Private Shared Sub IsFocused_Changed(ByVal d As DependencyObject, ByVal e As DependencyPropertyChangedEventArgs)Dim source As UIElement = DirectCast(d, UIElement)If CBool(e.NewValue) Thensource.OnGotFocus(New RoutedEventArgs(UIElement.GotFocusEvent, source))Elsesource.OnLostFocus(New RoutedEventArgs(UIElement.LostFocusEvent, source))End IfEnd SubPublic ReadOnly Property IsFocused As BooleanGetReturn CBool(MyBase.GetValue(UIElement.IsFocusedProperty))End GetEnd PropertyFriend Shared ReadOnly IsFocusedPropertyKey As DependencyPropertyKey = DependencyProperty.RegisterReadOnly("IsFocused", GetType(Boolean), GetType(UIElement), New PropertyMetadata(BooleanBoxes.FalseBox, New PropertyChangedCallback(AddressOf UIElement.IsFocused_Changed)))Public Custom Event LostFocus As RoutedEventHandlerAddHandler(ByVal value As RoutedEventHandler)Me.AddHandler(UIElement.LostFocusEvent, value)End AddHandlerRemoveHandler(ByVal value As RoutedEventHandler)Me.RemoveHandler(UIElement.LostFocusEvent, value)End RemoveHandlerEnd Event

    [SecurityCritical]private static void OnLostKeyboardFocusThunk(object sender, KeyboardFocusChangedEventArgs e){Invariant.Assert(!e.Handled, "Unexpected: Event has already been handled.");UIElement uiElement = sender as UIElement;if (uiElement != null){uiElement.OnLostKeyboardFocus(e);}else{ContentElement contentElement = sender as ContentElement;if (contentElement != null)contentElement.OnLostKeyboardFocus(e);else((UIElement3D) sender).OnLostKeyboardFocus(e);}}

    Private Sub TextBox1_LostFocus(ByVal sender As System.Object, ByVal e As System.Windows.RoutedEventArgs) Handles TextBox1.LostFocusIf Trim(TextBox1.Text).Equals("123") ThenTextBlock2.Text = "AAA"ElseTextBlock2.Text = String.EmptyMe.Dispatcher.BeginInvoke(DispatcherPriority.Render, New Action(Sub() TextBox1.Focus()))End IfEnd Sub