dotnet/wpf · error · InvalidOperationException

SR.Enumerator_NotStarted

Error message

SR.Enumerator_NotStarted

What it means

Calling Current on the MatchingLanguageEnumerator returned by XmlLanguage.GetMatchingLanguages().GetEnumerator() before MoveNext() was ever called. The WPF enumerator contract requires MoveNext() first; the library throws InvalidOperationException to signal the cursor is still at its initial position.

Solutions

  1. Call MoveNext() once and check its return value before reading Current
  2. Use foreach (XmlLanguage lang in xmlLanguage.GetMatchingLanguages()) so the compiler-generated loop handles ordering
  3. Guard with a boolean tracking whether MoveNext succeeded

Example fix

// before
var e = xmlLanguage.GetMatchingLanguages().GetEnumerator();
var first = e.Current; // throws
// after
var e = xmlLanguage.GetMatchingLanguages().GetEnumerator();
if (e.MoveNext()) { var first = e.Current; }
Defensive patterns

Strategy: try-catch

Validate before calling

var e = xmlLanguage.GetMatchingLanguages().GetEnumerator();
bool started = e.MoveNext();
if (!started) return; // empty: skip Current access

Try / catch

try { var first = e.Current; } catch (InvalidOperationException) { /* not started */ }

Prevention

When it happens

Trigger: Getting the enumerator from GetMatchingLanguages() and reading Current immediately without calling MoveNext(); using a cached enumerator after reset; manual iteration misuse that touches Current first.

Common situations: Hand-written loops over the language fallback list instead of foreach; porting code that assumed Current returns the first element before iteration starts.

Understand the failure class

Background: "Invalid state transition" errors: "status must be X, actually Y", "already rejected/charging/uninstalled", "cannot ... while running" — what they mean when a library rejects your call — this error's family across 31 libraries.

Related errors

AI-assisted analysis of dotnet/wpf@81131a70a4 (2026-09-14). Data as JSON: /api/errors/e4c57557a56bbc85. Report an issue: GitHub.

Appendix: source

Thrown at src/Microsoft.DotNet.Wpf/src/PresentationCore/System/Windows/Markup/XmlLanguage.cs:519

                _atStart = true;
                _maxCultureDepth = XmlLanguage.MaxCultureDepth;
            }

            public void Reset()
            {
                _current = _start;
                _pastEnd = false;
                _atStart = true;
                _maxCultureDepth = XmlLanguage.MaxCultureDepth;
            }

            public XmlLanguage Current
            {
                get
                {
                    if (_atStart)
                    {
                        throw new InvalidOperationException(SR.Enumerator_NotStarted);
                    }
                    if (_pastEnd)
                    {
                        throw new InvalidOperationException(SR.Enumerator_ReachedEnd);
                    }
 
                    return _current;
                }
            }

            public bool MoveNext()
            {
                if (_atStart)
                {
                    _atStart = false;
                    return true;
                }
                else if (_current.IetfLanguageTag.Length == 0)

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