WPF中如何绑定Thickness.Left并实现厚度数值运算？

需求与问题 #

需要对Thickness进行数值运算，让Image的Margin.Left与TemplatedParent的Padding.Left呈相对负值关系（即Margin.Left = -Padding.Left）。尝试用ThicknessDirectionConverter提取Padding.Left的值，再通过ThicknessConverter进行运算，但没能实现预期效果。

现有代码 #

XAML代码 #

<Image.Margin>
    <Binding Path="Padding" RelativeSource="{RelativeSource TemplatedParent}"
            Converter="{StaticResource ThicknessConverter}">
        <Binding.ConverterParameter>
            <x:Array Type="System:Object">
                <Thickness Left="{TemplateBinding Padding,Converter={StaticResource ThicknessDirectionConverter},ConverterParameter='l'}" Top="0" Right="0" Bottom="0"/>
                <System:String>L</System:String>
            </x:Array>
        </Binding.ConverterParameter>
    </Binding>
</Image.Margin> 

ThicknessConverter代码 #

public class ThicknessConverter : IValueConverter
{
        public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
        {
            if (value is Thickness thickness && parameter is object[] parameters && parameters.Length == 2 && parameters[0] is Thickness offset && 
                    parameters[1] is string operations)
            {
                //l,t,r,b for positive
                //L,T,B,R for negative
                //use contains
                if (operations.Contains('l')) thickness.Left += offset.Left;
                else if (operations.Contains('L')) thickness.Left -= offset.Left;

                if (operations.Contains('t')) thickness.Top += offset.Top;
                else if (operations.Contains('T')) thickness.Top -= offset.Top;

                if (operations.Contains('r')) thickness.Right += offset.Right;
                else if (operations.Contains('R')) thickness.Right -= offset.Right;

                if (operations.Contains('b')) thickness.Bottom += offset.Bottom;
                else if (operations.Contains('B')) thickness.Bottom -= offset.Bottom;

                return thickness;
                }
                return Binding.DoNothing;
            }

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

}

ThicknessDirectionConverter代码 #

public class ThicknessDirectionConverter : IValueConverter
{
    public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
    {
        if(value is Thickness thickness && parameter is string par && par[0] is char direction)
        {
            //l,t,r,b
            switch (direction)
            {
                case 'l':
                    double left = (double)thickness.Left;
                    return left;
                case 't':
                    double left = (double)thickness.Top;
                    return left;
                case 'r':
                    double left = (double)thickness.Right;
                    return left;
                case 'b':
                    double left = (double)thickness.Bottom;
                    return left;
            }
        }

            return Binding.DoNothing;
        }

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

问题根源 #

1. ConverterParameter不支持绑定：XAML中Binding.ConverterParameter是静态值，无法通过TemplateBinding动态传递数据，所以Thickness.Left的绑定不会生效，offset.Left始终为0。
2. ThicknessDirectionConverter存在语法错误：所有switch分支都重复定义了double left变量，编译会直接报错。

解决方案 #

方案1：简化转换器逻辑（推荐） #

直接修改ThicknessConverter，让它可以直接对指定方向的Thickness值取负，无需额外的提取转换器：

public class ThicknessConverter : IValueConverter
{
    public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
    {
        if (value is Thickness sourceThickness && parameter is string direction)
        {
            var result = new Thickness(0);
            switch (direction.ToLower())
            {
                case "l":
                    result.Left = -sourceThickness.Left;
                    break;
                case "t":
                    result.Top = -sourceThickness.Top;
                    break;
                case "r":
                    result.Right = -sourceThickness.Right;
                    break;
                case "b":
                    result.Bottom = -sourceThickness.Bottom;
                    break;
                case "all":
                    result = new Thickness(-sourceThickness.Left, -sourceThickness.Top, -sourceThickness.Right, -sourceThickness.Bottom);
                    break;
            }
            return result;
        }
        return Binding.DoNothing;
    }

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

对应的XAML简化为：

<Image.Margin>
    <Binding Path="Padding" RelativeSource="{RelativeSource TemplatedParent}"
             Converter="{StaticResource ThicknessConverter}"
             ConverterParameter="l"/>
</Image.Margin>

方案2：修复原转换器结构 #

如果要保留原有两个转换器的设计，需要用MultiBinding传递动态值，代替不支持绑定的ConverterParameter：

修改后的XAML

<Image.Margin>
    <MultiBinding Converter="{StaticResource ThicknessConverter}">
        <Binding Path="Padding" RelativeSource="{RelativeSource TemplatedParent}"/>
        <Binding Path="Padding" RelativeSource="{RelativeSource TemplatedParent}"
                 Converter="{StaticResource ThicknessDirectionConverter}"
                 ConverterParameter="l"/>
        <Binding Source="L"/>
    </MultiBinding>
</Image.Margin>

修改后的ThicknessConverter（改为IMultiValueConverter）

public class ThicknessConverter : IMultiValueConverter
{
    public object Convert(object[] values, Type targetType, object parameter, CultureInfo culture)
    {
        if (values.Length == 3 && values[0] is Thickness thickness && values[1] is double leftValue && values[2] is string operations)
        {
            var offset = new Thickness(leftValue, 0, 0, 0);
            if (operations.Contains('L')) thickness.Left -= offset.Left;
            return thickness;
        }
        return Binding.DoNothing;
    }

    public object[] ConvertBack(object value, Type[] targetTypes, object parameter, CultureInfo culture)
    {
        throw new NotImplementedException();
    }
}

修复ThicknessDirectionConverter的变量错误

public class ThicknessDirectionConverter : IValueConverter
{
    public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
    {
        if(value is Thickness thickness && parameter is string par && par[0] is char direction)
        {
            switch (direction)
            {
                case 'l':
                    return thickness.Left;
                case 't':
                    return thickness.Top;
                case 'r':
                    return thickness.Right;
                case 'b':
                    return thickness.Bottom;
            }
        }
        return Binding.DoNothing;
    }

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

内容的提问来源于stack exchange，提问作者Salmy