dotnet/reactive · error · ArgumentNullException

Value cannot be null. (Parameter 'source')

Error message

Value cannot be null. (Parameter 'source')

What it means

Cast<TSource,TResult> is an extension on IAsyncObservable<TSource> and throws ArgumentNullException when the source is null. Like all AsyncRx operators it validates inputs up front rather than failing inside the subscription pipeline.

Solutions

1. Ensure the source observable is non-null before calling Cast
2. Replace a null source with AsyncObservable.Empty<TSource>() when 'no source' is a valid state
3. Null-check the method returning the observable and handle the null branch explicitly

Example fix

// before
IAsyncObservable<object> src = maybeNull;
var ints = src.Cast<object, int>();
// after
if (src == null) src = AsyncObservable.Empty<object>(Scheduler.Default);
var ints = src.Cast<object, int>();

if (source == null) throw new InvalidOperationException("Cannot Cast a null source observable");

bool IsObservable<T>(IAsyncObservable<T> s) => s is not null;

try { var result = src.Cast<object, int>(); }
catch (ArgumentNullException ex) when (ex.ParamName == "source") { /* fall back to Empty source */ }

When it happens

Trigger: Invoking source.Cast<TSource,TResult>() on a null IAsyncObservable<TSource>, or Cast(null) via static call syntax.

Common situations: Chaining off a factory method that returned null; LINQ-style pipelines where an earlier operator produced null; conditional source assignment left unset.

Related errors

• Value cannot be null. (Parameter 'observer')
• Value cannot be null. (Parameter 'source')
• Value cannot be null. (Parameter 'handler')
• Value cannot be null. (Parameter 'first')
• removeHandler

AI-assisted analysis of dotnet/reactive@94b5d5ab91 (2026-09-15). Data as JSON: /api/errors/70cc1c0091996f95. Report an issue: GitHub.

Appendix: source

Thrown at AsyncRx.NET/System.Reactive.Async/Linq/Operators/Cast.cs:12

// 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. 

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

            return Create<TSource, TResult>(source, static (source, observer) => source.SubscribeSafeAsync(AsyncObserver.Cast<TSource, TResult>(observer)));
        }
    }

    public partial class AsyncObserver
    {
        public static IAsyncObserver<TSource> Cast<TSource, TResult>(IAsyncObserver<TResult> observer)
        {
            if (observer == null)
                throw new ArgumentNullException(nameof(observer));

            return Select<TSource, TResult>(observer, x => (TResult)(object)x);
        }
    }
}

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