LykosAI/StabilityMatrix · error · ArgumentException

ConvertBack Target type

Error message

ConvertBack Target type {targetType.Name} must be assignable to {typeof(TSource).Name}

What it means

NullableDefaultNumericConverter.ConvertBack validates that the binding target property's type (targetType) is assignable to the converter's generic source type TSource before unboxing the value. If Avalonia supplies a target type that is not compatible with TSource, the converter cannot safely unbox the value back to TSource, so it throws ArgumentException immediately. This is a programming/declaration error in the binding, not a runtime data issue.

Solutions

Update the converter's generic type arguments in the XAML/Code declaration so TSource matches (or is a base of) the bound property's type.
Change the bound view model property type to one assignable to TSource.
Replace the converter with one whose TSource matches the actual target type used in the binding.

Example fix

<!-- before -->
<TextBlock Text="{Binding Count, Converter={StaticResource NullableDefaultNumericConverter}}" />
<!-- where converter is NullableDefaultNumericConverter<double, int> but Count is long -->

// after
public class CountConverter : NullableDefaultNumericConverter<long, int> { }
// now TSource (long) is assignable from the bound property type

Defensive patterns

Strategy: type-guard

Validate before calling

bool IsConverterCompatible<TSource>(Type targetType) => typeof(TSource).IsAssignableFrom(targetType);

Type guard

static bool TargetMatches<TSource>(Type? targetType) => targetType is not null && typeof(TSource).IsAssignableFrom(targetType);

Try / catch

try
{
    var value = converter.ConvertBack(visualValue, targetPropertyType, param, culture);
}
catch (ArgumentException ex)
{
    logger.LogError(ex, "Converter generic type does not match binding target type");
    return BindingOperations.DoNothing;
}

Prevention

Match the converter's generic TSource to the view model property type when adding bindings
Recheck converter generic arguments after refactoring property types
Keep one converter instance per source/target type pair to avoid misuse

When it happens

Trigger: Using NullableDefaultNumericConverter<TSource,TTarget> in a XAML/Code binding where the bound property's type is not assignable to TSource, e.g. binding ConvertBack against a non-nullable or unrelated numeric type, or swapping/reordering the generic arguments on the converter declaration.

Common situations: Refactoring a view model property type (int -> long or decimal) without updating the converter's generic arguments; copy-pasting a converter usage from a different property type; accidentally applying the converter to a non-numeric property.

Understand the failure class

Background: "is not a compatible type" / "cannot merge" errors: when a value's type doesn't match what the library requires — this error's family across 65 libraries.

Related errors

AI-assisted analysis of LykosAI/StabilityMatrix@af93d6ef57 (2026-09-12). Data as JSON: /api/errors/b31e6537174df81e. Report an issue: GitHub.

Appendix: source

Thrown at StabilityMatrix.Avalonia/Converters/NullableDefaultNumericConverter.cs:71

        {
            // ReSharper disable once LocalizableElement
            throw new ArgumentException(
                $"Convert Target type {targetType.Name} must be assignable to {typeof(TTarget).Name}"
            );
        }

        return (TTarget?)System.Convert.ChangeType(value, typeof(TTarget));
    }

    /// <summary>
    /// Convert a nullable value type to a value type
    /// </summary>
    public object ConvertBack(object? value, Type targetType, object? parameter, CultureInfo culture)
    {
        if (!targetType.IsAssignableTo(typeof(TSource)))
        {
            // ReSharper disable once LocalizableElement
            throw new ArgumentException(
                $"ConvertBack Target type {targetType.Name} must be assignable to {typeof(TSource).Name}"
            );
        }

        return Unbox((TTarget?)value);
    }

    public enum ReturnBehavior
    {
        DefaultValue,
        Throw
    }
}

View on GitHub (pinned to af93d6ef57)