dotnet/reactive · error · ArgumentNullException

nameof(source)

Error message

nameof(source)

What it means

This is an ArgumentNullException thrown by AsyncObservable.Finally when the source observable is null. Finally attaches a guaranteed-invoked Action after the source terminates, and composition requires a real source to subscribe to, so a null source fails immediately at the call site.

Solutions

1. Ensure the source observable is non-null before calling Finally
2. Fix the factory/lookup that returned null
3. Guard optional sources: source?.Finally(action) ?? emptyObservable, or check null explicitly

Example fix

// before
var tracked = source.Finally(() => Console.WriteLine("done")); // source null
// after
var tracked = source?.Finally(() => Console.WriteLine("done")) ?? AsyncObservable.Empty<TSource>();

if (source is null) throw new ArgumentNullException(nameof(source));
var tracked = source.Finally(finallyAction);

bool HasSource<TSource>(IAsyncObservable<TSource> s) => s is not null;

try { var tracked = source.Finally(finallyAction); }
catch (ArgumentNullException ex) when (ex.ParamName == "source") { /* substitute an empty observable or skip wiring */ }

When it happens

Trigger: Calling source.Finally(action) where source is null — e.g. chained off a method that returned null, a registry lookup that missed, or an expression evaluated on an uninitialized observable.

Common situations: Cleanup wiring around streams that may be absent; refactoring where the source factory became nullable; pipelines built from config where a source key was missing.

Related errors

• nameof(source)
• source
• Value cannot be null. (Parameter 'disposable1')
• Value cannot be null. (Parameter 'disposable2')
• Value cannot be null. (Parameter 'source')

AI-assisted analysis of dotnet/reactive@94b5d5ab91 (2026-09-15). Data as JSON: /api/errors/dad250ad09a76b2b. Report an issue: GitHub.

Appendix: source

Thrown at AsyncRx.NET/System.Reactive.Async/Linq/Operators/Finally.cs:15

// 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.Disposables;
using System.Threading.Tasks;

namespace System.Reactive.Linq
{
    public partial class AsyncObservable
    {
        public static IAsyncObservable<TSource> Finally<TSource>(this IAsyncObservable<TSource> source, Action finallyAction)
        {
            if (source == null)
                throw new ArgumentNullException(nameof(source));
            if (finallyAction == null)
                throw new ArgumentNullException(nameof(finallyAction));

            return Create(
                source,
                finallyAction,
                static async (source, finallyAction, observer) =>
                {
                    var subscription = await source.SubscribeSafeAsync(observer).ConfigureAwait(false);

                    return AsyncDisposable.Create(async () =>
                    {
                        try
                        {
                            await subscription.DisposeAsync().ConfigureAwait(false);
                        }
                        finally
                        {

View on GitHub (pinned to 94b5d5ab91)