (cElements++ > MAX_ELEMENTS_IN_ROUTE)
                    {
                        throw new InvalidOperationException(SR.TreeLoop);
                    }

                    // Allow the element to adjust source
                    object newSource = null;
                    if (uiElement != null)
                    {
                        newSource = uiElement.AdjustEventSource(args);
                    }
                    else if (contentElement != null)
                    {
                        newSource = contentElement.AdjustEventSource(args);
                    }
                    else if (uiElement3D != null)
                    {
                        newSource = uiElement3D.AdjustEventSource(args);
                    }

                    // Add changed source information to the route

View on GitHub (pinned to 81131a70a4)