stride3d/stride · error · ArgumentException

The value of the ConvertBack method of this converter must…

Error message

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

What it means

SumSize.Convert adds two Size structures and validates that both the converted value and the parameter are instances of System.Windows.Size. The first check throws ArgumentException when the value is not a Size. Note the message text mistakenly says 'ConvertBack method' even though it is raised in Convert — a copy/paste artifact in the library.

Solutions

Pass System.Windows.Size instances for both value and parameter (e.g. bind to ActualSize-style properties of type Size).
Convert doubles into a Size before invoking the converter.
Use a different arithmetic converter for scalar types.

Example fix

// before
converter.Convert(element.Width, typeof(Size), new Size(1,1), culture); // double -> throws
// after
converter.Convert(new Size(element.Width, element.Height), typeof(Size), new Size(1,1), culture);

Defensive patterns

Strategy: type-guard

Validate before calling

bool ok = value is System.Windows.Size && parameter is System.Windows.Size;

Type guard

static bool IsSizePair(object value, object parameter) => value is System.Windows.Size && parameter is System.Windows.Size;

Try / catch

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

Prevention

Bind only properties of type Size through SumSize
Construct Size parameters explicitly in code or resources
Do not reuse SumSize for scalar Width/Height values
Beware the message text mentions ConvertBack although thrown from Convert

When it happens

Trigger: Calling SumSize.Convert with a value that is not a Size — e.g. a double, string, Vector, or null — while the parameter may or may not be valid.

Common situations: Binding an element's non-size property through SumSize; passing scalars expecting implicit conversion; reusing the converter for Width/Height (doubles) instead of full Size values.

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/a4a271d7de6e44a2. Report an issue: GitHub.

Appendix: source

Thrown at sources/presentation/Stride.Core.Presentation.Wpf/ValueConverters/SumSize.cs:24

using System.Windows.Data;
using Stride.Core.Annotations;

namespace Stride.Core.Presentation.ValueConverters
{
    /// <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))
            {

View on GitHub (pinned to 96fad776d2)