How to write a callback function in VB.NET called from C++ - c++

Question

I am working with a DLL written in C++ provided by an external source with documentation but no source code.
I need to instantiate a callback function in VB.NET that is to be called by the C++ module in certain cases. The documentation provided looks like;
Function I call to establish the call back;
extern "C" int EDIT_API Do_RunStuff(
const int MyID,
const char* my_tag1,
const char* my_tag2,
const char* some_data,
const int some_options,
int* some_count,
void* owner,
void* callback_func);
The template for the call back function is;
typedef void (*CallbackRunStuff)( void* ptr2object, const char* my_tag1,
const char* my_name, const char* some_code,
const char* error_type, const char* message );
So I created my VB.NET code;
Declare the function;
Declare Function Do_RunStuff Lib "SOME.DLL" (ByVal MyID As Integer, _
ByVal my_tag1 As String, _
ByVal my_tag2 As String, _
ByVal some_data As String, _
ByVal some_options As Integer, _
ByRef some_count As Integer, _
ByRef owner As IntPtr, _
ByRef callback_func As IntPtr) As Integer
Call the function;
Dim objEditsErrorHandler As New editsErrorHandlerDelegate(AddressOf editsErrorHandler)
Dim objThis As GCHandle = GCHandle.Alloc(Me)
Dim ptrEditsErrorHandler As IntPtr = Marshal.GetFunctionPointerForDelegate(objEditsErrorHandler)
Dim ptrThis As IntPtr = GCHandle.ToIntPtr(objThis)
intResult = Do_RunStuff(_AnID, strTag1, strTag2, strSomeData, someValue, SomeCount, ptrThis, ptrEditsErrorHandler)
Create delegate;
Public Delegate Sub editsErrorHandlerDelegate(ByRef objThis As IntPtr, ByVal strEditTag As String, ByVal strEditName As String, ByVal strAdminCode As String, ByVal strErrorType As String, ByVal strMessage As String)
Create Function;
Public Sub editsErrorHandler(ByRef objThis As IntPtr, ByVal strEditTag As String, ByVal strEditName As String, ByVal strAdminCode As String, ByVal strErrorType As String, ByVal strMessage As String)
Debug.Print("*****====================*****")
Debug.Print("Error Type: " & strErrorType)
Debug.Print("Message: " & strMessage)
End Sub
Everything works swimmingly, when no callback is needed. When it attempts to do a call back to editsErrorHandler the program aborts with;
An unhandled exception of type 'System.AccessViolationException' occurred in
I do not know where to go from here. The DLL's developer is a C++ person and does not have an answer.
Any Ideas?
EDIT 1
In the documentation they show how to call from C++
Call to module that may do call back;
FMyEngineClass->Do_RunStuff(my_tag1, my_tag2, some_data,
some_options, &some_count, this, ShowErrorMessages);
Definition of function that is called back;
static void ShowErrorMessages(void* caller, const char* my_tag1,
const char* some_name, const char* some_code,
const char* error_type, const char* message)

Answer 1

Well if anyone else finds this, my particular problem was one silly line;
<UnmanagedFunctionPointer(CallingConvention.Cdecl)>
That needs to go immediately before;
Public Delegate Sub editsErrorHandlerDelegate(ByRef objThis As IntPtr, ByVal strEditTag As String, ByVal strEditName As String, ByVal strAdminCode As String, ByVal strErrorType As String, ByVal strMessage As String)
like so;
<UnmanagedFunctionPointer(CallingConvention.Cdecl)>
Public Delegate Sub editsErrorHandlerDelegate(ByRef objThis As IntPtr, ByVal strEditTag As String, ByVal strEditName As String, ByVal strAdminCode As String, ByVal strErrorType As String, ByVal strMessage As String)

Answer 2

You don't need to explicitly use GCHandle to pass function pointers, and I seriously doubt you got the API declaration right (you seem to be getting away with it because you don't seem to use the parameters you got wrong).
I believe the following is correct:
<UnmanagedFunctionPointer(CallingConvention.Cdecl)> _
Public Delegate Sub editsErrorHandlerDelegate(ByVal objThis As IntPtr, <[In]()> ByVal strEditTag As String, <[In]()> ByVal strEditName As String, <[In]()> ByVal strAdminCode As String, <[In]()> ByVal strErrorType As String, <[In]()> ByVal strMessage As String)
Declare Function Do_RunStuff Lib "SOME.DLL" (ByVal MyID As Integer, _
ByVal my_tag1 As String, _
ByVal my_tag2 As String, _
ByVal some_data As String, _
ByVal some_options As Integer, _
ByRef some_count As Integer, _
ByVal owner As WhateverClassThisResidesIn, _
ByVal callback_func As editsErrorHandlerDelegate) As Integer
Public Sub editsErrorHandler(ByVal objThis As IntPtr, ByVal strEditTag As String, ByVal strEditName As String, ByVal strAdminCode As String, ByVal strErrorType As String, ByVal strMessage As String)
Debug.Print("*****====================*****")
Debug.Print("Error Type: " & strErrorType)
Debug.Print("Message: " & strMessage)
End Sub
Do_RunStuff(_AnID, strTag1, strTag2, strSomeData, someValue, someCount, Me, AddressOf editsErrorHandler)
If WhateverClassThisResidesIn is a structure, that will need to be ByRef owner As ThatStructure.