intenta con esto
Código vb.net:
Ver originaldim V as byte = 1
Private Sub TextBox1_Click(ByVal sender As Object, ByVal e As System.EventArgs) Handles TextBox1.Click
If V = 1 Then
V = 0
TextBox1.SelectAll()
End If
End Sub
Private Sub TextBox1_Enter(ByVal sender As Object, ByVal e As System.EventArgs) Handles TextBox1.Enter
TextBox1.SelectAll()
V = 0
End Sub
Private Sub TextBox1_Leave(ByVal sender As Object, ByVal e As System.EventArgs) Handles TextBox1.Leave
V = 1
End Sub
Private Sub TextBox1_GotFocus(ByVal sender As Object, ByVal e As System.EventArgs) Handles TextBox1.gotfocus
TextBox1.SelectAll()
End Sub