LuckyPennySoftware/MediatR · error · ArgumentException

Unknown strategy: {strategy}

Error message

Unknown strategy: {strategy}

What it means

Thrown by the sample Publisher when the requested PublishStrategy enum value is not a key in its PublishStrategies dictionary. The sample's constructor registers all six enum members (SyncContinueOnException, SyncStopOnException, Async, ParallelNoWait, ParallelWhenAll, ParallelWhenAny), so a miss means the dictionary was mutated or an out-of-range enum value was supplied.

Source

Thrown at samples/MediatR.Examples.PublishStrategies/Publisher.cs:47

    {
        return Publish(notification, DefaultStrategy, default(CancellationToken));
    }

    public Task Publish<TNotification>(TNotification notification, PublishStrategy strategy)
    {
        return Publish(notification, strategy, default(CancellationToken));
    }

    public Task Publish<TNotification>(TNotification notification, CancellationToken cancellationToken)
    {
        return Publish(notification, DefaultStrategy, cancellationToken);
    }

    public Task Publish<TNotification>(TNotification notification, PublishStrategy strategy, CancellationToken cancellationToken)
    {
        if (!PublishStrategies.TryGetValue(strategy, out var mediator))
        {
            throw new ArgumentException($"Unknown strategy: {strategy}");
        }

        return mediator.Publish(notification, cancellationToken);
    }

    private Task ParallelWhenAll(IEnumerable<NotificationHandlerExecutor> handlers, INotification notification, CancellationToken cancellationToken)
    {
        var tasks = new List<Task>();

        foreach (var handler in handlers)
        {
            tasks.Add(Task.Run(() => handler.HandlerCallback(notification, cancellationToken)));
        }

        return Task.WhenAll(tasks);
    }

    private Task ParallelWhenAny(IEnumerable<NotificationHandlerExecutor> handlers, INotification notification, CancellationToken cancellationToken)

View on GitHub (pinned to 916ef1b3d6)

Solutions

1. Pass one of the six defined PublishStrategy values directly rather than a numeric cast.
2. If you extended the enum, add the matching `PublishStrategies[NewStrategy] = new CustomMediator(...)` line in the Publisher constructor.
3. Do not mutate or clear the public PublishStrategies dictionary after construction.

Example fix

// before
publisher.Publish(pinged, (PublishStrategy)99); // unknown
// after
publisher.Publish(pinged, PublishStrategy.ParallelWhenAll);

if (!publisher.PublishStrategies.ContainsKey(strategy))
    throw new ArgumentOutOfRangeException(nameof(strategy), strategy, "PublishStrategy not registered in PublishStrategies.");

static bool IsKnownStrategy(PublishStrategy s) => Enum.IsDefined(typeof(PublishStrategy), s) && publisher.PublishStrategies.ContainsKey(s);

When it happens

Trigger: Calling Publish(notification, strategy) with a PublishStrategy value that is either an invalid numeric cast (e.g. (PublishStrategy)99) or whose dictionary entry was removed/cleared after the Publisher constructor ran.

Common situations: Casting an arbitrary int to PublishStrategy; clearing/overwriting the public PublishStrategies dictionary; extending the PublishStrategy enum with a new member but forgetting to register a CustomMediator for it in the constructor.

Related errors

• Name cannot be '2'
• Name cannot be '2'
• No assemblies found to scan. Supply at least one assembly to
• {openBehaviorType.Name} must implement {typeof(IStreamPipeli
• {implementationType.Name} must implement {typeof(IRequestPre

AI-assisted analysis of LuckyPennySoftware/MediatR@916ef1b3d6 (2026-08-13). Data as JSON: /api/errors/a729f3c50a42133c. Report an issue: GitHub.