What causes "This enumerator cannot be reset." in dotnet/efcore?

Buffer with ToList()/ToListAsync() and iterate the in-memory list.

dotnet/efcore · error · NotSupportedException

EnumerableResetNotSupported

EnumerableResetNotSupported

Error message

This enumerator cannot be reset.

What it means

Thrown from Enumerator.Reset() on SplitQueryingEnumerable (SplitQueryingEnumerable.cs:304), the enumerator behind an AsSplitQuery(). Reset is unsupported because the enumerator drives a forward-only parent reader plus coordinated child readers that cannot be rewound. Code 'EnumerableResetNotSupported' is attached to this specific site.

Source

Thrown at src/EFCore.Relational/Query/Internal/SplitQueryingEnumerable.cs:304

                if (_resultCoordinator != null)
                {
                    foreach (var dataReader in _resultCoordinator.DataReaders)
                    {
                        dataReader?.DataReader.Dispose();
                    }

                    _resultCoordinator.DataReaders.Clear();

                    _resultCoordinator = null;
                }

                _dataReader = null;
                _dbDataReader = null;
            }
        }

        public void Reset()
            => throw new NotSupportedException(CoreStrings.EnumerableResetNotSupported);
    }

    private sealed class AsyncEnumerator : IAsyncEnumerator<T>
    {
        private readonly RelationalQueryContext _relationalQueryContext;
        private readonly RelationalCommandResolver _relationalCommandResolver;
        private readonly IReadOnlyList<ReaderColumn?>? _readerColumns;
        private readonly Func<QueryContext, DbDataReader, ResultContext, SplitQueryResultCoordinator, T> _shaper;
        private readonly Func<QueryContext, IExecutionStrategy, SplitQueryResultCoordinator, Task>? _relatedDataLoaders;
        private readonly Type _contextType;
        private readonly IDiagnosticsLogger<DbLoggerCategory.Query> _queryLogger;
        private readonly bool _standAloneStateManager;
        private readonly bool _detailedErrorEnabled;
        private readonly IConcurrencyDetector? _concurrencyDetector;
        private readonly IExceptionDetector _exceptionDetector;
        private readonly CancellationToken _cancellationToken;

        private IRelationalCommand? _relationalCommand;

View on GitHub (pinned to dbf9771522)

Solutions

Buffer with ToList()/ToListAsync() and iterate the in-memory list.
Re-execute the IQueryable to obtain a fresh enumerator.
Avoid Reset; use foreach over the materialized results.
Feed reset-assuming components the buffered list, not the live split enumerable.

Example fix

// before
using var e = ctx.Orders.AsSplitQuery().Include(o => o.Items).GetEnumerator();
e.MoveNext();
e.Reset(); // throws EnumerableResetNotSupported

// after
var orders = ctx.Orders.AsSplitQuery().Include(o => o.Items).ToList();
foreach (var o in orders) { /* safe */ }

Defensive patterns

Strategy: validation

Validate before calling

var orders = ctx.Orders.AsSplitQuery().Include(o => o.Items).ToList(); // buffer for reuse

Type guard

static bool IsResettable<T>(IEnumerable<T> s) => s is IList<T> or Array;

Try / catch

try { e.Reset(); }
catch (NotSupportedException) { var buffered = query.ToList(); }

Prevention

Materialize split-query results before re-iterating.
Avoid Reset() on EF enumerators; use foreach over a buffered list.
Pass buffered lists to reset-assuming components.

When it happens

Trigger: Calling IEnumerator.Reset() on an enumerator from an AsSplitQuery() query. Triggered by code or libraries that re-iterate via Reset, or by manually holding and reusing an enumerator from a split query.

Common situations: Custom helpers applied to split-query enumerables; ported LINQ-to-Objects code; libraries that buffer via Reset.

Related errors

AI-assisted analysis of dotnet/efcore@dbf9771522 (2026-08-06). Data as JSON: /api/errors/b2ada8ff3d59fb25. Report an issue: GitHub.