dotnet/reactive · error · ArgumentNullException

new ArgumentNullException(nameof(source))

Error message

new ArgumentNullException(nameof(source))

What it means

The GetAwaiter extension on IAsyncObservable<TSource> lets you await an async observable as a single-value subject. It validates the source and throws ArgumentNullException(nameof(source)) when the observable is null, before creating the underlying SequentialAsyncAsyncSubject.

Solutions

1. Ensure the source is produced by AsyncObservable operators, which never return null — trace upstream to find where null was introduced.
2. Add an explicit null check or `?? throw new InvalidOperationException(...)` before awaiting.
3. Use nullable-reference-type annotations so the compiler flags the possibly-null source.

Example fix

// before
var result = await maybeSource; // maybeSource may be null
// after
if (maybeSource == null) throw new InvalidOperationException("source was not initialized");
var result = await maybeSource;

if (source is null) throw new InvalidOperationException("observable source was not initialized");

static bool IsAwaitableSource<TSource>(IAsyncObservable<TSource>? s) => s is not null;

try { var value = await source; } catch (ArgumentNullException ex) when (ex.ParamName == "source") { /* handle missing source */ }

When it happens

Trigger: Writing `await someSource` (or calling source.GetAwaiter()) where someSource is null — e.g. a lookup/cache miss returned null, a factory method returned null, or an optional dependency was never assigned.

Common situations: Chaining operator results where an earlier call returned null unexpectedly; awaiting a field initialized lazily but accessed before initialization; dictionary lookups with TryGetValue ignored.

Related errors

• new ArgumentNullException(nameof(source))
• new ArgumentNullException(nameof(keySelector))
• observer
• Value cannot be null. (Parameter 'subscribeAsync')
• Value cannot be null. (Parameter 'observer')

AI-assisted analysis of dotnet/reactive@94b5d5ab91 (2026-09-15). Data as JSON: /api/errors/0bf6281745502e61. Report an issue: GitHub.

Appendix: source

Thrown at AsyncRx.NET/System.Reactive.Async/Linq/Operators/GetAwaiter.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.Subjects;

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

            var subject = new SequentialAsyncAsyncSubject<TSource>();

            var subscribeTask = source.SubscribeSafeAsync(subject);

            subscribeTask.AsTask().ContinueWith(t =>
            {
                if (t.Exception != null)
                {
                    subject.OnErrorAsync(t.Exception); // NB: Should not occur due to use of SubscribeSafeAsync.
                }
            });

            return subject;
        }

        public static AsyncAsyncSubject<TSource> GetAwaiter<TSource>(this IConnectableAsyncObservable<TSource> source)
        {

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