What causes "The scheduler service '{0}' is not available." in dotnet/reactive?

Reference System.Reactive.PlatformServices so the default provider registers scheduler services

dotnet/reactive · critical · NotSupportedException

The scheduler service

Error message

The scheduler service '{0}' is not available.

What it means

Scheduler.Initialize resolves an IScheduler from the platform enlightenment provider by service name; when the provider returns no implementation it throws NotSupportedException with 'The scheduler service "{0}" is not available.' This is how Scheduler.ThreadPool, NewThread and TaskPool lazily bind to the platform at first access.

Solutions

Reference System.Reactive.PlatformServices so the default provider registers scheduler services
If using a custom PlatformEnlightenmentProvider, ensure GetService<IScheduler>(name) returns implementations for "ThreadPool", "NewThread", "TaskPool"
Use Scheduler.Default / immediate schedulers as a fallback instead of the missing service

Example fix

// before
class MyProvider : IPlatformEnlightenmentProvider
{
    public T GetService<T>(object[] args) where T : class => null; // scheduler services missing
}
// after
public T GetService<T>(object[] args) where T : class
{
    if (typeof(T) == typeof(IScheduler)) return new MyScheduler() as T;
    return new DefaultPlatformEnlightenmentProvider().GetService<T>(args);
}

Defensive patterns

Strategy: fallback

Validate before calling

var sched = PlatformEnlightenmentProvider.Current.GetService<IScheduler>("ThreadPool");
if (sched == null) sched = new DefaultSchedulerShim();

Type guard

static bool SchedulerServiceAvailable<T>() where T : class => PlatformEnlightenmentProvider.Current.GetService<T>("ThreadPool") != null;

Try / catch

try { sched = Scheduler.TaskPool; }
catch (NotSupportedException ex)
{
    // fall back to default scheduler or log environment misconfiguration
    sched = Scheduler.Default;
}

Prevention

Always reference System.Reactive.PlatformServices in executable projects
Test first access to Scheduler.ThreadPool/TaskPool in your host environment
When replacing IPlatformEnlightenmentProvider, delegate unknown services to the default provider

When it happens

Trigger: Accessing Scheduler.ThreadPool, Scheduler.NewThread or Scheduler.TaskPool in an environment where the platform enlightenment provider was not configured (e.g. custom PlatformEnlightenmentProvider without scheduler services, trimmed/AOT app missing the System.Reactive.PlatformServices registration).

Common situations: Replacing the default enlightenment provider with a custom one that omits IScheduler services; unit-test hosts that stub the provider; missing platform services package in some targets.

Related errors

AI-assisted analysis of dotnet/reactive@94b5d5ab91 (2026-09-15). Data as JSON: /api/errors/1967516b01127a7b. Report an issue: GitHub.

Appendix: source

Thrown at Rx.NET/Source/src/System.Reactive/Concurrency/Scheduler.cs:90

        /// <summary>
        /// Gets a scheduler that schedules work on a new thread using default thread creation options.
        /// </summary>
        [Obsolete(Constants_Core.ObsoleteSchedulerNewthread)]
        public static IScheduler NewThread => _newThread.Value;

        private static readonly Lazy<IScheduler> _taskPool = new(static () => Initialize("TaskPool"));

        /// <summary>
        /// Gets a scheduler that schedules work on Task Parallel Library (TPL) task pool using the default TaskScheduler.
        /// </summary>
        [Obsolete(Constants_Core.ObsoleteSchedulerTaskpool)]
        public static IScheduler TaskPool => _taskPool.Value;

        private static IScheduler Initialize(string name)
        {
            return PlatformEnlightenmentProvider.Current.GetService<IScheduler>(name)
                ?? throw new NotSupportedException(string.Format(CultureInfo.CurrentCulture, Strings_Core.CANT_OBTAIN_SCHEDULER, name));
        }
    }
}

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