如何根据内容而不是宽度等属性在WPF网格上触发ValidationRule？

<Grid>
    <Grid.Resources>
        <Style TargetType="TextBox">
            <Setter Property="Validation.ErrorTemplate">
                <Setter.Value>
                    <ControlTemplate>
                        <StackPanel>
                            <AdornedElementPlaceholder x:Name="placeholder"/>
                            <TextBlock Text="{Binding [0].ErrorContent}" Foreground="Red" Margin="5"/>
                        </StackPanel>
                    </ControlTemplate>
                </Setter.Value>
            </Setter>
        </Style>
    </Grid.Resources>
    
    <TextBox Text="{Binding UserName, UpdateSourceTrigger=PropertyChanged, ValidatesOnDataErrors=True}">
        <TextBox.Text>
            <Binding Path="UserName" UpdateSourceTrigger="PropertyChanged">
                <Binding.ValidationRules>
                    <local:CustomValidationRule/>
                </Binding.ValidationRules>
            </Binding>
        </TextBox.Text>
    </TextBox>
</Grid>

public class CustomValidationRule : ValidationRule
{
    public override ValidationResult Validate(object value, CultureInfo cultureInfo)
    {
        string input = value as string;

        if (string.IsNullOrEmpty(input))
        {
            return new ValidationResult(false, "输入不能为空");
        }

        // 根据内容进行其他验证逻辑

        return ValidationResult.ValidResult;
    }
}

public class ViewModel : INotifyPropertyChanged, IDataErrorInfo
{
    private string userName;
    public string UserName
    {
        get { return userName; }
        set
        {
            userName = value;
            OnPropertyChanged(nameof(UserName));
        }
    }

    // 实现其他属性和接口成员...

    public event PropertyChangedEventHandler PropertyChanged;
    protected virtual void OnPropertyChanged(string propertyName)
    {
        PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));
    }

    public string Error => throw new NotImplementedException();

    public string this[string columnName]
    {
        get
        {
            if (columnName == nameof(UserName))
            {
                if (string.IsNullOrEmpty(UserName))
                {
                    return "输入不能为空";
                }
                // 根据内容进行其他验证逻辑
            }

            return null;
        }
    }
}