dotnet/machinelearning · error · ArgumentNullException

engine (Parameter 'engine')

Error message

engine (Parameter 'engine')

What it means

ArgumentNullException in PoolLoader<TData,TPrediction>.Return: the engine parameter is null. Return must put a previously rented PredictionEngine back into its originating pool (or dispose it if that generation was hot-swapped), so a null reference cannot be identified with any rental and is rejected immediately. The message names 'engine' as the faulty parameter.

Source

Thrown at src/Microsoft.Extensions.ML/PoolLoader.cs:83

                throw new ObjectDisposedException(nameof(PoolLoader<TData, TPrediction>));
            }

            var engine = pool.Get();

            _rentedEngines.Remove(engine);
            _rentedEngines.Add(engine, pool);
            return engine;
        }

        /// <summary>
        /// Returns an engine to the generation it was rented from. If that generation has already
        /// been disposed by a hot-swap, the pool disposes the engine instead of retaining it.
        /// </summary>
        public void Return(PredictionEngine<TData, TPrediction> engine)
        {
            if (engine == null)
            {
                throw new ArgumentNullException(nameof(engine));
            }

            if (_rentedEngines.TryGetValue(engine, out var origin))
            {
                _rentedEngines.Remove(engine);
                origin.Return(engine);
            }
            else
            {
                engine.Dispose();
            }
        }

        private void LoadPool()
        {
            if (Volatile.Read(ref _disposed) != 0)
            {
                return;

View on GitHub (pinned to 7b76e69cf9)

Solutions

1. Guard for null before calling Return
2. Only return engines that were successfully rented from Get()

Example fix

// before
pool.ReturnPredictionEngine(engine); // engine may be null
// after
if (engine != null) pool.ReturnPredictionEngine(engine);

if (engine is null) throw new InvalidOperationException("No engine to return");

bool CanReturn(PredictionEngine<TData,TPred> e) => e != null;

null // prevent by null-checking before Return; no need to catch ArgumentNullException

When it happens

Trigger: Calling ReturnPredictionEngine / PoolLoader.Return(null), typically when a variable was never assigned or a Get() failed earlier.

Common situations: Code paths where engine creation failed but Return is still called in a finally block without a null check.

Related errors

• engine (Parameter 'engine')
• Value cannot be null. (Parameter 'idColumns')
• ArgumentNullException(nameof(column))
• null
• value

AI-assisted analysis of dotnet/machinelearning@7b76e69cf9 (2026-09-11). Data as JSON: /api/errors/a01e0525ca06126f. Report an issue: GitHub.