dotnet/wpf · error · InvalidOperationException

SR.Enumerator_ReachedEnd

Error message

SR.Enumerator_ReachedEnd

What it means

FreezableCollection<T>.Enumerator.Current throws InvalidOperationException(SR.Enumerator_ReachedEnd) when accessed after enumeration has completed (_index == -2, i.e. MoveNext returned false and iteration went past the last element). Once exhausted, the enumerator has no current element.

Solutions

  1. Only read Current when MoveNext() returned true; capture the last item inside the loop before it ends.
  2. Remember the last valid item in a local during iteration if you need it after the loop.
  3. Create a fresh enumerator to iterate again instead of reading Current from the exhausted one.

Example fix

// before
while (e.MoveNext()) { }
var last = e.Current; // throws
// after
T last = default;
while (e.MoveNext()) { last = e.Current; }
Defensive patterns

Strategy: validation

Validate before calling

if (!e.MoveNext()) return; // empty or exhausted: do not read Current
var item = e.Current;

Type guard

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

Try / catch

try { var cur = e.Current; }
catch (InvalidOperationException) { /* enumeration already finished */ }

Prevention

When it happens

Trigger: Reading Current after MoveNext() returned false — e.g. a do/while loop that reads Current after the terminating MoveNext, or accessing Current after a fully consumed foreach using the same enumerator object.

Common situations: Loops structured as 'while(true) { advanced = MoveNext(); var item = Current; ... }' where Current is read even when advanced is false, reusing an exhausted enumerator to 'get the last item'.

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/40cd3356fe6b3a92. Report an issue: GitHub.

Appendix: source

Thrown at src/Microsoft.DotNet.Wpf/src/PresentationCore/System/Windows/FreezableCollection.cs:1079

            /// off the end of the list. However, the IEnumerable.Current
            /// contract requires that we throw exceptions
            /// </summary>
            public T 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 T _current;
            private FreezableCollection<T> _list;
            private uint _version;
            private int _index;
            #endregion
        }

        private class SimpleMonitor : IDisposable
        {
            public void Enter()
            {

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