dotnet/reactive · error · ArgumentNullException

function

Error message

function

What it means

Start throws ArgumentNullException when the function argument is null. Start converts a synchronous Func<TSource> into an async observable and invokes it (via ToAsync) on a scheduler, so it requires a real delegate to execute.

Solutions

1. Pass a valid Func<TSource>, e.g. () => ComputeValue()
2. Null-check the function variable before calling Start
3. Fix the code path that was supposed to assign the function

Example fix

// before
Func<int> fn = null;
var obs = AsyncObservable.Start(fn);
// after
var obs = AsyncObservable.Start(() => ComputeValue());

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

bool IsValid<TSource>(Func<TSource> f) => f != null;

try { var obs = AsyncObservable.Start(fn); }
catch (ArgumentNullException ex) when (ex.ParamName == "function") { /* supply function */ }

When it happens

Trigger: Calling AsyncObservable.Start<TSource>((Func<TSource>)null).

Common situations: A delegate field or config-selected function was never assigned; passing a null Action/Func variable created for conditional execution.

Related errors

• eighth
• keySelector
• nameof(eventName)
• nameof(removeHandler)
• nameof(target)

AI-assisted analysis of dotnet/reactive@94b5d5ab91 (2026-09-15). Data as JSON: /api/errors/6473ba90c9020fd6. Report an issue: GitHub.

Appendix: source

Thrown at AsyncRx.NET/System.Reactive.Async/Linq/Operators/Start.cs:14

// Licensed to the .NET Foundation under one or more agreements.
// The .NET Foundation licenses this file to you under the MIT License.
// See the LICENSE file in the project root for more information. 

using System.Reactive.Concurrency;

namespace System.Reactive.Linq
{
    public partial class AsyncObservable
    {
        public static IAsyncObservable<TSource> Start<TSource>(Func<TSource> function)
        {
            if (function == null)
                throw new ArgumentNullException(nameof(function));

            return ToAsync(function)();
        }

        public static IAsyncObservable<TSource> Start<TSource>(Func<TSource> function, IAsyncScheduler scheduler)
        {
            if (function == null)
                throw new ArgumentNullException(nameof(function));
            if (scheduler == null)
                throw new ArgumentNullException(nameof(scheduler));

            return ToAsync(function, scheduler)();
        }

        public static IAsyncObservable<Unit> Start(Action action)
        {
            if (action == null)
                throw new ArgumentNullException(nameof(action));

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