Hi all,

I'm having problems with a delegate used as a function pointer in a pinvoke method that takes a callback function.  When I supply a delegate, the CLR seems to just die on me.  By die, the symptom is that during my NUnit test, NUnit simply stops running and disappears.  No catchable exception seems to be thrown.  I'm haven't tried writing a quick console app to see if I can get more details on any exceptions thrown.

the sequence of events is that when I call the vendor unmanaged function, my callback function is called twice (I'm expecting about 15 calls total).  As I leave the function the second time, NUnit simply crashes (no error or anything).  If I pass in a null reference for the callback, everything works fine.  It looks some kind of weird corruption in the CLR or something, but I really don't know.

Things I've tried so far that didn't work:
- GC.KeepAlive on the callback delegate instance.  However, this shouldn't be needed (I don't think) as the calling class has a reference to it so it should not be collected
- Played with the marshaling behavior on the delegate signature with MarshalAs(UnmanagedType.LpStr)

If anyone has any ideas where I should be looking, I'd be very grateful.

Joe K.

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More details
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I'm creating a wrapper for a vendor C API using pinvoke.   The API in question takes a function pointer like so:
(all parameter and function names changed to protect the innocent)

int doSomeStuff(
const char * const string1,
const char * const string2,
const char * const string3,
const char * const string4,
const char * const string5,
void (*logCallback)(const char *),
bool bool1,
char **outData);

The signature for the callback function looks like this:

void doLog(const char *message)

It is essentially used for me to intercept trace messages so I can get detailed failure (or success) info.

My managed signatures look like this:
------------------------
<DllImport("vendor.dll", CharSet:=CharSet.Ansi)> _
Private Shared Function doSomeStuff( _
   ByVal string1 As String, _
   ByVal string2 As String, _
   ByVal string3 As String, _
   ByVal string4 As String, _
   ByVal string5 As String, _
   ByVal logCallback As VendorLogCallback, _
   ByVal bool1 As Boolean, _
   ByRef outData As IntPtr _
   ) As Integer
End Function

'note that outData is defined as IntPtr instead of using StringBuilder because I need to use
'the vendors function to free the memory and need the pointer later.  

Friend Delegate Sub VendorLogCallback(ByVal message As String)

'This is the wrapper function that calls the vendor API.  parameters and variable names changed again
Private Shared Function VendorDoStuff( _
   ByVal string1 As String, _
   ByVal string2 As String, _
   ByVal string3 As String, _
   ByVal string4 As String, _
   ByVal string5 As String, _
   ByVal callBack As VendorLogCallback, _
   ByVal message As StringBuilder _
   ) As String

   Dim returnValue As Integer
   Dim returnPtr As IntPtr = IntPtr.Zero
   Dim returnString As String

   returnValue = doSomeStuff(string1, string2, string3, string4, string5, callBack, True, returnPtr)

   If returnValue = 0 Then
If returnPtr.Equals(IntPtr.Zero) Then
   'API did not make a change
   returnString = string1
Else
   'API did make a change!
   returnString = Marshal.PtrToStringAnsi(returnPtr)
   freeReturnData(returnPtr)
End If
   Else
Throw New InvalidOperationException("Stuff didn't work.  I'd like to know what the callback logger said..."))
   End If

   Return returnString

End Function

'this is the method that is the delegate instance above.  _messages is a stringbuilder field in my class
Private Sub LogCallback(ByVal message As String)
   _messages.Append(message)
End Sub
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In the wrapper function, the callback delegate instance is an instance field in the containing class, so it has a root and should not be collected.  The StringBuilder that is passed in is the instance where the logger method sticks the data from callback function.