ToolTip With Binding

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<Window x:Class="WpfApplication1.Window1"
  xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
  xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
  xmlns:local="clr-namespace:WpfApplication1" 
  Title="WithBinding" Height="135" Width="200">
  <Window.Resources>
    <local:Employee x:Key="Tom" Name="Tom" Age="11" />
    <local:AgeToForegroundConverter x:Key="ageConverter" />
    <local:Base16Converter x:Key="Base16Converter" />
  </Window.Resources>
  <StackPanel DataContext="{StaticResource Tom}">
    <TextBlock>Name:</TextBlock>
    <TextBox Text="{Binding Path=Name}" />
    <TextBox Text="{Binding Path=Name, Source={StaticResource Tom}}" />
    <TextBlock>Age:</TextBlock>
    <TextBox>
      <TextBox.Text>
        <Binding Path="Age" />
      </TextBox.Text>
    </TextBox>
    <TextBox Name="ageTextBox" Text="{Binding Path=Age}"
             Foreground="{Binding Path=Age, Converter={StaticResource ageConverter}}" />
    <TextBox Text="{Binding Path=Age, Converter={StaticResource Base16Converter}}"
             Foreground="{Binding Path=Age, Converter={StaticResource ageConverter}}" />
    <TextBox Name="ageTextBoxOneWay" Foreground="{Binding Path=Age, Mode=OneWay,Source={StaticResource Tom}, Converter={StaticResource ageConverter}}"
      ToolTip="{Binding RelativeSource={RelativeSource Self}, Path=(Validation.Errors)&#91;0&#93;.ErrorContent}">
      <TextBox.Text>
        <Binding Path="Age" UpdateSourceTrigger="PropertyChanged">
          <Binding.ValidationRules>
            <local:NumberRangeRule Min="0" Max="128" />
          </Binding.ValidationRules>
        </Binding>
      </TextBox.Text>
    </TextBox>
    <Button Name="birthdayButton" Foreground="{Binding Path=Foreground, ElementName=ageTextBox}">Birthday</Button>
  </StackPanel>
</Window>
//File:Window.xaml.cs

using System;
using System.Collections.Generic;
using System.Text;
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;
using System.ComponentModel;
using System.Diagnostics;


namespace WpfApplication1 {

  public class Employee : INotifyPropertyChanged {

    public event PropertyChangedEventHandler PropertyChanged;
    protected void Notify(string propName) {
      if( this.PropertyChanged != null ) {
        PropertyChanged(this, new PropertyChangedEventArgs(propName));
      }
    }

    string name;
    public string Name {
      get { return this.name; }
      set {
        this.name = value;
        Notify("Name");
      }
    }

    int age;
    public int Age {
      get { return this.age; }
      set {
        this.age = value;
        Notify("Age");
      }
    }

    public Employee() { }
    public Employee(string name, int age) {
      this.name = name;
      this.age = age;
    }
  }

  public partial class Window1 : Window {
    public Window1() {
      InitializeComponent();
      this.birthdayButton.Click += birthdayButton_Click;
    }
    void birthdayButton_Click(object sender, RoutedEventArgs e) {
      Employee emp = (Employee)this.FindResource("Tom");
      ++emp.Age;
      Console.WriteLine(emp.Name);
      Console.WriteLine(emp.Age);
    }
  }
  [ValueConversion(/* sourceType */ typeof(int), /* targetType */ typeof(Brush))]
  public class AgeToForegroundConverter : IValueConverter {
    public object Convert(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture) {
      if( targetType != typeof(Brush) ) { return null; }
      int age = int.Parse(value.ToString());
      return (age > 60 ? Brushes.Red : Brushes.Black);
    }

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

  public class Base16Converter : IValueConverter {
    public object Convert(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture) {
      return ((int)value).ToString("x");
    }

    public object ConvertBack(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture) {
      return int.Parse((string)value, System.Globalization.NumberStyles.HexNumber);
    }
  }

  public class NumberRangeRule : ValidationRule {
    int _min;
    public int Min {
      get { return _min; }
      set { _min = value; }
    }

    int _max;
    public int Max {
      get { return _max; }
      set { _max = value; }
    }

    public override ValidationResult Validate(object value, System.Globalization.CultureInfo cultureInfo) {
      int number;
      if( !int.TryParse((string)value, out number) ) {
        return new ValidationResult(false, "Invalid number format");
      }

      if( number < _min || number > _max ) {
        string s = string.Format("Number out of range ({0}-{1})", _min, _max);
        return new ValidationResult(false, s);
      }
      return ValidationResult.ValidResult;
    }
  }
}