It is me who posted the question. Since I use the tablet PC, so you can write on the combobox, even I already put a list of the number in the combobox, but if you type or using the pen to enter the information. The input will accept the characters.
One way of doing that would be to double-click the combobox. The "SelectedIndexChanged"-eventhandler will appear before you. In the right-hand corner of your screen you'll see a dropdownbox with the text: "SelectedInexChanged"). Click on the box and choose "TextChanged". The "TextChanged" eventhandler will now appear before you.
Now enter the following code (this is pseudo-code and you'll have to adjust it):
if not isNumeric(Combobox1.text) then
messageBox.show("Please enter eight or less numeric characters only")
Hope this helps 'cause I did it out the top of my head.
did you noted this a bit hiden functionalities ... for instance:
Private Shared Function IsCharacterAllowed(ByVal c As Char) As Boolean
If Char.IsControl(c) Then Return False
If Char.IsDigit(c) Then Return True
If Char.IsLetter(c) Then Return False 'Letters forbiden
If Char.IsSeparator(c) Then Return False
If Char.IsWhiteSpace(c) Then Return False
If Char.IsSymbol(c) Then Return False
If Char.IsPunctuation(c) Then Return True 'Comma is allowed