Using a callback from a C-Dll in VB6

Question

I have to import a few functions of a dll written in C into a VB6 project. I have an example written in C# but I don't really know how to do the same thing in VB6.

In C# it goes like this:

[UnmanagedFunctionPointer(CallingConvention.Cdecl)]
public delegate int someCallback(IntPtr ch, uint chL, IntPtr cbData);

[DllImport("someDLL.dll", CallingConvention = CallingConvention.Cdecl)]
private static extern int someFunction(IntPtr con, someCallback callback, IntPtr cbData);

Everything works fine in the example when calling someFunction.

The documentation of the dll just gives me this:

typedef int(SOMEAPI_CALL * someCallback)(const unsigned char *ch,
                                         unsigned int         chL,
                                         void                *cbData)

SOMEAPI_CALL someFunction(Con*         con,
                          someCallback callback,
                          void*        cbData)

There should be a way to do the same in VB6 but I don't have that much experience with this language. Searched the web for a good time but didn't find anything that could help me.

I know how to declare functions from that dll in my project but thats that. How to convert this UnmanagedFunctionPointer thingy into VB6 code, I just don't know.

Are you sure you are using VB6 and not VB.NET?

Konrad Rudolph
– Konrad Rudolph

2020-04-20 08:53:27 +00:00
Commented Apr 20, 2020 at 8:53 — Konrad Rudolph
– Konrad Rudolph, Commented Apr 20, 2020 at 8:53
Yes, very sure.

user10036485
– user10036485

2020-04-20 08:56:28 +00:00
Commented Apr 20, 2020 at 8:56 — user10036485
– user10036485, Commented Apr 20, 2020 at 8:56
Possible duplicate of VBA COM interop troubles

GSerg
– GSerg

2020-04-20 11:47:46 +00:00
Commented Apr 20, 2020 at 11:47 — GSerg
– GSerg, Commented Apr 20, 2020 at 11:47

Konrad Rudolph · Accepted Answer · 2020-04-20 12:59:36Z

1

VB6 has no attributes in the way VB.NET (and C#) does, and the annotation would be unnecessary anyway. You can just pass a function pointer to a C API function via the AddressOf operator:

Declare Function someFunction Lib "someDLL" ( _
    ByVal con As Long, _
    ByVal callback As Long, _
    ByVal data As Long _
) As Long

…

Call someFunction(con, AddressOf SomeCallback, data)

But even that won’t work since VB6 does not support native interop using the cdecl calling convention, it only supports stdcall. You will either need to recompile your DLL using the stdcall calling convention, or create a wrapper in C or IDL (there are some hacks using inline assembly to wrap individual cdecl calls but I wouldn’t recommend using these).

edited Apr 20, 2020 at 12:59

answered Apr 20, 2020 at 9:01

Konrad Rudolph

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9 Comments

user10036485 Over a year ago

Thanks. I tried it like this but i still get an error at the AddressOf part. Says that a sub, function or property are expected. Is there something missing?

Konrad Rudolph Over a year ago

@RabumAlal Well did you declare a suitable SomeCallback function?

user10036485 Over a year ago

Not yet, I'll try that. Question is, is that neccesary? Isnt that function inside that dll? Why declare? Sry for bad english and stupid questions. Still very new in all this.

StayOnTarget Over a year ago

@RabumAlal in the C# example you pass a C# delegate to the external function. You can think of a delegate as a stand-in for the address of the function you want to be called-back. So you are passing it a function in your own code.

GSerg Over a year ago

No, it wouldn't work with CallingConvention.Cdecl.

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