stride3d/stride · error · ArgumentException

The parameter of the ConvertBack method of this converter…

Error message

The parameter of the ConvertBack method of this converter must be a an instance of the Size structure.

What it means

SumSize.Convert's second validation throws ArgumentException when the converter parameter is not a System.Windows.Size; both operands must be Size so their Width/Height can be added. Like its sibling check, the message text says 'ConvertBack method' although it is raised inside Convert — a known copy/paste typo in the source.

Solutions

Set ConverterParameter to a Size, e.g. in XAML pass a value the parser creates as Size or construct it in code: new Size(w,h).
Create the parameter in code-behind or a resource with x:Type Size semantics.
Use the scalar-appropriate converter if the parameter is a plain number.

Example fix

// before
<MultiBinding Converter="{StaticResource SumSize}" ConverterParameter="10" />
// after
binding.ConverterParameter = new System.Windows.Size(10, 10);

Defensive patterns

Strategy: type-guard

Validate before calling

bool ok = parameter is System.Windows.Size;

Type guard

static bool IsSizeParameter(object parameter) => parameter is System.Windows.Size;

Try / catch

try { return sumSize.Convert(value, targetType, parameter, culture); }
catch (ArgumentException) { return Binding.DoNothing; }

Prevention

Set ConverterParameter to a Size instance, not a raw number
Create parameters in code-behind or via resources when XAML parsing is ambiguous
Use scalar converters for numeric parameters instead
Beware the message text mentions ConvertBack although thrown from Convert

When it happens

Trigger: Invoking SumSize.Convert (or configuring the converter in XAML) with ConverterParameter that is not a Size — e.g. ConverterParameter='10' (parsed as string/double), a Thickness, or a binding-supplied non-Size object.

Common situations: Supplying numeric XAML parameters expecting scalar addition; copying parameter style from NumericToThickness converters; passing a Thickness where a Size is expected.

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 stride3d/stride@96fad776d2 (2026-09-14). Data as JSON: /api/errors/1aec0bac8e1d9bba. Report an issue: GitHub.

Appendix: source

Thrown at sources/presentation/Stride.Core.Presentation.Wpf/ValueConverters/SumSize.cs:28

{
    /// <summary>
    /// This converter will sum a given <see cref="Size"/> with a <see cref="Size"/> passed as parameter. You can use the <see cref="MarkupExtensions.SizeExtension"/>
    /// markup extension to easily pass one, with the following syntax: {sd:Size (arguments)}. 
    /// </summary>
    [ValueConversion(typeof(Size), typeof(Size))]
    public class SumSize : ValueConverterBase<SumSize>
    {
        /// <inheritdoc/>
        [NotNull]
        public override object Convert(object value, Type targetType, object parameter, CultureInfo culture)
        {
            if (!(value is Size))
            {
                throw new ArgumentException("The value of the ConvertBack method of this converter must be a an instance of the Size structure.");
            }
            if (!(parameter is Size))
            {
                throw new ArgumentException("The parameter of the ConvertBack method of this converter must be a an instance of the Size structure.");
            }

            var sizeValue = (Size)value;
            var sizeParameter = (Size)parameter;
            var result = new Size(sizeValue.Width + sizeParameter.Width, sizeValue.Height + sizeParameter.Height);
            return result;
        }

        /// <inheritdoc/>
        [NotNull]
        public override object ConvertBack(object value, Type targetType, object parameter, CultureInfo culture)
        {
            if (!(value is Size))
            {
                throw new ArgumentException("The value of the ConvertBack method of this converter must be a an instance of the Size structure.");
            }
            if (!(parameter is Size))
            {

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