dotnet/wpf · error · ArgumentException

SR.Format(SR.UnexpectedParameterType, o.GetType()…

Error message

SR.Format(SR.UnexpectedParameterType, o.GetType(), typeof(Setter))

What it means

TriggerActionCollection (Trigger) validates that items added to a TriggerActionCollection are Setter instances via CheckChildIsSetter. Passing any other object type throws ArgumentException with SR.UnexpectedParameterType naming the actual type and expected Setter type.

Solutions

  1. Add only Setter instances to this collection; use Trigger.Setters for property setters.
  2. If you meant an action, use the correct collection (e.g. EventTrigger.Actions for TriggerAction objects).
  3. Check the object type before adding: if (item is Setter) collection.Add(item);

Example fix

// before
trigger.Setters.Add(new TriggerAction()); // throws
// after
trigger.Setters.Add(new Setter(UIElement.OpacityProperty, 0.5));
Defensive patterns

Strategy: type-guard

Validate before calling

if (item is not Setter) throw new ArgumentException($"Expected Setter, got {item?.GetType().Name}");

Type guard

static bool IsSetter(object o) => o is Setter;

Try / catch

try { collection.Add(item); }
catch (ArgumentException ex) { log.Error($"Wrong collection for {item.GetType().Name}: {ex.Message}"); }

Prevention

When it happens

Trigger: Adding a non-Setter (e.g. a TriggerAction, EventSetter in the wrong collection, or arbitrary object) to an EventTrigger/Trigger action collection, or calling the internal validation path with a wrong-typed item.

Common situations: Programmatic style construction where the wrong collection is used (Setters vs Actions); refactoring that moved setters into action collections.

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 dotnet/wpf@81131a70a4 (2026-09-14). Data as JSON: /api/errors/49c12a3cd9b8a54e. Report an issue: GitHub.

Appendix: source

Thrown at src/Microsoft.DotNet.Wpf/src/PresentationFramework/System/Windows/Trigger.cs:181

        ///</param>
        void IAddChild.AddText (string text)
        {
            // Verify Context Access
            VerifyAccess();

            XamlSerializerUtil.ThrowIfNonWhiteSpaceInAddText(text, this);
        }

        // Shared by PropertyTrigger, MultiPropertyTrigger, DataTrigger, MultiDataTrigger
        internal static Setter CheckChildIsSetter( object o )
        {
            ArgumentNullException.ThrowIfNull(o);

            Setter setter = o as Setter;

            if (setter == null)
            {
                throw new ArgumentException(SR.Format(SR.UnexpectedParameterType, o.GetType(), typeof(Setter)), nameof(o));
            }

            return setter;
        }

        internal sealed override void Seal()
        {
            if (IsSealed)
            {
                return;
            }

            if (_property != null)
            {
                // Ensure valid condition
                if (!_property.IsValidValue(_value))
                {
                    throw new InvalidOperationException(SR.Format(SR.InvalidPropertyValue, _value, _property.Name ));

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