AvaloniaUI/Avalonia · error · ArgumentException

Object of type ' ' cannot be converted to type ' '.

Error message

Object of type '{value?.GetType()}' cannot be converted to type '{_property.PropertyType}'.

What it means

InpcPropertyAccessorPlugin.Accessor.SetValue writes a bound value by converting it to the property type via TypeUtilities.TryConvert. If conversion is impossible (no assignable or convertible relationship and no registered converter), it throws ArgumentException at binding-write time. This only fires on TwoWay/OneWayToSource bindings that actually push a value back.

Solutions

Add an IValueConverter that converts the source value to the target property type.
Make the source property assignable/convertible to the target type.
Use FallbackValue / TargetNullValue for null or unconvertible inputs.
Set the binding mode to OneWay if a write-back is not required.

Example fix

<!-- before -->
<Binding Path="Age"/>  <!-- Age is int, source is string -->
<!-- after -->
<Binding Path="Age" Converter="{StaticResource StringToIntConverter}"/>

Defensive patterns

Strategy: validation

Validate before calling

// Before writing back, check TypeUtilities can convert the value:
if (!TypeUtilities.TryConvert(targetPropertyType, value, null, out var _))
{
    // supply a converter or skip the write
}

Type guard

// Guard the source value against the target property type before binding:
static bool CanConvertTo(Type? sourceType, Type targetType) =>
    sourceType is null || targetType.IsAssignableFrom(sourceType) ||
    TypeUtilities.TryConvert(targetType, Activator.CreateInstance(sourceType)!, null, out _);

Try / catch

try { accessor.SetValue(value, priority); }
catch (ArgumentException ex) when (ex.Message.Contains("cannot be converted"))
{
    // log, use FallbackValue, or report a BindingNotification
}

Prevention

Provide an IValueConverter for non-assignable source/target type pairs.
Match source and target property types where possible.
Use FallbackValue/TargetNullValue for null or unconvertible inputs.
Prefer OneWay bindings when write-back conversion is uncertain.

When it happens

Trigger: Binding a string source to an int property where the string is not numeric; binding an enum to an incompatible type; null pushed into a non-nullable value-type property without a converter.

Common situations: Missing IValueConverter for a non-assignable type pair; culture-specific parse failures; null-to-value-type mismatches; OneWay binding accidentally set to TwoWay.

Related errors

AI-assisted analysis of AvaloniaUI/Avalonia@11c5427268 (2026-08-13). Data as JSON: /api/errors/5810aec165451743. Report an issue: GitHub.

Appendix: source

Thrown at src/Avalonia.Base/Data/Core/Plugins/InpcPropertyAccessorPlugin.cs:128

            }

            public override Type? PropertyType => _property.PropertyType;

            public override object? Value
            {
                get
                {
                    var o = GetReferenceTarget();
                    return o != null ? _property.GetValue(o) : null;
                }
            }

            public override bool SetValue(object? value, BindingPriority priority)
            {
                if (_property.CanWrite)
                {
                    if (!TypeUtilities.TryConvert(_property.PropertyType, value, null, out var converted))
                        throw new ArgumentException($"Object of type '{value?.GetType()}' " +
                            $"cannot be converted to type '{_property.PropertyType}'.");

                    _eventRaised = false;
                    _property.SetValue(GetReferenceTarget(), converted);

                    if (!_eventRaised)
                    {
                        SendCurrentValue();
                    }

                    return true;
                }

                return false;
            }

            void IWeakEventSubscriber<PropertyChangedEventArgs>.
                OnEvent(object? notifyPropertyChanged, WeakEvent ev, PropertyChangedEventArgs e)

View on GitHub (pinned to 11c5427268)