<Window x:Class="MainWindow"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:local="clr-namespace:WpfApplication1"
    Title="MainWindow" Height="350" Width="525">
    <Grid>
        <StackPanel>
            <TextBox Validation.Error="TextBox_Error" Name="TestBox" >
                <TextBox.Text>
                    <Binding Path="Test" NotifyOnValidationError="True"  >
                        <Binding.ValidationRules>
                            <local:ValiText />
                        </Binding.ValidationRules>
 
                    </Binding>
                </TextBox.Text>
            </TextBox>
            <Button  Height="25"/>
        </StackPanel>
 
    </Grid>
</Window>

Imports System.ComponentModel
 
Public Class MainWindow
    Implements INotifyPropertyChanged
 
#Region "PropChanged"
 
    Public Event PropertyChanged(ByVal sender As Object, ByVal e As _
      System.ComponentModel.PropertyChangedEventArgs) Implements _
      System.ComponentModel.INotifyPropertyChanged.PropertyChanged
 
    Private Sub OnPropChanged(ByVal pro As String)
        RaiseEvent PropertyChanged(Me, New PropertyChangedEventArgs(pro))
    End Sub
 
#End Region
 
    Sub New()
 
        ' Dieser Aufruf ist für den Designer erforderlich.
        InitializeComponent()
 
        ' Fügen Sie Initialisierungen nach dem InitializeComponent()-Aufruf 
        ' hinzu.
        Me.DataContext = Me
    End Sub
 
    Public Property Test As String
 
    Private Sub TextBox_Error(ByVal sender As System.Object, ByVal e As _
      System.Windows.Controls.ValidationErrorEventArgs)
        MsgBox(e.Action.ToString)
    End Sub
 
End Class
 
Public Class ValiText
    Inherits ValidationRule
 
    Public Overrides Function Validate(ByVal value As Object, ByVal cultureInfo _
      As System.Globalization.CultureInfo) As _
      System.Windows.Controls.ValidationResult
        Dim IntValue As Integer = 0
 
        Try
            IntValue = Convert.ToInt16(value)
        Catch generatedExceptionName As Exception
            Return New ValidationResult(False, "Please provide a number")
        End Try
 
        If IntValue < 0 Then
            Return New ValidationResult(False, "Please provide a number greater" & _
              "than 0")
        End If
 
        Return New ValidationResult(True, Nothing)
 
 
    End Function
End Class

Jedes Programm,
in das sich ein Fehler einschleichen kann,wird auch einen enthalten.