dotnet/reactive · error · ArgumentNullException

nameof(type)

Error message

nameof(type)

What it means

This is an ArgumentNullException raised by the static-event overload FromEventPattern(Type type, string eventName, IAsyncScheduler scheduler) when the type argument is null. This overload subscribes to static .NET events via reflection on the given Type, so a null Type cannot be reflected over and is rejected synchronously before any observable is created.

Solutions

1. Use typeof(SenderType) instead of a string-based Type lookup to get a compile-time-guaranteed non-null Type
2. If using Type.GetType, verify the assembly-qualified name and check the result for null before calling FromEventPattern
3. Ensure plugin/reference assemblies are present so the type actually resolves

Example fix

// before
var t = Type.GetType("MyApp.Services.Messenger"); // null: wrong namespace
var events = AsyncObservable.FromEventPattern(t, "MessageReceived");
// after
var events = AsyncObservable.FromEventPattern(typeof(MyApp.Services.Messenger), "MessageReceived");

if (type is null)
    throw new ArgumentNullException(nameof(type), "Event source type not resolved");
var obs = AsyncObservable.FromEventPattern(type, eventName);

static bool HasSourceType([NotNullWhen(true)] Type? t) => t is not null;

try
{
    var obs = AsyncObservable.FromEventPattern(type, eventName);
}
catch (ArgumentNullException ex) when (ex.ParamName == "type")
{
    // string-based Type lookup failed; log the bad type name
    throw new TypeLoadException($"Could not resolve event source type '{typeByName}'", ex);
}

When it happens

Trigger: Calling the public FromEventPattern(Type type, string eventName) or explicit-scheduler overload (AsyncRx.NET/System.Reactive.Async/Linq/Operators/FromEventPattern.cs:139) with type == null, e.g. passing the result of Type.GetType("Bad.Namespace.Type") which returns null when the type name/assembly is wrong.

Common situations: Type.GetType with a mistyped or assembly-unqualified name returning null; loading types from plugins where the assembly failed to load; reflection-based registration code that forwards a null type it obtained elsewhere.

Related errors

• Value cannot be null. (Parameter 'scheduler')
• Value cannot be null. (Parameter 'source')
• Value cannot be null.
• addHandler
• removeHandler

AI-assisted analysis of dotnet/reactive@94b5d5ab91 (2026-09-15). Data as JSON: /api/errors/ef83c4440c17292a. Report an issue: GitHub.

Appendix: source

Thrown at AsyncRx.NET/System.Reactive.Async/Linq/Operators/FromEventPattern.cs:139

        public static IAsyncObservable<EventPattern<TSender, TEventArgs>> FromEventPattern<TSender, TEventArgs>(object target, string eventName, IAsyncScheduler scheduler)
        {
            if (target == null)
                throw new ArgumentNullException(nameof(target));
            if (eventName == null)
                throw new ArgumentNullException(nameof(eventName));
            if (scheduler == null)
                throw new ArgumentNullException(nameof(scheduler));

            return FromEventPatternCore<TSender, TEventArgs, EventPattern<TSender, TEventArgs>>(target.GetType(), target, eventName, scheduler, (o, e) => new EventPattern<TSender, TEventArgs>(o, e));
        }

        public static IAsyncObservable<EventPattern<object>> FromEventPattern(Type type, string eventName) => FromEventPattern(type, eventName, GetSchedulerForCurrentContext());

        public static IAsyncObservable<EventPattern<object>> FromEventPattern(Type type, string eventName, IAsyncScheduler scheduler)
        {
            if (type == null)
                throw new ArgumentNullException(nameof(type));
            if (eventName == null)
                throw new ArgumentNullException(nameof(eventName));
            if (scheduler == null)
                throw new ArgumentNullException(nameof(scheduler));

            return FromEventPatternCore<object, object, EventPattern<object>>(type, null, eventName, scheduler, (o, e) => new EventPattern<object>(o, e));
        }

        public static IAsyncObservable<EventPattern<TEventArgs>> FromEventPattern<TEventArgs>(Type type, string eventName) => FromEventPattern<TEventArgs>(type, eventName, GetSchedulerForCurrentContext());

        public static IAsyncObservable<EventPattern<TEventArgs>> FromEventPattern<TEventArgs>(Type type, string eventName, IAsyncScheduler scheduler)
        {
            if (type == null)
                throw new ArgumentNullException(nameof(type));
            if (eventName == null)
                throw new ArgumentNullException(nameof(eventName));
            if (scheduler == null)
                throw new ArgumentNullException(nameof(scheduler));

View on GitHub (pinned to 94b5d5ab91)