dotnet/wpf · error · InvalidOperationException

SR.Enumerator_NotStarted

Error message

SR.Enumerator_NotStarted

What it means

The enumerator's Current property tracks position via _index: -1 means MoveNext() has never been called, so no current element exists and it throws InvalidOperationException(SR.Enumerator_NotStarted). .NET enumerators must be advanced before Current is valid; MaterialCollection implements this fail-fast behavior explicitly.

Solutions

  1. Call MoveNext() before reading Current: if (e.MoveNext()) { var m = e.Current; }
  2. Prefer foreach, which handles MoveNext/Current sequencing correctly.
  3. If you need the first element without an enumerator, use materials[0] after checking Count > 0.

Example fix

// before
var e = materials.GetEnumerator();
var first = e.Current;
// after
var e = materials.GetEnumerator();
var first = e.MoveNext() ? e.Current : null;
Defensive patterns

Strategy: type-guard

Validate before calling

bool hasCurrent = enumerator.MoveNext(); // must be true before reading Current

Type guard

static bool TryPeekFirst<T>(IEnumerator<T> e, out T current) { if (e.MoveNext()) { current = e.Current; return true; } current = default; return false; }

Try / catch

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

Prevention

When it happens

Trigger: Reading enumerator.Current (or calling Current manually on IEnumerator) immediately after GetEnumerator() without a prior MoveNext(), e.g. var e = materials.GetEnumerator(); var m = e.Current;

Common situations: Hand-rolled iteration code that skips the initial MoveNext; code translated from languages where iterators point at the first element before advancing; peeking at the 'first' item directly.

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/9d3850f0469db852. Report an issue: GitHub.

Appendix: source

Thrown at src/Microsoft.DotNet.Wpf/src/PresentationCore/System/Windows/Media3D/Generated/MaterialCollection.cs:882

            /// <summary>
            /// Current element
            ///
            /// The behavior of IEnumerable&lt;T>.Current is undefined
            /// before the first MoveNext and after we have walked
            /// off the end of the list. However, the IEnumerable.Current
            /// contract requires that we throw exceptions
            /// </summary>
            public Material Current
            {
                get
                {
                    if (_index > -1)
                    {
                        return _current;
                    }
                    else if (_index == -1)
                    {
                        throw new InvalidOperationException(SR.Enumerator_NotStarted);
                    }
                    else
                    {
                        Debug.Assert(_index == -2, "expected -2, got " + _index + "\n");
                        throw new InvalidOperationException(SR.Enumerator_ReachedEnd);
                    }
                }
            }

            #endregion

            #region Data
            private Material _current;
            private MaterialCollection _list;
            private uint _version;
            private int _index;
            #endregion
        }

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