dotnet/reactive · error · ArgumentNullException

scheduler

Error message

scheduler

What it means

StartAsync<TSource>(Func<ValueTask<TSource>>, IAsyncScheduler) throws ArgumentNullException when the scheduler parameter is null. The scheduler governs where the async work runs; the library requires an explicit instance rather than treating null as 'immediate'.

Solutions

1. Supply a real scheduler, e.g. ImmediateAsyncScheduler.Instance
2. Use the single-argument overload StartAsync(functionAsync) which defaults to ImmediateAsyncScheduler.Instance
3. Repair the configuration that yielded a null scheduler

Example fix

// before
StartAsync(fetch, null);
// after
StartAsync(fetch, ImmediateAsyncScheduler.Instance);

if (scheduler == null) scheduler = ImmediateAsyncScheduler.Instance; // or choose the 1-arg overload

static bool IsValidScheduler(IAsyncScheduler s) => s is not null;

When it happens

Trigger: Calling StartAsync(functionAsync, null) with a valid function but a null IAsyncScheduler.

Common situations: Unconfigured scheduler dependency, tests passing null, copy-paste from code that used the overload without a scheduler.

Related errors

• null (Parameter 'scheduler')
• Value cannot be null. (Parameter 'observer')
• Value cannot be null. (Parameter 'values')
• scheduler (Value cannot be null)
• null (Parameter 'values')

AI-assisted analysis of dotnet/reactive@94b5d5ab91 (2026-09-15). Data as JSON: /api/errors/3ec42fc82e903d0e. Report an issue: GitHub.

Appendix: source

Thrown at AsyncRx.NET/System.Reactive.Async/Linq/Operators/StartAsync.cs:21

// See the LICENSE file in the project root for more information. 

using System.Reactive.Concurrency;
using System.Reactive.Disposables;
using System.Threading;
using System.Threading.Tasks;

namespace System.Reactive.Linq
{
    public partial class AsyncObservable
    {
        public static IAsyncObservable<TSource> StartAsync<TSource>(Func<ValueTask<TSource>> functionAsync) => StartAsync(functionAsync, ImmediateAsyncScheduler.Instance);

        public static IAsyncObservable<TSource> StartAsync<TSource>(Func<ValueTask<TSource>> functionAsync, IAsyncScheduler scheduler)
        {
            if (functionAsync == null)
                throw new ArgumentNullException(nameof(functionAsync));
            if (scheduler == null)
                throw new ArgumentNullException(nameof(scheduler));

            Task<TSource> task;

            try
            {
                task = functionAsync().AsTask();
            }
            catch (Exception ex)
            {
                return Throw<TSource>(ex);
            }

            return task.ToAsyncObservable(scheduler);
        }

        public static IAsyncObservable<TSource> StartAsync<TSource>(Func<CancellationToken, ValueTask<TSource>> functionAsync) => StartAsync(functionAsync, ImmediateAsyncScheduler.Instance);

        public static IAsyncObservable<TSource> StartAsync<TSource>(Func<CancellationToken, ValueTask<TSource>> functionAsync, IAsyncScheduler scheduler)

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