Cannot access a disposed object in instance VB.net -

i have problem project, have 2 form, 1 form mdi container = true , 1 form child form. have code in parent form below

   public class frminduk     dim afrmdaftarbarang frmdaftarbarang = frmdaftarbarang.instance     private sub databarangtoolstripmenuitem_click(byval sender system.object, byval e system.eventargs) handles databarangtoolstripmenuitem.click          afrmdaftarbarang             .mdiparent = me             'dim afrmdaftarbarang frmdaftarbarang = frmdaftarbarang.instance             .show()             .focus()          end     end sub end class 

and than, in child form have code below

public class frmdaftarbarang private shared aninstance frmdaftarbarang public shared readonly property instance() frmdaftarbarang             if aninstance nothing             aninstance = new frmdaftarbarang          end if         return aninstance      end end property  end class 

when run project, no problem . project can run well. truble come when call frmdatabarang, , close, when call frmdatabarang again error lice picture bellow

i hope can me :(

you trying use singleton pattern, did imperfectly. mdi child form can created more once, have set singleton instance nothing when gets destroyed. formclosing event:

protected overrides sub onformclosed(byval e formclosedeventargs)     aninstance = nothing     mybase.onformclosed(e) end sub 

you need reasonable when existing instance displayed again. might not in right state. code really belongs in mdi parent, can limp along this:

            if aninstance nothing             aninstance = new frmdaftarbarang         else             if aninstance.windowstate = formwindowstate.minimized                 aninstance.windowstate = formwindowstate.normal             end if         end if         return aninstance 

do note gets ugly when use singleton things other child activation. rather bad idea use pattern. fine on mdiparent, there can ever 1 parent , when gets closed program terminates anyway. same rules don't apply child. move child instance management code parent class, that's belongs.