dotnet/reactive · error · ArgumentNullException

Value cannot be null. (Parameter 'source')

Error message

Value cannot be null. (Parameter 'source')

What it means

The ToArray operator throws ArgumentNullException with message "Value cannot be null. (Parameter 'source')" when the source IAsyncObservable<TSource> is null. ToArray buffers all elements into a TSource[] emitted on completion, and buffering requires a valid source, validated at the operator call.

Solutions

1. Ensure the source is non-null; substitute AsyncObservable.Empty<TSource>() for an intentionally empty stream.
2. ThrowIfNull the source where the observable is produced so the failure points to the real defect.
3. Fix the producing method/branch to always return a valid observable.

Example fix

// before
var items = GetStream(); // null on failure
var arr = await items.ToArray();

// after
var items = GetStream() ?? AsyncObservable.Empty<int>();
var arr = await items.ToArray();

if (source is null)
    throw new InvalidOperationException("ToArray requires a non-null source observable.");

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

try
{
    return await source.ToArray();
}
catch (ArgumentNullException ex) when (ex.ParamName == "source")
{
    return Array.Empty<T>();
}

When it happens

Trigger: Calling source.ToArray() where source is null: a null-returning factory, an uninitialized pipeline variable, or a conditional query builder that skipped assignment.

Common situations: Methods that return null instead of AsyncObservable.Empty<T>() on failure; async initialization ordering where the query is built before the source is created; concatenating pipeline stages where one stage returns null.

Related errors

• ArgumentNullException
• Value cannot be null. (Parameter 'source')
• source
• Value cannot be null. (Parameter 'clock')
• Value cannot be null. (Parameter 'onCompletedAsync')

AI-assisted analysis of dotnet/reactive@94b5d5ab91 (2026-09-15). Data as JSON: /api/errors/74a4a8b3754105d8. Report an issue: GitHub.

Appendix: source

Thrown at AsyncRx.NET/System.Reactive.Async/Linq/Operators/ToArray.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.Collections.Generic;

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

            return Create<TSource, TSource[]>(source, static (source, observer) => source.SubscribeSafeAsync(AsyncObserver.ToArray(observer)));
        }
    }

    public partial class AsyncObserver
    {
        public static IAsyncObserver<TSource> ToArray<TSource>(IAsyncObserver<TSource[]> observer)
        {
            if (observer == null)
                throw new ArgumentNullException(nameof(observer));

            return Aggregate<TSource, List<TSource>, TSource[]>(observer, new List<TSource>(), (xs, x) => { xs.Add(x); return xs; }, xs => xs.ToArray());
        }
    }
}

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