dotnet/reactive · error · ArgumentNullException

Value cannot be null. (Parameter 'asyncLock')

Error message

Value cannot be null. (Parameter 'asyncLock')

What it means

Observer.Synchronize<T>(IObserver<T>, AsyncLock) throws ArgumentNullException when the asyncLock parameter is null. The AsyncLock provides the mutual exclusion for notifications delivered to the observer; without it the wrapper cannot function, so Rx validates it upfront.

Solutions

1. Create and pass an AsyncLock instance: new AsyncLock()
2. Ensure the AsyncLock field is initialized before use (readonly field initialized inline)
3. Do not null out the AsyncLock on dispose if it will be referenced again; keep a fresh instance per observer chain
4. Verify you are targeting the correct overload (object gate vs AsyncLock) and supply the matching type

Example fix

// before
AsyncLock gate = _lock; // null after dispose
var synced = Observer.Synchronize(observer, gate);
// after
using var asyncLock = new AsyncLock();
var synced = Observer.Synchronize(observer, asyncLock);

if (observer is null) throw new ArgumentNullException(nameof(observer));
if (asyncLock is null) asyncLock = new AsyncLock();

static bool HasAsyncLock(AsyncLock? l) => l is not null;

try { var synced = Observer.Synchronize(observer, asyncLock); }
catch (ArgumentNullException ex) when (ex.ParamName == "asyncLock")
{
    asyncLock = new AsyncLock();
    var synced = Observer.Synchronize(observer, asyncLock);
}

When it happens

Trigger: Calling Observer.Synchronize(observer, null) — commonly when the AsyncLock is created lazily, disposed and nulled earlier, or accidentally passing null in place of a gate/lock object.

Common situations: AsyncLock disposed with the owning class and nulled, then reused; a factory method returning null; copy-paste between the object-gate and AsyncLock overloads.

Related errors

• predicate
• Value cannot be null. (Parameter 'source5')
• Value cannot be null. (Parameter 'gate')
• Value cannot be null. (Parameter 'source')
• Value cannot be null. (Parameter 'scheduler')

AI-assisted analysis of dotnet/reactive@94b5d5ab91 (2026-09-15). Data as JSON: /api/errors/43cef115fd7b1d2c. Report an issue: GitHub.

Appendix: source

Thrown at Rx.NET/Source/src/System.Reactive/Observer.Extensions.cs:281

        /// <summary>
        /// Synchronizes access to the observer such that its callback methods cannot be called concurrently, using the specified asynchronous lock to protect against concurrent and reentrant access.
        /// This overload is useful when coordinating multiple observers that access shared state by synchronizing on a common asynchronous lock.
        /// </summary>
        /// <typeparam name="T">The type of the elements received by the source observer.</typeparam>
        /// <param name="observer">The observer whose callbacks should be synchronized.</param>
        /// <param name="asyncLock">Gate object to synchronize each observer call on.</param>
        /// <returns>An observer that delivers callbacks to the specified observer in a synchronized manner.</returns>
        /// <exception cref="ArgumentNullException"><paramref name="observer"/> or <paramref name="asyncLock"/> is null.</exception>
        public static IObserver<T> Synchronize<T>(IObserver<T> observer, AsyncLock asyncLock)
        {
            if (observer == null)
            {
                throw new ArgumentNullException(nameof(observer));
            }

            if (asyncLock == null)
            {
                throw new ArgumentNullException(nameof(asyncLock));
            }

            return new AsyncLockObserver<T>(observer, asyncLock);
        }

        /// <summary>
        /// Schedules the invocation of observer methods on the given scheduler.
        /// </summary>
        /// <typeparam name="T">The type of the elements received by the source observer.</typeparam>
        /// <param name="observer">The observer to schedule messages for.</param>
        /// <param name="scheduler">Scheduler to schedule observer messages on.</param>
        /// <returns>Observer whose messages are scheduled on the given scheduler.</returns>
        /// <exception cref="ArgumentNullException"><paramref name="observer"/> or <paramref name="scheduler"/> is null.</exception>
        public static IObserver<T> NotifyOn<T>(this IObserver<T> observer, IScheduler scheduler)
        {
            if (observer == null)
            {
                throw new ArgumentNullException(nameof(observer));

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