EnableExplicit

Define EventID
Define Quit
Define a

If OpenWindow(0,0,0,500,250,"Window",#PB_Window_SystemMenu|#PB_Window_ScreenCentered)
  ListViewGadget(1,10,10,480,200,#PB_ListView_Multiselect)
  For a=1 To 10
    AddGadgetItem(1,-1,"Testitem "+Str(a),0,0)
  Next
  ButtonGadget(2,10,220,100,20,"Ermitteln",0)
  
Repeat
EventID=WaitWindowEvent()
If EventID=#PB_Event_Gadget
  Select EventGadget()
    Case 2
      MessageRequester("","Anzahl der ausgewählten Items: "+Str(SendMessage_(GadgetID(1),#LB_GETSELCOUNT,0,0)),0)
  EndSelect
EndIf
If EventID=#PB_Event_CloseWindow
  Quit=1
EndIf
Until Quit=1
EndIf