默认是当目标发生改变时候，通过绑定改变源时候进行校验，因为WPF认为源是安全的，如果想让源改变时候，也进行校验则设置验证规则的ValidatesOnTargetUpdated =true

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows;
using System.Windows.Controls;
using System.Windows.Data;
using System.Windows.Documents;
using System.Windows.Input;
using System.Windows.Media;
using System.Windows.Media.Imaging;
using System.Windows.Shapes;

namespace XamlTest
{
    /// <summary>
    /// Interaction logic for Window16.xaml
    /// </summary>
    public partial class Window16 : Window
    {
        public Window16()
        {
            InitializeComponent();
            Binding b = new Binding();
            b.Source= slider;
            b.Path = new PropertyPath("Value");
            b.UpdateSourceTrigger = UpdateSourceTrigger.PropertyChanged;
            RangeValidationRule vr = new RangeValidationRule();
            //vr.ValidatesOnTargetUpdated =true;
            b.NotifyOnValidationError = true;
            b.ValidationRules.Add(vr);
            txt.SetBinding(TextBox.TextProperty, b);
            txt.AddHandler(Validation.ErrorEvent,new RoutedEventHandler(this.ValidationError));
        }

private void ValidationError(object sender, RoutedEventArgs e)
        {
            if (Validation.GetErrors(txt).Count>0)
            {
                txt.ToolTip = Validation.GetErrors(txt)[0].ErrorContent.ToString();
            }
        }
    }
}