Hi everyone,
I'm a newbie .NET developer with vb6 experience. I was toying myself with
the Howl open source library for Zeroconf networks and I stumbled across an
exception I can't quite figure out completely.
The library is a .dll and i'm using PInvoke (<DllImport>) to declare the
functions in the library; I've only declared the following so far:
<DllImport("ren dezvous.dll")> Public Function sw_rendezvous_i nit(ByRef
session As IntPtr) As Integer
End Function
<DllImport("ren dezvous.dll")> Public Function sw_rendezvous_f ina(ByVal
session As IntPtr) As Integer
End Function
'sw_result
'sw_rendezvous_ browse_services (
' sw_rendezvous session,
' sw_const_string type,
' sw_const_string domain,
' sw_rendezvous_b rowse_handler handler,
' sw_rendezvous_b rowse_reply reply,
' sw_opaque extra,
' sw_rendezvous_b rowse_id *id)
<DllImport("ren dezvous.dll")> _
Public Function sw_rendezvous_b rowse_services( ByVal session As
IntPtr, _
ByVal type As String, _
ByVal domain As String, _
ByRef handler As IntPtr, _
ByVal reply As sw_rendezvous_b rowse_reply, _
ByRef extra As IntPtr, _
ByRef id As IntPtr) As Integer
End Function
'typedef sw_result
'(*sw_rendezvou s_browse_reply) (
' sw_rendezvous_b rowse_handler handler,
' sw_rendezvous rendezvous,
' sw_rendezvous_b rowse_id id,
' sw_rendezvous_b rowse_status status,
' sw_const_string name,
' sw_const_string type,
' sw_const_string domain,
' sw_opaque extra);
Public Delegate _
Function sw_rendezvous_b rowse_reply( _
ByVal handler As IntPtr, _
ByVal rendezvous As IntPtr, _
ByVal id As Integer, _
ByVal status As Integer, _
ByVal type As String, _
ByVal domain As String, _
ByVal extra As IntPtr) As Integer
I did some research in Howl's headers and found out that most of the custom
types are simply either pointers (sw_opaque is void*, sw_const_string is
char*) or simple handles (such as sw_rendezvous or
sw_rendezvous_b rowse_id). Now, I tried putting this code in a form to see
what happens:
Imports Howl
Public Class Form1
Inherits System.Windows. Forms.Form
[cut on wf designer's code, there are no controls on the form]
Private Sub Form1_Load(ByVa l sender As System.Object, ByVal e As
System.EventArg s) Handles MyBase.Load
Dim rv As IntPtr, id As IntPtr, Repl As sw_rendezvous_b rowse_reply
= AddressOf Something.myRep ly
If (Howl.sw_rendez vous_init(rv) = 0) Then
MsgBox(Howl.sw_ rendezvous_brow se_services(rv, _
"_http._tcp .", Nothing, _
Nothing, Repl, Nothing, id))
End If
End Sub
End Class
Module Something
Public Function myReply( _
ByVal handler As IntPtr, _
ByVal rendezvous As IntPtr, _
ByVal id As Integer, _
ByVal status As Integer, _
ByVal name As String, _
ByVal domain As String, _
ByVal extra As IntPtr) As Integer
Stop
Return 0
End Function
End Module
Now, I have a Mac connected to my LAN which allows me to produce
Zeroconf/Rendezvous services for testing. The good part of this snippet is
that the init works and gives an IntPtr back; also, the call to
sw_rendezvous_b rowse_services apparently works because it returns 0
(SW_OKAY) which is always correctly displayed by the MsgBox, and because
myReply gets called (VB stops at the "Stop"); i can even inspect the
contents of myReply's args and they contain the proper data in IntPtr,
Integer or String form ? the service name, the domain, everything.
The problem is that, immediatly after returning from myReply, the whole
thing goes down crashing and burning reporting a NullReferenceEx ception on
the "End If" line of Form1_Load!
I tried googling for a solution but I couldn't find any. What's wrong?
- e "theta" v
