AvaloniaUI/Avalonia · error · InvalidOperationException

Transition has no property specified.

Error message

Transition has no property specified.

What it means

Thrown by Transition.Apply when the transition's Property has not been set. Apply needs a concrete AvaloniaProperty to bind the animated observable onto, so a null Property makes animation impossible. This is the internal override reached via the ITransition.Apply path when Property is null.

Source

Thrown at src/Avalonia.Base/Animation/Transition.cs:30

            PropertyProperty.Changed.AddClassHandler<Transition<T>>((x, e) => x.OnPropertyPropertyChanged(e));
        }

        private void OnPropertyPropertyChanged(AvaloniaPropertyChangedEventArgs e)
        {
            if ((e.NewValue is AvaloniaProperty newValue) && !newValue.PropertyType.IsAssignableFrom(typeof(T)))
                throw new InvalidCastException
                    ($"Invalid property type \"{typeof(T).Name}\" for this transition: {GetType().Name}.");
        }

        /// <summary>
        /// Apply interpolation to the property.
        /// </summary>
        internal abstract IObservable<T> DoTransition(IObservable<double> progress, T oldValue, T newValue);

        internal override IDisposable Apply(Animatable control, IClock clock, object? oldValue, object? newValue)
        {
            if (Property is null)
                throw new InvalidOperationException("Transition has no property specified.");

            var transition = DoTransition(new TransitionInstance(clock, Delay, Duration), (T)oldValue!, (T)newValue!);
            return control.Bind<T>((AvaloniaProperty<T>)Property, transition, Data.BindingPriority.Animation);
        }
    }
}

View on GitHub (pinned to 11c5427268)

Solutions

1. Set transition.Property = Control.PropertyNameField before applying or adding to a Transitions collection.
2. In XAML, include Property="Width" (or similar) on every <DoubleTransition>/<ColorTransition> element.
3. Validate Property is non-null in a debug assertion when building transitions dynamically.

Example fix

// before
var t = new DoubleTransition { Duration = TimeSpan.FromSeconds(0.3) };
control.Transitions = new Transitions { t }; // Property not set

// after
var t = new DoubleTransition { Duration = TimeSpan.FromSeconds(0.3), Property = Visual.OpacityProperty };

if (transition.Property is null)
    transition.Property = Visual.OpacityProperty; // set a real target

static bool IsTransitionReady(TransitionBase t) => t.Property is not null;

if (transition.Property is null) throw new InvalidOperationException("Set Property first");
control.Transitions!.Add(transition);

When it happens

Trigger: Creating a typed Transition<T> (e.g. DoubleTransition) and adding it to Transitions or calling Apply without setting its Property; defining a transition in XAML without a Property attribute.

Common situations: XAML transitions missing the 'Property=' setter; programmatic transitions built fluently where the Property call was forgotten; reusing a transition instance after clearing its Property.

Related errors

• Transition has no property specified.
• Cannot animate direct property {property} on {display}.
• Transition elements have different parents.
• Animator has no property specified.
• Animator has no property specified.

AI-assisted analysis of AvaloniaUI/Avalonia@11c5427268 (2026-08-13). Data as JSON: /api/errors/8e4c52be45c6dcbc. Report an issue: GitHub.