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Asked: 2026-05-16T18:01:51+00:00

I have acquired a DLL that was created in Visual Basic from a third

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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!

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1 Answer

  1. Editorial Team

    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.

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