'Open New Project. Add 2 text Box on it.
'Write down the codes below in the form

Private Sub Form_Load()
Text1.Text = "0123456789"
Text2.Text = "abcdef"
End Sub

Private Sub Text1_GotFocus()
Text1.SelStart = 0
Text1.SelLength = Len(Text1.Text)
End Sub

Private Sub Text2_GotFocus()
Text2.SelStart = 0
Text2.SelLength = Len(Text2.Text)
End Sub

'Click Text box 1 and Text Box 2 alternately.You can see that everytime the textbox get focus
'All character inside will be selected.