Public Class MyTextBox : Inherits TextBox
     Public Sub New()
          AddHandler Me.KeyPress, AddressOf HandleKeyPress
     End Sub
     Private Sub HandleKeyPress(sender As Object, e As KeyPressEventArgs)
          If Char.IsLetter(e.KeyChar) AndALso Not Char.IsUpper(e.KeyChar) Then
               e.Handled = True
               Me.AppendText (Char.ToUpper(e.KeyChar)) 
               Mybase.OnKeyPress(new KeyPressEventArgs(Char.ToUpper(e.KeyChar)))
          End If
     End Sub
End Class

Dim t As New mytextbox
t.Parent = Me