VB 2012 TextBox.Clear() not working - vb.net

Question

I'm working on an encryption program and it uses a "PIN" to calculate some stuff for the encryption. I have a textbox where the user can insert the "PIN". I'd like to prevent people from entering anything but numbers. I added this on the KeyPress event:
If Not Char.IsControl(e.KeyChar) Then
If Not Char.IsNumber(e.KeyChar) Then
MsgBox("Invalid character", , "WARNING!")
TextBox3.Clear()
End If
End If
It shows the msgbox and it doesn't write to the textbox until i close th emsgbox. The typed character appears in the textbox. When I write another one it works the same as before, but it only replaces the last character instead of writing another one. Is there something I'm missing because that looks like a bug to me?

Answer 1

Set the ES_NUMBER windows style for your TextBox:
Public Class Form1
Public Const GWL_STYLE As Integer = (-16)
Public Const ES_NUMBER As Integer = &H2000
Public Declare Function GetWindowLong Lib "user32" Alias "GetWindowLongA" _
(ByVal handle As IntPtr, ByVal nIndex As Integer) As Integer
Public Declare Function SetWindowLong Lib "user32" Alias "SetWindowLongA" _
(ByVal handle As IntPtr, ByVal nIndex As Integer, ByVal dwNewLong As Integer) As Integer
Public Sub SetNumbersOnlyTextBox(ByVal TB As TextBox)
SetWindowLong(TB.Handle, GWL_STYLE, GetWindowLong(TB.Handle, GWL_STYLE) Or ES_NUMBER)
End Sub
Private Sub Form1_Load(sender As System.Object, e As System.EventArgs) Handles MyBase.Load
SetNumbersOnlyTextBox(TextBox3)
End Sub
End Class

Answer 2

It shows the msgbox and it doesn't write to the textbox until i close th emsgbox.
Yes, that's what modal dialogs do. They block the caller from updates until closed. That's the point; the user cannot interact with the parent until they clear the modal child.
Why not simply clear the textbox first? Better yet; don't show an annoying dialog at all. Simply disallow the user from entering invalid characters by setting e.Handled to true. However, it's a bit trickier than it sounds as you need to allow for the backspace and delete keys, disable pasting, etc.
Here's an example of a NumericTextbox: http://msdn.microsoft.com/en-us/library/ms229644(v=vs.80).aspx

Answer 3

You just need to set the Handled property to true instead of clear:
e.Handled = True

Answer 4

As MarkPM notes above, if its a key you don't want you can set e.handle=true (as you intercept the key on the keypress event) to have the system eat it.
Along with this, in stead of a pop-up, you can have a label on the form that says "Only numbers can be entered here" or something like that. Set it up so that the color of the text is red. Also set it up so the label is not normally visible.
Finally, also in the keypress event, beyond setting e.handle=true for unwanted keys, when an unwanted key comes along make the label that says "Only numbers can be entered here" visible - you can also set up a timed event to turn the label's visibility off after a few seconds. You can also throw a Beep() into the mix if you like :-)
This is less invasive then a pop-up and moves things along nicely for the user.