dotnet/wpf · error · XamlInternalException

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

Error message

SR.Format(SR.MissingCase, _mode.ToString(), "WriteEndMember")

What it means

XamlInternalException with SR.MissingCase thrown from DeferredWriter.WriteEndMember when the deferring mode matches no switch case. The default arm signals the writer's DeferringMode state machine has diverged into an unhandled state during a member-end write.

Solutions

Fix the producer so mode transitions follow the Off/TemplateStarting/TemplateDeferring/TemplateReady lifecycle exactly
Recreate the DeferredWriter after any failure instead of continuing to write
Apply the latest .NET/WPF servicing updates to rule out a runtime bug
Catch XamlInternalException at the serialization boundary; the message names the unexpected _mode for diagnosis

Defensive patterns

Strategy: try-catch

Try / catch

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

Prevention

Follow the documented Off/TemplateStarting/TemplateDeferring/TemplateReady lifecycle
Never reuse a DeferredWriter after a failure — create a new one
Use single-threaded access to deferring writers
Update the .NET/WPF runtime to rule out known defects

When it happens

Trigger: WriteEndMember invoked while _mode is a DeferringMode value absent from the switch — the writer's internal mode tracking reached an untracked state, so the end-member event cannot be routed.

Common situations: Node streams from custom readers that leave the mode machine mid-transition after errors; concurrency or writer reuse issues corrupting _mode; framework updates introducing states not handled on this code path.

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/c5f6c3712ba684b1. Report an issue: GitHub.

Appendix: source

Thrown at src/Microsoft.DotNet.Wpf/src/System.Xaml/System/Xaml/InfosetObjects/DeferredWriter.cs:183

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

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

            case DeferringMode.TemplateDeferring:
                _deferredWriter.WriteEndMember();
                _handled = true;
                break;

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

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

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

            case DeferringMode.TemplateStarting:
                // This handles the case of SM template; V object; EM
                Debug.Assert(_deferredTreeDepth == 0);
                if (value is XamlNodeList)

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