Calling a Visual Basic DLL in C++

Question

I have acquired a DLL that was created in Visual Basic from a third party vendor(Sensor DLL.dll). This DLL contains functions for talking to a sensor, and I need to call these functions from a Visual C++ program I am writing. The vendor will not provide a header file, and I do not know Visual Basic. If I had a header file this would be a 15 minute project... instead I am still struggling with it a week later. Please Help!

I am told one function (Get_Data) in the DLL is of the form:

Public Function Get_Data(ByVal Handle As String) As String

I have tried several methods for calling this Get_Data function with no success:

Method 1) the DllImport attribute

#using <mscorlib.dll>
using namespace System::Runtime::InteropServices; 

namespace Sensor
{

[DllImport("Sensor DLL.dll", EntryPoint = "Get_Data", CharSet = System::Runtime::InteropServices::CharSet::Unicode)] BSTR Get_Data(BSTR Handle); }

//then I call the function
Sensor::Get_Data(Handle);

This method seems to be the closest I have gotten to a sloution. It compiles, but gives the following error when it runs:

An unhandled exception of type 'System.EntryPointNotFoundException' occurred

Additional information: Unable to find an entry point named 'Get_Data' in DLL 'Sensor DLL.dll'.

I have tried various datatype combinations/permutations besides BSTR including BSTR*, wchar_t, int, etc. It is possible that I missed one, but each datatype returns the same error.

Method 2) dllimport storage-class attribute

__declspec(dllimport) BSTR Get_Data(BSTR Handle);

//then I call the function
Get_Data(Handle);

This method is confusing to me because I don't specify the DLL I want to import from. I have copied the DLL to the project folder and I have added it to the project manually, so hopefully that means it can be found. When I compile the linker returns the following errors:

error LNK2028: unresolved token (0A00034F) "wchar_t * __cdecl Get_Data(wchar_t *)" (?Get_Data@@$$FYAPA_WPA_W@Z) referenced in function "int __cdecl main(void)" (?main@@$$HYAHXZ)

error LNK2019: unresolved external symbol "wchar_t * __cdecl Get_Data(wchar_t *)" (?Get_Data@@$$FYAPA_WPA_W@Z) referenced in function "int __cdecl main(void)" (?main@@$$HYAHXZ)

I suspected maybe this meant I should be using wchar_t or wchar_t* instead of BSTR, but changing to either datatype results in the same error.

Method 3) GetProcAddress

typedef BSTR (*Get_Data_Ptr)(BSTR Handle);  
HINSTANCE LoadMe;
LoadMe = LoadLibraryA("Sensor DLL.dll");

if (!LoadMe)
    std::cout << "\nDLL failed to load!\n";

Get_Data_Ptr LibMainGet_Data;  
LibMainGet_Data = (Get_Data_Ptr)GetProcAddress(LoadMe,"Get_Data");

//then I call the function
LibMainGet_Data(Handle);

This will compile, but gives the following error when run:

An unhandled exception of type 'System.AccessViolationException' occurred

Additional information: Attempted to read or write protected memory. This is often an indication that other memory is corrupt.

When I mouse over the various parts of this code in debug mode it seems that, like the first method, it was also unable to find the 'Get_Data' entry point in the DLL.

Has anyone called functions from a VB DLL using C++ when you haven't made the DLL yourself and you don't have .idl files, etc? Does anyone have a working example like this you could share?
Thanks!

Answer 1

A VB6 DLL is normally a COM server. You do in fact have the equivalent of a .h file, it has a type library embedded in it. Start this off with Project + Properties, Common Properties, Framework and References. Add New Reference button, Browse tab, select the DLL.

Next, View + Object Browser. You should see the generated Interop library in the list. Open the node to see what is there. You write normal managed code, like gcnew, to create the COM object and call the interface methods. You do need some minimum documentation on the available methods to have a guess at how they should be called.

Answer 2

I believe the missing piece is the calling convention. C++ has its own function calling convention different than VB6 (I assume VB6 since you haven't stated VB.NET explicitly). VB6 uses STDCALL convention whereas C++, depending on the vendor, uses a different calling convention termed __cdecl, which is why you see the __cdecl in the compiler error line for method #2. It assumes your external function is using that calling convention by default. Calling convention is a set of rules describing how functions call one another; specifically about how registers are used, what order parameters are delivered in, how by-value / by-reference is determined, etc.

I would suggest sticking with method #3 since method #1 is for Managed C++ which is not standard C++, and method #2 is unfamiliar to me and looks a bit ambiguous. What you want to try is declaring the function pointer typedef to use STDCALL.

typedef BSTR (__stdcall *Get_Data_Ptr)(BSTR Handle);

Answer 3

In the OLE/COM viewer, in order to view the COM type library in a dll/exe/... you have to open it by using "File->View TypeLib" instead of "File->Bind to File"

Answer 4

It sounds like the DLL isn't actually exporting a function named Get_Data. Open up a command prompt and use dumpbin to get the list of exports of the DLL, e.g.:

dumpbin /exports "Sensor DLL.dll"

(dumpbin.exe is located in VC\bin within your Visual Studio install folder, which istypically something like C:\Program Files\Microsoft Visual Studio 10.0).

Then, replace Get_Data with the actual entry point and see if you have any better luck.

Answer 5

A Visual basic program normally needs a runtime to execute.

If you have a COM object (implemented in VB) use the COM API to communicate with it from C++. You will have to register the COM first. Here is a thread that explains hot to do that: http://forums.devx.com/archive/index.php/t-87059.html

If you use a .NET language, use the method of Hans Passant with a reference that will create an interop dll for you. This is far much easier.

• Method 1: Do not do that, if you have a COM object that you want to use from a .NET environment, reference it.
• Method 2: You get errors because you lack the the .lib file to properly link to the DLL (statically dynamically linking)
• Method 3: Would be a pure dynamic solution but you have to know the exact names of the methods in the DLL. These may vary according to the parameters and calling convention used. This is very similar (actually identical, I would say) to the issue you face with your Method 1 solution. The name of the method is for yure not "Get_Data" but something else. With a tool like the dependency viewer you can have a look at the exported names.

Even Method 3 with the right names is likely to fail because if it is a COM object you will need some environment called Appartment to use the COM objects. You "enter" this appartment by calling CoInitialize. This creates some magical stuff in the TLS (Thread Local Storage) to perform the COM magic. I hope this explains why your tries will be pointless if the DLL you have is happening to be a COM component, what is quite likely according to the ATL like naming we can see.

EDIT: I forgot to say that you can also easily see what is inside the dll if it is a COM with the OLE/COM Viewer (normally if you have a compiler you will have such a tool around).