DLL not exporting string in visual studio 2013 VB code - c++

Question

I'm writing a small application in VB.NET in Visual Studio 2013, and so far what I've written is the following code:
Public Class MainMenu
Private Declare Function Version_Get Lib "mypath/mydll.dll" () As String
' Before anyone asks, yes, the DLL is present in the mypath folder
Private Sub MainMenu_Load(sender As Object, e As EventArgs) Handles Me.Load
Dim Temp As String
Dim Dummy As Integer
On Error GoTo Handler
Temp = Version_Get
Dummy = 1 ' This line is never reached in debug mode
Handler:
If Not IsNothing(Err.GetException()) Then
MsgBox("Error " & Str(Err.Number) & " generated by the application " & Err.Source & ControlChars.CrLf & Err.Description, vbCritical, "Error")
End
End If
End Sub
End Class
Where the DLL is supposed to return a BSTR (this is of course extern "C" etc., but for the sake of comprehension I'm writing the function pure and simple):
File .h
#ifdef EVALFUNC_EXPORTS
#define EVALFUNC_API __declspec(dllexport)
#else
#define EVALFUNC_API __declspec(dllimport)
#endif
extern "C"
{
EVALFUNC_API BSTR __stdcall Version_Get();
}
File .cpp
BSTR __stdcall Version_Get()
{
CRegKey Key;
CString sValue;
BSTR Str;
LONG nA = Key.Open(HKEY_LOCAL_MACHINE, _T("SOFTWARE\\my Software"), KEY_READ);
// Before anyone asks, yes, the application is present in the system
ULONG nValueLength = 0;
LONG nB = Key.QueryStringValue(_T("Version"), NULL, &nValueLength);
if (nValueLength > 0) LONG nC = Key.QueryStringValue(_T("Version"), sValue.GetBufferSetLength(nValueLength - 1), &nValueLength);
Str = sValue.AllocSysString();
return Str;
}
The problem is that even if I wrote an error handler, the code encounters no error, and Version_Get makes my code crash without any error (the MainMenu form gets loaded anyway).
I tried the DLL on another VB environment (Excel) with the same declaration.
Private Declare Function Version_Get Lib "mypath/mydll.dll" () As String
In that case the string variable is filled with the correct text.
What am I doing wrong?

Answer 1

I had to create a class to import the Dll by marshalling the BSTR String:
Imports System.Runtime.InteropServices
Public Class ImportDll
<DllImport("myDll.dll", SetLastError:=True, CharSet:=CharSet.Unicode, CallingConvention:=CallingConvention.StdCall)> _
Public Shared Function Version_Get() As <MarshalAs(UnmanagedType.BStr)> String
End Function
End Class
And could call the code as follows:
Version = ImportDll.Version_Get