dotnet/reactive · error · ArgumentNullException

Value cannot be null. (Parameter 'onNext')

Error message

Value cannot be null. (Parameter 'onNext')

What it means

ArgumentNullException thrown by the three-handler Do<TSource> extension when the onNext expression (Expression<Func<TSource, Task>>) is null. All handler arguments become nodes of the query expression tree sent to the provider, so onNext is validated immediately after source. The message names 'onNext'.

Solutions

1. Provide a no-op onNext: _ => Task.CompletedTask
2. Use a Do overload that takes fewer handlers if onNext is genuinely unneeded
3. Fix the handler factory to emit a completed-task lambda instead of null
4. Assert handler non-nullness at the point handlers are constructed

Example fix

// before
query.Do(null, OnError, OnDone);
// after
query.Do(_ => Task.CompletedTask, OnError, OnDone);

if (onNext is null) onNext = _ => Task.CompletedTask;

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

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

When it happens

Trigger: Calling Do(source, null, onError, onCompleted) — usually when only error/completion logging was wired and the per-item handler was left null, or a handler selector returned null.

Common situations: Telemetry taps where item-level handling is optional; refactors that removed the body of onNext; mapping code from an observer interface where onNext was a no-op represented as null.

Related errors

• Value cannot be null. (Parameter 'onCompleted')
• 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/159c7ccc4936a841. Report an issue: GitHub.

Appendix: source

Thrown at Ix.NET/Source/System.Interactive.Async.Providers/System/Linq/AsyncQueryableEx.Generated.cs:1076

                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));
            if (onCompleted == null)
                throw new ArgumentNullException(nameof(onCompleted));

            return source.Provider.CreateQuery<TSource>(Expression.Call(Do__TSource__4__2(typeof(TSource)), source.Expression, onNext, onError, onCompleted));
        }

        private static MethodInfo? s_Expand__TSource__2__0;
        
        private static MethodInfo Expand__TSource__2__0(Type TSource) =>
            (s_Expand__TSource__2__0 ??
            (s_Expand__TSource__2__0 = new Func<IAsyncQueryable<object>, Expression<Func<object, CancellationToken, ValueTask<IAsyncEnumerable<object>>>>, IAsyncQueryable<object>>(Expand<object>).GetMethodInfo()!.GetGenericMethodDefinition())).MakeGenericMethod(TSource);

        public static IAsyncQueryable<TSource> Expand<TSource>(this IAsyncQueryable<TSource> source, Expression<Func<TSource, CancellationToken, ValueTask<IAsyncEnumerable<TSource>>>> selector)
        {
            if (source == null)
                throw new ArgumentNullException(nameof(source));

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