WPF中如何将ViewModel数据传入ValidationRule？我的实现为何返回Null？

我是C#和WPF编程新手，现有两个TextBox用于用户输入验证：

<TextBox
    x:Name="TextBox1">
    <TextBox.Text>
        <Binding
            NotifyOnValidationError="True"
            Path="String1"
            UpdateSourceTrigger="PropertyChanged"
            ValidatesOnDataErrors="True">
            <Binding.ValidationRules>
                <local:ValidationRule1 ValidatesOnTargetUpdated="True" />
            </Binding.ValidationRules>
        </Binding>
    </TextBox.Text>
</TextBox>
<TextBox
    x:Name="TextBox2">
    <TextBox.Text>
        <Binding
            NotifyOnValidationError="True"
            Path="Int1"
            UpdateSourceTrigger="PropertyChanged"
            ValidatesOnDataErrors="True">
            <Binding.ValidationRules>
                <local:ValidationRule2 ValidatesOnTargetUpdated="True" />
            </Binding.ValidationRules>
        </Binding>
    </TextBox.Text>
</TextBox>

TextBox1用于输入字符串，TextBox2用于输入整数。ValidationRule1仅验证TextBox1不为空。我希望通过ValidationRule2检查两个TextBox的组合值是否已存在于数据库中，因此需要将TextBox1的字符串传递给ValidationRule2，尝试用Wrapper和BindingProxy实现后，ValidationRule2中的String1属性始终为Null，请问哪里出错了？

我的实现代码如下：

Wrapper代码 #

public class Wrapper : DependencyObject
{
    public static readonly DependencyProperty NewSitzeinteilungSachnummerProperty = DependencyProperty.Register("NewSitzeinteilungSachnummer", typeof(string), typeof(Wrapper));

    public string NewSitzeinteilungSachnummer
    {
        get { return (string)GetValue(NewSitzeinteilungSachnummerProperty); }
        set { SetValue(NewSitzeinteilungSachnummerProperty, value); }
    }
}

BindingProxy代码 #

public class BindingProxy : Freezable
{
    protected override Freezable CreateInstanceCore()
    {
        return new BindingProxy();
    }

    public object Data
    {
        get { return (object)GetValue(DataProperty); }
        set { SetValue(DataProperty, value); }
    }

    public static readonly DependencyProperty DataProperty =
        DependencyProperty.Register("Data", typeof(object), typeof(BindingProxy), new PropertyMetadata());
}

调整后的XAML #

<TextBox
    x:Name="TextBox2">
    <TextBox.Resources>
        <local2:BindingProxy x:Key="proxy" Data="{Binding}"/>
    </TextBox.Resources>
    <TextBox.Text>
        <Binding
            NotifyOnValidationError="True"
            Path="Int1"
            UpdateSourceTrigger="PropertyChanged"
            ValidatesOnDataErrors="True">
            <Binding.ValidationRules>
                <local1:ValidationRule2 ValidatesOnTargetUpdated="True" >
                    <local1:ValidationRule2.Wrapper>
                        <local1:Wrapper String1="{Binding String1, Source={StaticResource proxy}}"/>
                    </local1:ValidationRule2.Wrapper>
                </local1:ValidationRule2>
            </Binding.ValidationRules>
        </Binding>
    </TextBox.Text>
</TextBox>

1. Wrapper依赖属性与CLR属性不匹配 #

你定义的Wrapper依赖属性是NewSitzeinteilungSachnummerProperty，对应CLR属性是NewSitzeinteilungSachnummer，但XAML中绑定的是String1，属性名完全不对应，导致绑定失效，值始终为Null。

修正后的Wrapper代码：

public class Wrapper : DependencyObject
{
    public static readonly DependencyProperty String1Property = 
        DependencyProperty.Register(nameof(String1), typeof(string), typeof(Wrapper));

    public string String1
    {
        get { return (string)GetValue(String1Property); }
        set { SetValue(String1Property, value); }
    }
}

2. BindingProxy的作用域与绑定路径错误 #

你把BindingProxy放在TextBox的Resources中，此时{Binding}的DataContext是TextBox本身，而非ViewModel，导致Proxy的Data属性无法获取到ViewModel实例。另外，绑定路径应该指向Proxy的Data.String1，而非直接String1。

修正后的BindingProxy放置位置（放在Window/UserControl的Resources中，确保DataContext是ViewModel）：

<Window.Resources>
    <local:BindingProxy x:Key="viewModelProxy" Data="{Binding}"/>
</Window.Resources>

修正后的TextBox2 XAML：

<TextBox x:Name="TextBox2">
    <TextBox.Text>
        <Binding
            NotifyOnValidationError="True"
            Path="Int1"
            UpdateSourceTrigger="PropertyChanged"
            ValidatesOnDataErrors="True">
            <Binding.ValidationRules>
                <local:ValidationRule2 ValidatesOnTargetUpdated="True" >
                    <local:ValidationRule2.Wrapper>
                        <local:Wrapper String1="{Binding Data.String1, Source={StaticResource viewModelProxy}}"/>
                    </local:ValidationRule2.Wrapper>
                </local:ValidationRule2>
            </Binding.ValidationRules>
        </Binding>
    </TextBox.Text>
</TextBox>

3. ValidationRule2的Wrapper属性定义 #

确保ValidationRule2正确暴露Wrapper属性，以便绑定生效：

public class ValidationRule2 : ValidationRule
{
    public Wrapper Wrapper { get; set; }

    public override ValidationResult Validate(object value, CultureInfo cultureInfo)
    {
        if (Wrapper == null || string.IsNullOrEmpty(Wrapper.String1))
        {
            return new ValidationResult(false, "TextBox1的值未传入");
        }

        string stringValue = Wrapper.String1;
        int? intValue = int.TryParse(value.ToString(), out int result) ? result : null;

        // 替换为实际数据库检查逻辑
        bool isDuplicate = CheckCombinationExists(stringValue, intValue);

        if (isDuplicate)
        {
            return new ValidationResult(false, "该组合值已存在于数据库中");
        }

        return ValidationResult.ValidResult;
    }

    private bool CheckCombinationExists(string str, int? num)
    {
        // 实现数据库查询逻辑
        return false;
    }
}

内容的提问来源于stack exchange，提问作者user25180321