codice:
    Public Function DaysInMonth%(ByVal MeseAnno$)
        Dim v$() = (MeseAnno.Split("/"c))
        Return DateTime.DaysInMonth(Int32.Parse(v(1)), Int32.Parse(v(0)))
    End Function
e la richiami così:
Dim s$ = Me.TextBox3.Text
MsgBox(DaysInMonth(s))