Silverlight textbox overrides

Question

I'm trying to get a textbox keydown event to trap the backspace key down event. I have that working by adding a Class that overrides the textbox. What i don't know how to do is have that communicate with the class where the textbox is in the user control.

When a user types in the text box... say abcd or backspace, i need to update something on the usercontrol. let's just say i want to have something that displays how many characters are in the textbox. can someone help me with that. Here is what i have so far

Option Strict On
Imports System.Text.RegularExpressions

Partial Public Class Page
    Inherits UserControl

    Public Sub New()
        InitializeComponent()
        Dim textbox As New MyTextBox() With {.Width = 300, .Height = 100}
        LayoutRoot.Children.Add(textbox)
    End Sub
End Class

Public Class MyTextBox
    Inherits TextBox
    Protected Overrides Sub OnKeyDown(ByVal e As KeyEventArgs)
        MyBase.OnKeyDown(e)
        If e.Key = Key.Back Then
            e.Handled = True
            MyBase.OnKeyDown(e)

        ElseIf e.Key = Key.Delete Then
            e.Handled = True
            MyBase.OnKeyDown(e)
        End If
    End Sub
End Class

thanks shannon

Answer 1

You may be thinking the wrong way. Since you're in Silverlight, you can get this information with binding expressions. Just as a simple example, you said you want to know how many characters are in a text box. You can achieve this with the following xaml:

    <TextBox x:Name="txtInput" />
    <TextBlock Text="{Binding ElementName=txtInput, Path=Text.Length}" />

Answer 2

You shouldn't need to subclass TextBox to do this. Instead, add handler for the TextBox.TextChanged event right in your UserControl class. When this is called, the sender of the event should be your TextBox. You can then get the text from it and do what you need to do.

Update: Based on the comment, the following should work:

Partial Public Class Page
    Inherits UserControl

    Private TextBox1 as TextBox

    Public Sub New()
        InitializeComponent()
        TextBox1 = New TextBox() With {.Width = 300, .Height = 100}
        LayoutRoot.Children.Add(textbox)
    End Sub

    Private Sub OnTextChanged(sender as Object, e as TextChangedEventArgs) Handles TextBox1.TextChanged
        If e.Key = Key.Back Then
            e.Handled = True
        ElseIf e.Key = Key.Delete Then
            e.Handled = True
        End If
    End Sub
End Class

(My VB is a bit rusty, so the event handler syntax might not be completely correct.)

The basic idea is to get notified when text changes in the TextBox within your UserControl. This way you can modify the other parts of the UserControl as necessary.