dotnet/efcore · error · NotSupportedException

This enumerator cannot be reset.

Error message

This enumerator cannot be reset.

What it means

Thrown from Enumerator.Reset() on the FromSql query enumerable (FromSqlQueryingEnumerable.cs:296). EF's relational query enumerators wrap a forward-only DbDataReader that streams results from the database; once consumed, the reader cannot be rewound. IEnumerator.Reset is therefore explicitly unsupported and throws NotSupportedException. This is by design: EF enumerables are single-pass and do not buffer the result set.

Source

Thrown at src/EFCore.Relational/Query/Internal/FromSqlQueryingEnumerable.cs:296

            enumerator._indexMap = BuildIndexMap(enumerator._columnNames, enumerator._dataReader.DbDataReader);

            enumerator._relationalQueryContext.InitializeStateManager(enumerator._standAloneStateManager);

            return false;
        }

        public void Dispose()
        {
            if (_dataReader is not null)
            {
                _relationalQueryContext.Connection.ReturnCommand(_relationalCommand!);
                _dataReader?.Dispose();
                _dataReader = 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 IReadOnlyList<string> _columnNames;
        private readonly Func<QueryContext, DbDataReader, int[], T> _shaper;
        private readonly Type _contextType;
        private readonly IDiagnosticsLogger<DbLoggerCategory.Query> _queryLogger;
        private readonly bool _standAloneStateManager;
        private readonly bool _detailedErrorsEnabled;
        private readonly IConcurrencyDetector? _concurrencyDetector;
        private readonly IExceptionDetector _exceptionDetector;

        private IRelationalCommand? _relationalCommand;
        private RelationalDataReader? _dataReader;

View on GitHub (pinned to dbf9771522)

Solutions

1. Do not call Reset(); instead, materialize results once with ToList()/ToListAsync() and re-iterate the in-memory list.
2. Re-execute the query from the IQueryable to get a fresh enumerator (this re-issues the SQL).
3. If you need multiple passes, call AsEnumerable() (or AsAsyncEnumerable()) after ToList to move to an in-memory sequence that supports Reset.
4. Replace manual enumerator usage with foreach / await foreach.

Example fix

// before - calling Reset on an EF enumerator
var enumerator = context.Blogs.FromSqlRaw("SELECT * FROM Blogs").GetEnumerator();
enumerator.MoveNext();
enumerator.Reset(); // throws NotSupportedException

// after - buffer once, iterate the list freely
var blogs = context.Blogs.FromSqlRaw("SELECT * FROM Blogs").ToList();
var enumerator = blogs.GetEnumerator(); // List<T>.Enumerator supports Reset-free iteration
// or just foreach over the list as many times as needed

// Never call Reset() on an EF enumerable. Materialize once and iterate the list.
var materialized = ctx.Set<T>().FromSqlRaw(sql).ToList(); // List<T> is freely re-iterable

// Detect resettable vs single-pass enumerators before calling Reset.
static bool IsResettable<T>(IEnumerable<T> source) => source is IList<T> or Array or ICollection<T>;
// Usage: if (IsResettable(seq)) e.Reset(); else re-enumerate.

try { e.Reset(); }
catch (NotSupportedException) {
    // EF query enumerators cannot be reset; materialize and reuse instead.
    var list = query.ToList();
}

When it happens

Trigger: Code that calls .Reset() on an IEnumerator obtained from an EF query (e.g. via a helper, a LINQ operator implementation, or a library that assumes Reset works). Also triggered by enumerating the same query twice via the same enumerator instance, or by frameworks that call Reset during custom aggregation/caching.

Common situations: Porting code that used LINQ-to-Objects (where Reset is supported) to EF; third-party libraries that buffer/re-iterate enumerators; misuse of GetAsyncEnumerator()/GetEnumerator() directly instead of re-running the query.

Related errors

• This enumerator cannot be reset.
• This enumerator cannot be reset.
• EnumerableResetNotSupported
• This enumerator cannot be reset.
• This enumerator cannot be reset.

AI-assisted analysis of dotnet/efcore@dbf9771522 (2026-08-06). Data as JSON: /api/errors/97b6c82616eeb412. Report an issue: GitHub.