How do I implement a TextChanging property on a TextBox? - vb.net

Question

I figured there would be a question like this already, but I didn't have any luck searching. I saw a question where someone asked the same thing but the answer was to use the TextChanged event. That's not what I want though.
TextBox has an event for TextChanged that occurs after the Text property has been changed. I need my control to raise an event before the Text property is actually changed to validate data. If it is valid the Text can be changed, if it is not valid the Text does not get changed.
Here's what I tried:
Public Class TextChangingEventArgs
Inherits System.ComponentModel.CancelEventArgs
Private p_sNewValue As String
Public Sub New()
p_sNewValue = String.Empty
End Sub
Public Sub New(sNewValue As String)
p_sNewValue = sNewValue
End Sub
Public Property NewValue As String
Get
Return p_sNewValue
End Get
Set(value As String)
p_sNewValue = value
End Set
End Property
End Class
Public Class BetterTextBox
Inherits TextBox
Public Event TextChanging(sender As Object, e As TextChangingEventArgs)
Public Overrides Property Text As String
Get
Return MyBase.Text
End Get
Set(value As String)
Dim e As New TextChangingEventArgs(value)
RaiseEvent TextChanging(Me, e)
If e.Cancel = False Then
MyBase.Text = value
End If
End Set
End Property
End Class
My in my Form I handle the TextChanging event:
Private Sub BetterTextBox1_TextChanging(sender As System.Object, e As TextChangingEventArgs) Handles BetterTextBox1.TextChanging
e.Cancel = Not Regex.IsMatch(e.NewValue, REGEX_PATTERN)
End Sub
This works for programmatically setting the Text value of the BetterTextBox control, but it does not work when you are typing into the text box.
Does anyone know what I need to do to get this to work?

Answer 1

Since you are already inheriting from TextBox, you can override WndProc and check for paste messages. This should resolve the right-click > Paste problem. You could then handle "regular," typed text in KeyPressed or KeyDown, as others have suggested.
Here's an example of what I'm talking about:
Private Const WM_PASTE As Integer = &H302
Protected Overrides Sub WndProc(ByRef m As Message)
If m.Msg = WM_PASTE AndAlso Clipboard.ContainsText() AndAlso ShouldAllowPaste(Clipboard.GetText()) Then
MyBase.WndProc(m)
ElseIf m.Msg <> WM_PASTE Then
MyBase.WndProc(m)
End If
End Sub

Answer 2

Try this:
Protected Overrides Sub WndProc(ByRef m As System.Windows.Forms.Message)
If m.Msg = 8465 Then
If HIWORD(m.WParam) = 1024 Then
' raise your TextChanging event
End If
End If
MyBase.WndProc(m)
End Sub
Public Function HIWORD(ByVal n As Integer) As UInteger
Return CUInt(n >> 16) And CUInt(&HFFFF)
End Function

Answer 3

I would suggest you should try with keydown event. It occurs before textchanged or even before
keypress.

Answer 4

Try the KeyPress event of the Textbox instead.
EDIT : to disable right-click in the Textbox, you can do something like this.
Put this code inside the Textbox's MouseDown event.
If e.Button = MouseButtons.Right Then
MessageBox.Show("can't paste!")
Exit Sub
End If
however I noticed you could still paste text using the key combination Ctrl + V. You can write this code to stop that in the KeyPress event.
If Keys.ControlKey And Keys.V Then
MessageBox.Show("can't paste!")
Exit Sub
End If