Wrong DLL calling convention - c++

Question

I'm trying to call a C++ COM DLL from within VB6.
The C++ code is:
STDMETHODIMP CSonic::sonicChangeShortSpeed(
SHORT* samples,
LONG *numSamples,
FLOAT speed,
FLOAT pitch,
FLOAT rate,
FLOAT volume,
LONG useChordPitch,
LONG sampleRate,
LONG numChannels
)
I call it from VB6 like this:
Private Declare Function sonicChangeShortSpeed Lib "SonicLIB.dll" Alias "#1" (
ByRef samples As Integer,
ByRef numSamples As Long,
ByVal speed As Double,
ByVal pitch As Double,
ByVal rate As Double,
ByVal volume As Double,
ByVal useChordPitch As Long,
ByVal sampleRate As Long,
ByVal numChannels As Long)
As Long
In my code, I use:
Dim nIntegers() As Integer
ReDim nIntegers(2047)
Dim lSamples As Long
Dim dblSpeed As Double
Dim dblPitch As Double
Dim dblRate As Double
Dim dblVol As Double
Dim lUseChordPitch As Long
Dim lSampleRate As Long
Dim lNumChannels As Long
lSamples = 2048
dblSpeed = 0.5
dblPitch = 0
dblRate = 1
dblVol = 1
lUseChordPitch = 1
lSampleRate = 48000
lNumChannels = 1
Dim lRet As Long
lRet = sonicChangeShortSpeed(nIntegers(0), lSamples, dblSpeed, dblPitch, dblRate, dblVol, lUseChordPitch, lSampleRate, lNumChannels)
The last line produces the "Wrong calling convention" error.
Does anybody see my mistake?
Thank you!

Answer 1

If this is a COM DLL, you need to add it to your VB6 project as a reference, and then you can access the classes and other COM definitions contained in the DLL.
Alternately you could just register the DLL without adding it as a reference and then use 'late binding' with CreateObject() calls.
But if you are not sure if it is a COM DLL, you could check by:
Try to register it on the command like using regsvr32.exe (may need to be admin). If this reports success, that means it was a successfully registered COM DLL.
Drag and drop it into the program Oleview which comes with Visual Studio. If the DLL is COM it will normally have a type library which Oleview will display. If it is not COM, this will produce an error.
If it is NOT a COM DLL then I do not think your approach will necessarily work. You are trying to call a class method as though it were a normal 'C' style function. Perhaps that would work for a static C++ method -- but that does not appear to be the case here.
The traditional import/export table of a DLL knows nothing about classes. You may need to write a C++ wrapped function around your class object which can then be exported and used in VB6. The wrapper would have to handle object creation, destruction, etc.