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

Do not call Reset(); re-execute the LINQ query for a fresh enumeration.

dotnet/efcore · error · NotSupportedException

This enumerator cannot be reset.

Error message

This enumerator cannot be reset.

What it means

Thrown when calling Reset() on the IEnumerator<T> from a SingleQueryingEnumerable — the standard query enumerable for non-split relational queries. The enumerator wraps a single DbDataReader that is consumed once and cannot be rewound, so Reset() throws NotSupportedException as permitted by the IEnumerator contract.

Solutions

Do not call Reset(); re-execute the LINQ query for a fresh enumeration.
Materialize with ToList()/ToListAsync() if multiple passes are needed.
Report the Reset() call to the library or framework that makes it.

Example fix

// before
using var e = context.Blogs.Where(b => b.Active).GetEnumerator();
e.Reset(); // throws

// after
var blogs = context.Blogs.Where(b => b.Active).ToList();

Defensive patterns

Strategy: validation

Prevention

Never call Reset() on EF Core query enumerators — re-execute the query for fresh results.
Materialize results with ToList()/ToListAsync() before passing to libraries or data-binding frameworks.
Document that EF enumerators are forward-only in your codebase.

When it happens

Trigger: Calling .Reset() on the enumerator obtained from any standard LINQ-to-Entities query's GetEnumerator() (single-query mode). This is usually triggered by library code, not direct application code.

Common situations: A collection adapter, testing mock framework, or UI data binding component that calls Reset() on enumerators. Attempting to enumerate a query result twice with the same enumerator handle.

Related errors

AI-assisted analysis of dotnet/efcore@3a2006ef56 (2026-08-11). Data as JSON: /api/errors/bf62064123587e5d. Report an issue: GitHub.

Appendix: source

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

            enumerator._relationalQueryContext.InitializeStateManager(enumerator._standAloneStateManager);

            return false;
        }

        public void Dispose()
        {
            if (_dataReader is not null)
            {
                _relationalQueryContext.Connection.ReturnCommand(_relationalCommand!);
                _dataReader.Dispose();
                _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, SingleQueryResultCoordinator, 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 readonly CancellationToken _cancellationToken;

        private IRelationalCommand? _relationalCommand;
        private RelationalDataReader? _dataReader;

View on GitHub (pinned to 3a2006ef56)