dotnet/reactive · error · ArgumentNullException

Value cannot be null. (Parameter 'onCompleted')

Error message

Value cannot be null. (Parameter 'onCompleted')

What it means

ArgumentNullException thrown by the AsyncQueryableEx.Do<TSource> overload taking onNext/onError/onCompleted handlers when the onCompleted expression is null. Do builds a query operator whose three handler arguments are translated into the provider expression tree, so each must be non-null and is validated in order. The message names 'onCompleted'.

Solutions

1. Pass a no-op completion expression: () => Task.CompletedTask
2. If only onNext is needed, use the single-handler Do overload instead
3. Fix handler-factory code so it emits a completed-task lambda rather than null for onCompleted
4. Wrap observer adaptation to always materialize all three handlers

Example fix

// before
query.Do(x => Log(x), e => LogErr(e), null);
// after
query.Do(x => Log(x), e => LogErr(e), () => Task.CompletedTask);

if (onCompleted is null) onCompleted = () => Task.CompletedTask;

static bool HasCompleted(Expression<Func<Task>>? h) => h is not null;

try { q = source.Do(onNext, onError, onCompleted); }
catch (ArgumentNullException ex) when (ex.ParamName == "onCompleted") { q = source.Do(onNext, onError, () => Task.CompletedTask); }

When it happens

Trigger: Calling Do(source, onNext, onError, null) — typically because source/onNext/onError were validated earlier and only onCompleted was left null, e.g. a partial-handler factory that supplies only some callbacks.

Common situations: Logging/tap pipelines where the completion handler was deemed optional; adapters mapping Rx-style observers to queryable Do with a missing onCompleted; code generators emitting null for empty handlers.

Related errors

• Value cannot be null. (Parameter 'onNext')
• Value cannot be null. (Parameter 'onError')
• Value cannot be null. (Parameter 'selector')
• Value cannot be null. (Parameter 'onCompletedAsync')
• Value cannot be null. (Parameter 'onNext')

AI-assisted analysis of dotnet/reactive@94b5d5ab91 (2026-09-15). Data as JSON: /api/errors/79c35d5ddd836088. Report an issue: GitHub.

Appendix: source

Thrown at Ix.NET/Source/System.Interactive.Async.Providers/System/Linq/AsyncQueryableEx.Generated.cs:1060

            return source.Provider.CreateQuery<TSource>(Expression.Call(Do__TSource__4__0(typeof(TSource)), source.Expression, onNext, onError, Expression.Constant(onCompleted, typeof(Action))));
        }

        private static MethodInfo? s_Do__TSource__4__1;
        
        private static MethodInfo Do__TSource__4__1(Type TSource) =>
            (s_Do__TSource__4__1 ??
            (s_Do__TSource__4__1 = new Func<IAsyncQueryable<object>, Expression<Func<object, CancellationToken, Task>>, Expression<Func<Exception, CancellationToken, Task>>, Expression<Func<CancellationToken, Task>>, IAsyncQueryable<object>>(Do<object>).GetMethodInfo()!.GetGenericMethodDefinition())).MakeGenericMethod(TSource);

        public static IAsyncQueryable<TSource> Do<TSource>(this IAsyncQueryable<TSource> source, Expression<Func<TSource, CancellationToken, Task>> onNext, Expression<Func<Exception, CancellationToken, Task>> onError, Expression<Func<CancellationToken, Task>> onCompleted)
        {
            if (source == null)
                throw new ArgumentNullException(nameof(source));
            if (onNext == null)
                throw new ArgumentNullException(nameof(onNext));
            if (onError == null)
                throw new ArgumentNullException(nameof(onError));
            if (onCompleted == null)
                throw new ArgumentNullException(nameof(onCompleted));

            return source.Provider.CreateQuery<TSource>(Expression.Call(Do__TSource__4__1(typeof(TSource)), source.Expression, onNext, onError, onCompleted));
        }

        private static MethodInfo? s_Do__TSource__4__2;
        
        private static MethodInfo Do__TSource__4__2(Type TSource) =>
            (s_Do__TSource__4__2 ??
            (s_Do__TSource__4__2 = new Func<IAsyncQueryable<object>, Expression<Func<object, Task>>, Expression<Func<Exception, Task>>, Expression<Func<Task>>, IAsyncQueryable<object>>(Do<object>).GetMethodInfo()!.GetGenericMethodDefinition())).MakeGenericMethod(TSource);

        public static IAsyncQueryable<TSource> Do<TSource>(this IAsyncQueryable<TSource> source, Expression<Func<TSource, Task>> onNext, Expression<Func<Exception, Task>> onError, Expression<Func<Task>> onCompleted)
        {
            if (source == null)
                throw new ArgumentNullException(nameof(source));
            if (onNext == null)
                throw new ArgumentNullException(nameof(onNext));
            if (onError == null)
                throw new ArgumentNullException(nameof(onError));

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