dotnet/wpf · error · XamlInternalException

SR.Format(SR.MissingCase, _mode.ToString(), methodName)

Error message

SR.Format(SR.MissingCase, _mode.ToString(), methodName)

What it means

XamlInternalException with SR.MissingCase thrown from DeferredWriter.WriteObject when the deferring state machine hits a _mode value not covered by its switch. This is a defensive default arm: the DeferringMode enum reached an unhandled state for this write operation.

Solutions

Fix the producer of the node stream so the writer's deferring mode transitions (Off/TemplateStarting/TemplateDeferring/TemplateReady) stay consistent with writes
Check the installed .NET/WPF version — a framework bug may be at fault; apply servicing updates
Reproduce with a minimal node stream and file it/inspect DeferredWriter mode transitions
Catch XamlInternalException at the save boundary and log the full node stream for diagnosis

Defensive patterns

Strategy: try-catch

Try / catch

try { deferringWriter.WriteStartObject(type); }
catch (XamlInternalException ex) { log.Error("Unhandled DeferringMode: {0}", ex.Message); }

Prevention

Keep deferring-mode transitions consistent with the documented lifecycle
Pin and update the .NET/WPF runtime to serviced versions
Never interleave writes from multiple writers/readers on one DeferredWriter
Log full node streams when reproducing internal exceptions

When it happens

Trigger: WriteStartObject or WriteGetObject invoked while DeferredWriter._mode holds a DeferringMode value with no switch case (any state other than Off, TemplateReady, TemplateStarting, TemplateDeferring as handled), indicating the writer's internal mode tracking diverged from the node stream.

Common situations: Newer DeferringMode values introduced by framework updates flowing through an older switch; corrupted or interleaved node streams from custom readers that leave the writer in an unexpected mode; bugs in custom IXamlNodePresenter implementations that toggle deferring modes manually.

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

AI-assisted analysis of dotnet/wpf@81131a70a4 (2026-09-14). Data as JSON: /api/errors/01ad1c62f3e98555. Report an issue: GitHub.

Appendix: source

Thrown at src/Microsoft.DotNet.Wpf/src/System.Xaml/System/Xaml/InfosetObjects/DeferredWriter.cs:103

                    _mode = DeferringMode.TemplateDeferring;
                    goto case DeferringMode.TemplateDeferring;

                case DeferringMode.TemplateDeferring:
                    if (fromMember)
                    {
                        _deferredWriter.WriteGetObject();
                    }
                    else
                    {
                        _deferredWriter.WriteStartObject(xamlType);
                    }

                    _deferredTreeDepth += 1;
                    _handled = true;
                    break;

                default:
                    throw new XamlInternalException(SR.Format(SR.MissingCase, _mode.ToString(), methodName));
            }
        }

        public override void WriteEndObject()
        {
            _handled = false;
            switch (_mode)
            {
            case DeferringMode.Off:
                break;

            case DeferringMode.TemplateReady:
                throw new XamlInternalException(SR.Format(SR.TemplateNotCollected, "WriteEndObject"));

            case DeferringMode.TemplateDeferring:
                _deferredWriter.WriteEndObject();
                _handled = true;
                _deferredTreeDepth -= 1;

View on GitHub (pinned to 81131a70a4)