What causes "SR.Enumerator_NotStarted" in dotnet/wpf?

Call MoveNext() at least once and check its return value before reading Current

dotnet/wpf · error · InvalidOperationException

SR.Enumerator_NotStarted

Error message

SR.Enumerator_NotStarted

What it means

The VisualCollection Enumerator's Current property throws InvalidOperationException(SR.Enumerator_NotStarted) when MoveNext has never been called before accessing Current. The enumerator's _index is still -1, so there is no current element to return.

Solutions

Call MoveNext() at least once and check its return value before reading Current
Prefer foreach over manual enumerator usage, which enforces this ordering automatically
Restructure manual loops to follow the standard while (enumerator.MoveNext()) { use Current } pattern

Example fix

// before
var e = visualCollection.GetEnumerator();
var first = e.Current; // throws: not started
// after
var e = visualCollection.GetEnumerator();
if (e.MoveNext()) { var first = e.Current; }

Defensive patterns

Strategy: try-catch

Validate before calling

// no pre-call validation possible; use foreach or check via enumerator state

Try / catch

try { var cur = e.Current; }
catch (InvalidOperationException) { /* MoveNext not yet called; call it first */ }

Prevention

Prefer foreach, which never reads Current before MoveNext
In manual loops, always branch on MoveNext()'s return before Current
Do not read Current outside the iteration body

When it happens

Trigger: Creating an enumerator via VisualCollection.GetEnumerator() and reading Current (or the foreach iteration variable pattern manually) before the first MoveNext() call.

Common situations: Hand-written enumerator loops that read Current in an initialization step; custom code that stores enumerator.Current outside a foreach; foreach is safe — only manual enumerator use hits this.

Related errors

AI-assisted analysis of dotnet/wpf@81131a70a4 (2026-09-14). Data as JSON: /api/errors/b13fc7248c5a20e9. Report an issue: GitHub.

Appendix: source

Thrown at src/Microsoft.DotNet.Wpf/src/PresentationCore/System/Windows/Media/VisualCollection.cs:947

                get
                {
                    return this.Current;
                }
            }

            /// <summary>
            /// Gets the current Visual.
            /// </summary>
            public Visual Current
            {
                get
                {
                    if (_index < 0)
                    {
                        if (_index == -1)
                        {
                            // Not started.
                            throw new InvalidOperationException(SR.Enumerator_NotStarted);
                        }
                        else
                        {
                            // Reached the end.
                            Debug.Assert(_index == -2);
                            throw new InvalidOperationException(SR.Enumerator_ReachedEnd);
                        }
                    }

                    return _currentElement;
                }
            }


            /// <summary>
            /// Sets the enumerator to its initial position, which is before the first element in the collection.
            /// </summary>
            public void Reset()

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