dotnet/reactive · error · ArgumentNullException

throw new ArgumentNullException(nameof(iteratorMethod))

Error message

throw new ArgumentNullException(nameof(iteratorMethod))

What it means

QbservableEx.Create(provider, iteratorMethod) requires the generator method as an Expression<Func<IEnumerable<IObservable<object>>>> so it can be serialized into the query expression tree. A null iteratorMethod throws ArgumentNullException named 'iteratorMethod'. The expression is needed because qbservable queries are transmitted as expression trees, not compiled delegates.

Solutions

Supply a real expression lambda, e.g. () => new[] { Observable.Return(1) } typed as IEnumerable<IObservable<object>>
If the iterator is optional, branch: skip Create entirely when iteratorMethod is null
Fix the factory/reflective builder that returned a null expression

Example fix

// before
Qbservable.Create(provider, iteratorMethod); // iteratorMethod is null
// after
Expression<Func<IEnumerable<IObservable<object>>>> iteratorMethod =
    () => new[] { Observable.Return((object)42) };
Qbservable.Create(provider, iteratorMethod);

Defensive patterns

Strategy: validation

Validate before calling

if (iteratorMethod == null)
    throw new InvalidOperationException("iteratorMethod expression is required for Qbservable.Create");

Type guard

bool HasIterator(Expression<Func<IEnumerable<IObservable<object>>>> m) => m is not null;

Try / catch

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

Prevention

Always pass literal expression lambdas to Create, not nullable variables
Remember Qbservable APIs require Expression<>, not plain Func<>
Skip the Create call entirely when the iterator is not defined

When it happens

Trigger: Calling Qbservable.Create(provider, null) — usually when the iterator lambda was built conditionally, stored in a nullable variable, or produced by reflection/factory code that returned null.

Common situations: Building iterator methods dynamically and passing an unassigned variable; refactoring where the lambda was removed but the call remained; conditional query construction where only some branches define the iterator.

Related errors

AI-assisted analysis of dotnet/reactive@94b5d5ab91 (2026-09-15). Data as JSON: /api/errors/fbfb7fe5542b4e28. Report an issue: GitHub.

Appendix: source

Thrown at Rx.NET/Source/src/System.Reactive/Linq/QbservableEx.Homoicon.cs:967

            );
        }

#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>
        /// <param name="provider">Query provider used to construct the <see cref="IQbservable{T}"/> data source.</param>
        /// <typeparam name="TResult">The type of the elements in the produced sequence.</typeparam>

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