TextBox uses the ExceptionValidationRule and UpdateSourceExceptionFilter handler : TextBox « Windows Presentation Foundation « VB.Net Tutorial

1. VB.Net Tutorial
2. Windows Presentation Foundation
3. TextBox

<Window xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
  xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
  xmlns:c="clr-namespace:WpfApplication1"
  x:Class="WpfApplication1.Window1">
    <Window.Resources>
        <c:MyDataSource x:Key="ods"/>
        <ControlTemplate x:Key="validationTemplate">
            <DockPanel>
                <TextBlock Foreground="Red" FontSize="20">!</TextBlock>
                <AdornedElementPlaceholder/>
            </DockPanel>
        </ControlTemplate>
        <Style x:Key="textBoxInError" TargetType="{x:Type TextBox}">
            <Style.Triggers>
                <Trigger Property="Validation.HasError" Value="true">
                    <Setter Property="ToolTip"
              Value="{Binding RelativeSource={x:Static RelativeSource.Self},Path=(Validation.Errors)[0].ErrorContent}"/>
                </Trigger>
            </Style.Triggers>
        </Style>
    </Window.Resources>
    <StackPanel>
        <TextBox Name="textBox3" Width="50" FontSize="15"
             Validation.ErrorTemplate="{StaticResource validationTemplate}"
             Style="{StaticResource textBoxInError}">
            <TextBox.Text>
                <Binding Path="Age3" Source="{StaticResource ods}" UpdateSourceTrigger="PropertyChanged">
                    <Binding.ValidationRules>
                        <ExceptionValidationRule/>
                    </Binding.ValidationRules>
                </Binding>
            </TextBox.Text>
        </TextBox>

        <CheckBox Name="cb" Checked="UseCustomHandler" Unchecked="DisableCustomHandler">Enable Custom Handler (see ToolTip)</CheckBox>
    </StackPanel>
</Window>

//File:Window.xaml.vb
Imports System
Imports System.ComponentModel
Imports System.Windows
Imports System.Windows.Controls
Imports System.Windows.Documents
Imports System.Windows.Navigation
Imports System.Windows.Shapes
Imports System.Windows.Data
Imports System.Globalization
Imports System.Collections.ObjectModel

Namespace WpfApplication1
  Public Partial Class Window1
    Inherits Window
    Public Sub New()
      InitializeComponent()
    End Sub

    Private Sub UseCustomHandler(sender As Object, e As RoutedEventArgs)
      Dim myBindingExpression As BindingExpression = textBox3.GetBindingExpression(TextBox.TextProperty)
      Dim myBinding As Binding = myBindingExpression.ParentBinding
      myBinding.UpdateSourceExceptionFilter = New UpdateSourceExceptionFilterCallback(AddressOf ReturnExceptionHandler)
      myBindingExpression.UpdateSource()
    End Sub

    Private Sub DisableCustomHandler(sender As Object, e As RoutedEventArgs)
      Dim myBinding As Binding = BindingOperations.GetBinding(textBox3, TextBox.TextProperty)
      myBinding.UpdateSourceExceptionFilter = DirectCast([Delegate].Remove(myBinding.UpdateSourceExceptionFilter, New UpdateSourceExceptionFilterCallback(AddressOf ReturnExceptionHandler)), UpdateSourceExceptionFilterCallback)
      BindingOperations.GetBindingExpression(textBox3, TextBox.TextProperty).UpdateSource()
    End Sub

    Private Function ReturnExceptionHandler(bindingExpression As Object, exception As Exception) As Object
      Return "This is from the UpdateSourceExceptionFilterCallBack."
    End Function
  End Class
  Public Class AgeRangeRule
    Inherits ValidationRule
    Private _min As Integer
    Private _max As Integer

    Public Sub New()
    End Sub

    Public Property Min() As Integer
      Get
        Return _min
      End Get
      Set
        _min = value
      End Set
    End Property

    Public Property Max() As Integer
      Get
        Return _max
      End Get
      Set
        _max = value
      End Set
    End Property

    Public Overrides Function Validate(value As Object, cultureInfo As CultureInfo) As ValidationResult
      Dim age As Integer = 0

      Try
        If DirectCast(value, String).Length > 0 Then
          age = Int32.Parse(DirectCast(value, [String]))
        End If
      Catch e As Exception
        Return New ValidationResult(False, "Illegal characters or " & e.Message)
      End Try

      If (age < Min) OrElse (age > Max) Then
        Return New ValidationResult(False, "Please enter an age in the range: " & Min & " - " & Max & ".")
      Else
        Return New ValidationResult(True, Nothing)
      End If
    End Function
  End Class

  Public Class MyDataSource
    Private _age As Integer
    Private _age2 As Integer
    Private _age3 As Integer

    Public Sub New()
      Age = 0
      Age2 = 0
    End Sub

    Public Property Age() As Integer
      Get
        Return _age
      End Get
      Set
        _age = value
      End Set
    End Property
    Public Property Age2() As Integer
      Get
        Return _age2
      End Get
      Set
        _age2 = value
      End Set
    End Property

    Public Property Age3() As Integer
      Get
        Return _age3
      End Get
      Set
        _age3 = value
      End Set
    End Property
  End Class
End Namespace

16.4.TextBox
	16.4.1.	You cannot use TextBox and Image at the same time for Button Content
	16.4.2.	Single line and Multiline TextBox
	16.4.3.	TextBox Column
	16.4.4.	Bind value to TextBox
	16.4.5.	An upside down TextBox
	16.4.6.	Style with Data Trigger for TextBox
	16.4.7.	TextBox with custom ErrorTemplate and ToolTip
	16.4.8.	TextBox uses the ExceptionValidationRule and UpdateSourceExceptionFilter handler
	16.4.9.	TextBox with UpdateSourceExceptionFilter handler
	16.4.10.	TextBox focus listener
	16.4.11.	TextBox MouseLeftButtonDown action and PreviewMouseLeftButtonDown action
	16.4.12.	Mark the text control as being changed to prevent any text content or selection changed events
	16.4.13.	Listen to TextBox text changed event
	16.4.14.	Handler for the PreviewKeyDown event on the TextBox
	16.4.15.	Format TextBox with MenuItem: normal, bold, italic
	16.4.16.	TextBox: set text, select all, clear, prepend, insert, append, cut, paste, undo
	16.4.17.	Set TextBox to editable
	16.4.18.	Scroll TextBox
	16.4.19.	Use TextBox.CommandBindingst to bind command
	16.4.20.	Use Dictionary to record which textbox has been changed and not saved
	16.4.21.	Set TextBox ContextMenu to null
	16.4.22.	TextBox Selection start, end and selected text
	16.4.23.	Scrollable TextBox Column
	16.4.24.	Check Spelling Error