Dim di As new DirectoryInfo(Server.MapPath("~/foto"))
If Not di.GetFiles(".jpg") Is Nothing Then
Dim totaleImmagini = di.GetFiles(".jpg").Length
End If

Partial Class mappe_prova_variabile 
Inherits System.Web.UI.Page 
Private Sub Page_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load 
Dim di As New IO.DirectoryInfo(Server.MapPath("../../public/allegati/foto")) 
If Not di.GetFiles("*.jpg") Is Nothing Then
Dim totale As Integer = a + di.GetFiles("*.jpg").Length 
Response.Write(totale) 
End If

End Sub 
End Class