dotnet/wpf · error · XamlInternalException

SR.Format(SR.TemplateNotCollected, "WriteMember")

Error message

SR.Format(SR.TemplateNotCollected, "WriteMember")

What it means

XamlInternalException with SR.TemplateNotCollected thrown from DeferredWriter.WriteStartMember when the writer is in DeferringMode.TemplateReady. Starting a member in TemplateReady state means the template content was never collected through the TemplateStarting -> StartDeferredList transition, so member writes are invalid.

Solutions

  1. Emit the member writes only after the writer entered TemplateDeferring (i.e. after StartDeferredList ran)
  2. Drive serialization through XamlDeferringReader/XamlDeferringWriter rather than calling WriteStartMember manually in TemplateReady
  3. Reset the writer (new DeferredWriter) so _mode is Off for non-template content
  4. Add a pre-write assertion/check of the deferring mode in custom writer code

Example fix

// before
writer.WriteStartMember(member); // _mode == TemplateReady -> TemplateNotCollected
// after
// let the template-start path run StartDeferredList first (TemplateStarting -> TemplateDeferring)
writer.WriteStartMember(member); // now valid inside deferred region
Defensive patterns

Strategy: try-catch

Try / catch

try { deferringWriter.WriteStartMember(member); }
catch (XamlInternalException ex) { log.Error("WriteStartMember in TemplateReady state: {0}", ex.Message); }

Prevention

When it happens

Trigger: WriteStartMember called while _mode == TemplateReady — for instance the template-start node (x:ClassModifier-style template content start) was consumed into TemplateReady but the next write was a plain member write instead of entering the deferred list.

Common situations: Custom node-stream rewriters that drop or reorder the node that triggers StartDeferredList; writing template content with a raw writer API without going through the deferring reader's template flow; test harnesses emitting synthetic node streams around templates.

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/05bc0a0d69779d00. Report an issue: GitHub.

Appendix: source

Thrown at src/Microsoft.DotNet.Wpf/src/System.Xaml/System/Xaml/InfosetObjects/DeferredWriter.cs:154

        public override void WriteStartMember(XamlMember property)
        {
            _handled = false;
            switch (_mode)
            {
            case DeferringMode.Off:
                if (property.DeferringLoader is not null)
                {
                    _mode = DeferringMode.TemplateStarting;

                    // We assume in WriteValue that this property can never be multi-valued
                    Debug.Assert(!property.IsDirective && !property.IsUnknown);
                }

                break;

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

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

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

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

View on GitHub (pinned to 81131a70a4)