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

Do not call Reset on EF Core Cosmos query enumerators; re-execute the query instead.

dotnet/efcore · error · NotSupportedException

This enumerator cannot be reset.

Error message

This enumerator cannot be reset.

What it means

Thrown by Reset() on the ReadItem query enumerator. EF Core's Cosmos enumerators are forward-only, single-pass async enumerators that consume a network response stream; resetting and re-reading is not supported, so Reset throws NotSupportedException.

Source

Thrown at src/EFCore.Cosmos/Query/Internal/CosmosShapedQueryCompilingExpressionVisitor.ReadItemQueryingEnumerable.cs:186

                    else
                    {
                        _queryLogger.QueryIterationFailed(_contextType, exception);
                    }

                    throw;
                }
            }

            public ValueTask DisposeAsync()
            {
                _response = null;
                _hasExecuted = false;

                return default;
            }

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

            private bool ShapeResult()
            {
                var hasNext = _response is not null;

                _cosmosQueryContext.InitializeStateManager(_standAloneStateManager);

                Current
                    = hasNext
                        ? _shaper(_cosmosQueryContext, _response.Value, ordinal: 0, out _)
                        : default;

                _hasExecuted = true;

                return hasNext;
            }
        }
    }

View on GitHub (pinned to dbf9771522)

Solutions

Do not call Reset on EF Core Cosmos query enumerators; re-execute the query instead.
Cache the materialized result (e.g. ToListAsync) if you need to iterate multiple times.
Avoid enumerator-sharing patterns; consume the async enumerator once.

Example fix

// before
using var e = query.GetAsyncEnumerator();
while (e.MoveNextAsync().Result) { /* ... */ }
e.Reset();
// after
var list = await query.ToListAsync();
foreach (var item in list) { /* iterate as many times as needed */ }

Defensive patterns

Strategy: validation

Validate before calling

// Never reset; materialize once if multiple iterations are needed
var list = await query.ToListAsync();
// iterate list freely; do not call Reset on any enumerator

Prevention

Never call Reset on EF Core query enumerators.
Materialize with ToListAsync when you need multiple passes.
Avoid sharing enumerators across consumers.

When it happens

Trigger: Calling .Reset() on an IEnumerator obtained from a Cosmos query result. Typically indirect, via a library or LINQ operator that materializes enumerators and calls Reset.

Common situations: Interoperating with code that assumes resettable enumerators (rare). Custom wrappers around EF query results that call Reset. Misuse of enumerator semantics.

Related errors

AI-assisted analysis of dotnet/efcore@dbf9771522 (2026-08-06). Data as JSON: /api/errors/f423f75ea04464ac. Report an issue: GitHub.