[ValueConversion(typeof(double), typeof(double))]
public class DecreaseDoubleConverter : ValueGenericConverter<double>
{
   protected override Func<object, CultureInfo, double> ConvertMethod =>
   (value, culture) =>
   {
      if (value == null)
         return 0;
      var result = System.Convert.ToDouble(value);
      return double.IsNaN(result) ? 0 : result;
   };
}

public abstract class ValueGenericConverter<T> : BaseGenericConverter<T>, IValueConverter where T : new()
{
    public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
    {
        Debug.Assert(targetType.IsAssignableFrom(typeof(T)), string.Format("targetType should be {typeof(T).FullName}"));

        var tResult = new T();
        if (!Convert(value, culture, ref tResult))
            return DependencyProperty.UnsetValue;

        ApplyParameter(parameter, culture, ref tResult);
        return System.Convert.ChangeType(tResult, targetType);
    }
}

public abstract class BaseGenericConverter<T>
{
   protected abstract Func<object, CultureInfo, T> ConvertMethod { get; }
   protected Func<T, T, T> ApplyParameterMethod = null;
}