SignalR/SignalR · error · InvalidOperationException

Multiple activators for type {0} are registered. Please call

Error message

Multiple activators for type {0} are registered. Please call GetServices instead.

What it means

Thrown by DefaultDependencyResolver.GetService when more than one activator is registered for the requested Type. GetService is single-instance and ambiguous when multiple registrations exist; the resolver refuses to silently pick one and directs the caller to GetServices (plural) instead.

Source

Thrown at src/Microsoft.AspNet.SignalR.Core/DefaultDependencyResolver.cs:128

        }

        public virtual object GetService(Type serviceType)
        {
            if (serviceType == null)
            {
                throw new ArgumentNullException("serviceType");
            }

            IList<Func<object>> activators;
            if (_resolvers.TryGetValue(serviceType, out activators))
            {
                if (activators.Count == 0)
                {
                    return null;
                }
                if (activators.Count > 1)
                {
                    throw new InvalidOperationException(String.Format(CultureInfo.CurrentCulture, Resources.Error_MultipleActivatorsAreaRegisteredCallGetServices, serviceType.FullName));
                }
                return Track(activators[0]);
            }
            return null;
        }

        public virtual IEnumerable<object> GetServices(Type serviceType)
        {
            IList<Func<object>> activators;
            if (_resolvers.TryGetValue(serviceType, out activators))
            {
                if (activators.Count == 0)
                {
                    return null;
                }
                return activators.Select(Track).ToList();
            }
            return null;

View on GitHub (pinned to 693053b89a)

Solutions

1. Use GetServices(serviceType) to retrieve all registered instances when multiples are expected.
2. If a single instance is required, deregister duplicates so only one activator remains.
3. Audit Register calls (including third-party HostStartup classes) for the conflicting type.

Example fix

// before
var single = resolver.GetService(typeof(IMyService));

// after
var all = resolver.GetServices(typeof(IMyService));
var single = all.FirstOrDefault();

var all = resolver.GetServices(serviceType).ToList();
var single = all.Count == 1 ? all[0] : all.FirstOrDefault();

var services = resolver.GetServices(serviceType)?.ToList() ?? new List<object>();
object single = services.Count > 1 ? null : services.FirstOrDefault();

try
{
    return resolver.GetService(serviceType);
}
catch (InvalidOperationException)
{
    // multiple registrations; fall back to plural resolution
    return resolver.GetServices(serviceType).FirstOrDefault();
}

When it happens

Trigger: Registering two or more factories for the same Type via Register, then calling GetService on that Type.

Common situations: Multiple modules/libraries each register the same service type (e.g., several IHubPipeline modules registered as the pipeline), or a plugin composition that double-registers.

Related errors

• serviceType
• DisconnectTimeout cannot be configured after the KeepAlive.
• request
• hubIncomingInvokerContext
• Outgoing authorization can only be required for an entire Hu

AI-assisted analysis of SignalR/SignalR@693053b89a (2026-08-13). Data as JSON: /api/errors/04e2c74e8f6f9e90. Report an issue: GitHub.