SignalR/SignalR · error · ArgumentNullException

onData

Error message

onData

What it means

HubProxyExtensions.On throws ArgumentNullException('onData') when the callback Action is null. The subscription wraps onData and invokes it per event payload, so a null handler is meaningless and would NRE later — the guard fails fast.

Source

Thrown at src/Microsoft.AspNet.SignalR.Client/HubProxyExtensions.cs:63

        /// <param name="proxy">The <see cref="IHubProxy"/>.</param>
        /// <param name="eventName">The name of the event.</param>
        /// <param name="onData">The callback</param>
        /// <returns>An <see cref="IDisposable"/> that represents this subscription.</returns>
        public static IDisposable On(this IHubProxy proxy, string eventName, Action onData)
        {
            if (proxy == null)
            {
                throw new ArgumentNullException("proxy");
            }

            if (String.IsNullOrEmpty(eventName))
            {
                throw new ArgumentNullException("eventName");
            }

            if (onData == null)
            {
                throw new ArgumentNullException("onData");
            }

            Subscription subscription = proxy.Subscribe(eventName);

            Action<IList<JToken>> handler = args =>
            {
                ExecuteCallback(eventName, args.Count, 0, onData);
            };

            subscription.Received += handler;

            return new DisposableAction(() => subscription.Received -= handler);
        }

        /// <summary>
        /// Registers for an event with the specified name and callback
        /// </summary>
        /// <param name="proxy">The <see cref="IHubProxy"/>.</param>

View on GitHub (pinned to 693053b89a)

Solutions

1. Always pass a concrete Action; use () => { } as a no-op if you only need side-effect-free subscription.
2. If the handler comes from a factory, validate it is non-null before subscribing.
3. Prefer method-group references over nullable delegate fields.

Example fix

// before
proxy.On("ping", null);
// after
proxy.On("ping", () => HandlePing());

if (onData == null) throw new ArgumentNullException(nameof(onData));
proxy.On(eventName, onData);

static bool HasHandler(Action h) => h != null;

When it happens

Trigger: Passing null as the callback; handler resolved from a factory that returned null; conditional handler assignment that skipped the real lambda.

Common situations: Refactor left the handler unassigned; DI/factory returned null; copy-paste dropped the lambda body.

Related errors

• eventName
• proxy
• name
• prepareRequest
• connection

AI-assisted analysis of SignalR/SignalR@693053b89a (2026-08-13). Data as JSON: /api/errors/176f758316dcd098. Report an issue: GitHub.