Avalonia/samples/ControlCatalog/Converter/MathSubtractConverter.cs

using System;
using System.Globalization;
using Avalonia.Data.Converters;

namespace ControlCatalog.Converter;

public class MathSubtractConverter : IValueConverter
{
    public object Convert(object? value, Type targetType, object? parameter, CultureInfo culture)
    {
        if (value is double dv && parameter is double dp)
        {
            return dv - dp;
        }
        return double.NaN;
    }

    public object ConvertBack(object? value, Type targetType, object? parameter, CultureInfo culture)
    {
        throw new NotSupportedException();
    }
}