dotnet/reactive · error · ArgumentNullException

throw new ArgumentNullException(nameof(provider))

Error message

throw new ArgumentNullException(nameof(provider))

What it means

QbservableEx.Create(provider, iteratorMethod) requires an IQbservableProvider to build the remote query. A null provider is rejected immediately with ArgumentNullException named 'provider'. Rx throws eagerly so invalid queries fail at composition rather than at subscription time on the provider.

Solutions

1. Pass a valid IQbservableProvider, e.g. from an existing query's .Provider or Qbservable.Provider
2. Resolve the provider from your DI/ServiceProvider and verify registration is non-null
3. Guard the call site with a null check on the provider before invoking Create

Example fix

// before
Qbservable.Create(provider, iteratorMethod);
// after
if (provider == null) throw new InvalidOperationException("No IQbservableProvider configured");
Qbservable.Create(provider, iteratorMethod);

if (provider == null)
    throw new InvalidOperationException("IQbservableProvider is not configured");

bool HasProvider(IQbservableProvider p) => p is not null;

try
{
    var query = Qbservable.Create(provider, iteratorMethod);
}
catch (ArgumentNullException ex) when (ex.ParamName == "provider")
{
    logger.LogError(ex, "Provider was null when composing Create query");
}

When it happens

Trigger: Calling Qbservable.Create(null, iteratorMethod) — typically when the provider came from a null source.Provider, an uninitialized service locator, or a conditional expression like qbservable?.Provider.

Common situations: Chaining .Provider off a query built conditionally; DI container returning null for IQbservableProvider; using the extension before the queryable source was initialized in test scaffolding.

Related errors

• throw new ArgumentNullException(nameof(iteratorMethod))
• throw new ArgumentNullException(nameof(sixteenth))
• throw new ArgumentNullException(nameof(source))
• eleventh
• fifteenth

AI-assisted analysis of dotnet/reactive@94b5d5ab91 (2026-09-15). Data as JSON: /api/errors/ab7478dd0eab5035. Report an issue: GitHub.

Appendix: source

Thrown at Rx.NET/Source/src/System.Reactive/Linq/QbservableEx.Homoicon.cs:965

                    GetSourceExpression(sixteenth)
                )
            );
        }

#if !STABLE
        /// <summary>
        /// Subscribes to each observable sequence returned by the iteratorMethod in sequence and produces a Unit value on the resulting sequence for each step of the iteration.
        /// </summary>
        /// <param name="provider">Query provider used to construct the <see cref="IQbservable{T}"/> data source.</param>
        /// <param name="iteratorMethod">Iterator method that drives the resulting observable sequence.</param>
        /// <returns>An observable sequence obtained by running the iterator and returning Unit values for each iteration step.</returns>
        /// <exception cref="ArgumentNullException">
        /// <paramref name="iteratorMethod" /> is null.</exception>
        [Experimental]
        public static IQbservable<Unit> Create(this IQbservableProvider provider, Expression<Func<IEnumerable<IObservable<object>>>> iteratorMethod)
        {
            if (provider == null)
                throw new ArgumentNullException(nameof(provider));
            if (iteratorMethod == null)
                throw new ArgumentNullException(nameof(iteratorMethod));

            return provider.CreateQuery<Unit>(
                Expression.Call(
                    null,
                    new Func<IQbservableProvider, Expression<Func<IEnumerable<IObservable<object>>>>, IQbservable<Unit>>(Create).Method,
                    Expression.Constant(provider, typeof(IQbservableProvider)),
                    iteratorMethod
                )
            );
        }
#endif

#if !STABLE
        /// <summary>
        /// Subscribes to each observable sequence returned by the iteratorMethod in sequence and returns the observable sequence of values sent to the observer given to the iteratorMethod.
        /// </summary>

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