dotnet/reactive · error · ArgumentNullException

Value cannot be null. (Parameter 'onErrorAsync')

Error message

Value cannot be null. (Parameter 'onErrorAsync')

What it means

UsingAsync is the ValueTask-based variant of Using for asynchronous resource acquisition. It guards the async resource factory eagerly; a null Func<ValueTask<TResource>> cannot be awaited at subscription time, so ArgumentNullException(nameof(resourceFactory)) is thrown at composition (Using.cs:63).

Solutions

1. Pass a non-null Func<ValueTask<TResource>>, e.g. async () => await ConnectAsync()
2. Wrap sync creation as () => new ValueTask<TResource>(Create())
3. Null-check async factory delegates resolved at runtime before composing

Example fix

// before
Func<ValueTask<DbConnection>> factory = useAsync ? null : MakeFactory();
var op = AsyncObservable.UsingAsync(factory, conn => QueryAsync(conn));
// after
var op = AsyncObservable.UsingAsync(async () => await ConnectAsync(), conn => QueryAsync(conn));

if (resourceFactory is null) throw new ArgumentNullException(nameof(resourceFactory));
var op = AsyncObservable.UsingAsync(resourceFactory, observableFactory);

static bool ValidUsingAsyncArgs<TResource,TResult>(Func<ValueTask<TResource>> rf, Func<TResource,ValueTask<IAsyncObservable<TResult>>> of) => rf != null && of != null;

try
{
    var op = AsyncObservable.UsingAsync(resourceFactory, observableFactory);
}
catch (ArgumentNullException ex) when (ex.ParamName == "resourceFactory")
{
    // fall back to sync creation: () => new ValueTask<TResource>(CreateSync())
}

When it happens

Trigger: Calling AsyncObservable.UsingAsync<TResult, TResource>(null, observableFactory) with TResource : IDisposable.

Common situations: Async factory supplied only when a connection string/config is present; migration from sync Using to UsingAsync leaving one argument null; factory retrieved from an optional service provider.

Related errors

• Value cannot be null. (Parameter 'onNextAsync')
• Value cannot be null. (Parameter 'onErrorAsync')
• Value cannot be null. (Parameter 'onCompletedAsync')
• null (Parameter 'scheduler')
• null (Parameter 'values')

AI-assisted analysis of dotnet/reactive@94b5d5ab91 (2026-09-15). Data as JSON: /api/errors/3741a2592a617abd. Report an issue: GitHub.

Appendix: source

Thrown at AsyncRx.NET/System.Reactive.Async/AsyncObservableExtensions.cs:29

    {
        public static ValueTask<IAsyncDisposable> SubscribeAsync<T>(this IAsyncObservable<T> source, Func<T, ValueTask> onNextAsync)
        {
            if (source == null)
                throw new ArgumentNullException(nameof(source));
            if (onNextAsync == null)
                throw new ArgumentNullException(nameof(onNextAsync));

            return source.SubscribeAsync(new AsyncObserver<T>(onNextAsync, ex => new ValueTask(Task.FromException(ex)), () => default));
        }

        public static ValueTask<IAsyncDisposable> SubscribeAsync<T>(this IAsyncObservable<T> source, Func<T, ValueTask> onNextAsync, Func<Exception, ValueTask> onErrorAsync)
        {
            if (source == null)
                throw new ArgumentNullException(nameof(source));
            if (onNextAsync == null)
                throw new ArgumentNullException(nameof(onNextAsync));
            if (onErrorAsync == null)
                throw new ArgumentNullException(nameof(onErrorAsync));

            return source.SubscribeAsync(new AsyncObserver<T>(onNextAsync, onErrorAsync, () => default));
        }

        public static ValueTask<IAsyncDisposable> SubscribeAsync<T>(this IAsyncObservable<T> source, Func<T, ValueTask> onNextAsync, Func<ValueTask> onCompletedAsync)
        {
            if (source == null)
                throw new ArgumentNullException(nameof(source));
            if (onNextAsync == null)
                throw new ArgumentNullException(nameof(onNextAsync));
            if (onCompletedAsync == null)
                throw new ArgumentNullException(nameof(onCompletedAsync));

            return source.SubscribeAsync(new AsyncObserver<T>(onNextAsync, ex => new ValueTask(Task.FromException(ex)), onCompletedAsync));
        }

        public static ValueTask<IAsyncDisposable> SubscribeAsync<T>(this IAsyncObservable<T> source, Func<T, ValueTask> onNextAsync, Func<Exception, ValueTask> onErrorAsync, Func<ValueTask> onCompletedAsync)
        {

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