dotnet/reactive · error · ArgumentNullException

Argument cannot be null (Parameter name: source)

Error message

Argument cannot be null (Parameter name: source)

What it means

FirstOrDefault<TSource>(IAsyncObservable<TSource>) (the no-predicate overload) throws ArgumentNullException when source is null. AsyncRx's operator API is extension-method based; a null source would cause a NullReferenceException only at Subscribe time, so the library guards eagerly and names the offending parameter. This keeps the stack trace at your call site.

Solutions

1. Fix the upstream factory/lookup so it returns a non-null IAsyncObservable; throw your own descriptive exception if the source cannot be resolved.
2. Verify you are calling the operator on the observable itself (source.FirstOrDefaultAsync()) and not passing a null into a static helper.
3. Guard before the call: if (source == null) throw new InvalidOperationException(...).

Example fix

// before
var result = sources[name].FirstOrDefaultAsync(); // sources[name] may be null

// after
if (!sources.TryGetValue(name, out var src)) throw new KeyNotFoundException($"source '{name}' missing");
var result = src.FirstOrDefaultAsync();

if (source is null) throw new InvalidOperationException("source observable not resolved");
var first = source.FirstOrDefaultAsync();

static bool HasSource<T>(IAsyncObservable<T>? s) => s is not null;

try { var first = src.FirstOrDefaultAsync(); }
catch (ArgumentNullException ex) when (ex.ParamName == "source") { /* log and use fallback stream */ }

When it happens

Trigger: Calling source.FirstOrDefault() / FirstOrDefaultAsync(source) where the IAsyncObservable<TSource> variable is null — e.g. a factory or dictionary lookup returned null before the extension call.

Common situations: Chained operator pipelines where an earlier operator unexpectedly returned null; config-driven source selection where the configured source key resolves to nothing; caching of observables that was never populated.

Related errors

• Argument cannot be null (Parameter name: predicate)
• Argument cannot be null (Parameter name: observer)
• Value cannot be null. (Parameter 'onErrorAsync')
• Value cannot be null. (Parameter 'onCompletedAsync')
• Value cannot be null. (Parameter 'error')

AI-assisted analysis of dotnet/reactive@94b5d5ab91 (2026-09-15). Data as JSON: /api/errors/c8f5e8b90d492796. Report an issue: GitHub.

Appendix: source

Thrown at AsyncRx.NET/System.Reactive.Async/Linq/Operators/FirstOrDefault.cs:18

// 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.Threading.Tasks;

namespace System.Reactive.Linq
{
    public partial class AsyncObservable
    {
        public static IAsyncObservable<TSource> FirstOrDefaultAsync<TSource>(this IAsyncObservable<TSource> source) => FirstOrDefault(source);
        public static IAsyncObservable<TSource> FirstOrDefaultAsync<TSource>(this IAsyncObservable<TSource> source, Func<TSource, bool> predicate) => FirstOrDefault(source, predicate);
        public static IAsyncObservable<TSource> FirstOrDefaultAsync<TSource>(this IAsyncObservable<TSource> source, Func<TSource, ValueTask<bool>> predicate) => FirstOrDefault(source, predicate);

        public static IAsyncObservable<TSource> FirstOrDefault<TSource>(this IAsyncObservable<TSource> source)
        {
            if (source == null)
                throw new ArgumentNullException(nameof(source));

            return Create(source, static (source, observer) => source.SubscribeSafeAsync(AsyncObserver.FirstOrDefault(observer)));
        }

        public static IAsyncObservable<TSource> FirstOrDefault<TSource>(this IAsyncObservable<TSource> source, Func<TSource, bool> predicate)
        {
            if (source == null)
                throw new ArgumentNullException(nameof(source));
            if (predicate == null)
                throw new ArgumentNullException(nameof(predicate));

            return Create(
                source,
                predicate,
                static (source, predicate, observer) => source.SubscribeSafeAsync(AsyncObserver.FirstOrDefault(observer, predicate)));
        }

        public static IAsyncObservable<TSource> FirstOrDefault<TSource>(this IAsyncObservable<TSource> source, Func<TSource, ValueTask<bool>> predicate)

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