Changing File Extension

To test this code, make a form with two textboxes (leave the default names of Text1 and Text2). Paste the following code into the double-click event of Text2. Run the project and try it with various things (or nothing!) in Text1.

I've commented the code heavily, hope everything's clear.

Dim flag As Integer  'flag is used for the condition in the Do/Loop
Dim x As Integer
Dim temp As String

'x is being used as a "counter" variable
'we set its length here
x = Len(Text1)

'next line errortraps to make sure that it's not an
'empty textbox
If x > 0 Then

Do While Not flag 'as long as flag is False...

'next line picks out one character from the string.
'the first pass through the loop will get the last
'character in the string, the next pass will get
'the next-to-the-last, etc. because we decrement
'x by one in the next "If/End If" statement
temp = Mid$(Text1, x, 1)

If temp = "." Then
  flag = True
  x = x - 1
Else
  x = x - 1
End If

'the next line gracefully exits the loop if
'there's no period in Text1 at all
If x = 0 Then flag = True
Loop

'the next line first tests to make sure that
'there was a period found in Text1... if so
'it adds everything to the left of the period
'and ".ext" and puts the resultant string into
'Text2. You can remove the "If x > 0 Then" if
'you think there might be an "extensionless"
'filename in Text1.
If x > 0 Then Text2 = Left$(Text1, x) & ".ext"

End If