PrismLibrary/Prism · error · ArgumentNullException

ArgumentNullException(nameof(actionReference))

Error message

ArgumentNullException(nameof(actionReference))

What it means

The non-generic EventSubscription constructor stores an IDelegateReference that must wrap a System.Action. It throws ArgumentNullException when actionReference is null and ArgumentException (Resources.InvalidDelegateRerefenceTypeException) when the reference's Target is not an Action.

Solutions

  1. Pass a DelegateReference whose Target is a System.Action
  2. Verify the delegate type before creating the DelegateReference
  3. Use the public PubSubEvent.Subscribe API instead of constructing EventSubscription manually

Example fix

// before
var sub = new EventSubscription(new DelegateReference((Func<int>)() => 1, false));
// after
var sub = new EventSubscription(new DelegateReference((Action)(() => { }), false));
Defensive patterns

Strategy: validation

Validate before calling

if (actionReference?.Target is Action) { var sub = new EventSubscription(actionReference); }

Type guard

bool IsValidActionRef(IDelegateReference r) => r?.Target is Action;

Try / catch

try { var sub = new EventSubscription(actionRef); }
catch (ArgumentException ex) { log.Error($"actionReference target must be Action: {ex.Message}"); }
catch (ArgumentNullException) { log.Error("actionReference was null"); }

Prevention

When it happens

Trigger: new EventSubscription(null); or new EventSubscription(new DelegateReference(someFuncOrNonActionDelegate, false)) where the wrapped target is a Func, Predicate, or custom delegate rather than Action.

Common situations: Building custom PubSubEvent subscription internals; passing a DelegateReference created for a differently-typed delegate after refactoring the event payload/signature.

Understand the failure class

Background: Type mismatch errors: IllegalArgumentException, TypeError and type guards across 150 open-source libraries — this error's family across 150 libraries.

Related errors

AI-assisted analysis of PrismLibrary/Prism@358118cd64 (2026-09-15). Data as JSON: /api/errors/1bb6214175710b62. Report an issue: GitHub.

Appendix: source

Thrown at src/Prism.Events/EventSubscription.cs:24

{
    /// <summary>
    /// Provides a way to retrieve a <see cref="Delegate"/> to execute an action depending
    /// on the value of a second filter predicate that returns true if the action should execute.
    /// </summary>
    public class EventSubscription : IEventSubscription
    {
        private readonly IDelegateReference _actionReference;

        ///<summary>
        /// Creates a new instance of <see cref="EventSubscription"/>.
        ///</summary>
        ///<param name="actionReference">A reference to a delegate of type <see cref="System.Action"/>.</param>
        ///<exception cref="ArgumentNullException">When <paramref name="actionReference"/> or <see paramref="filterReference"/> are <see langword="null" />.</exception>
        ///<exception cref="ArgumentException">When the target of <paramref name="actionReference"/> is not of type <see cref="System.Action"/>.</exception>
        public EventSubscription(IDelegateReference actionReference)
        {
            if (actionReference == null)
                throw new ArgumentNullException(nameof(actionReference));
            if (!(actionReference.Target is Action))
                throw new ArgumentException(string.Format(CultureInfo.CurrentCulture, Resources.InvalidDelegateRerefenceTypeException, typeof(Action).FullName), nameof(actionReference));

            _actionReference = actionReference;
        }

        /// <summary>
        /// Gets the target <see cref="System.Action"/> that is referenced by the <see cref="IDelegateReference"/>.
        /// </summary>
        /// <value>An <see cref="System.Action"/> or <see langword="null" /> if the referenced target is not alive.</value>
        public Action Action
        {
            get { return (Action)_actionReference.Target; }
        }

        /// <summary>
        /// Gets or sets a <see cref="SubscriptionToken"/> that identifies this <see cref="IEventSubscription"/>.
        /// </summary>

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