Unable to pass a string from VBA (Excel) to my COM object - vba

Question

I am experimenting with creating a COM interface for my application in order to allow eg. VBA to drive certain parts of my application.
I have my COM library up and running and installed, even to the part where a routine called in Excel can be debugged in the Delphi IDE.
Here's the VBA code that I activate from Excel using a button:
Sub TestAMQMOLE_OpenProject()
Dim vConnection As String
Dim vAMQM As Object
Dim vAMProject As Object
vConnection = "$(MYSRC)\LCCAMQM38\UnitTestData\AnalyseSortingAndGrouping"
Set vAMQM = CreateObject("LCCAMQM_AX.LCCAMQM_Application")
vAMQM.Connect
Set vAMQMProject = vAMQM.OpenProject(vConnection) 'This parameter does not get through
Set vAMQMProject.Active = True
Set vAMQMProject = Nothing
vAMQM.Disconnect
Set vAMQM = Nothing
End Sub
And the part in Delphi handling it looks like this:
function TLCCAMQM_Application.OpenProject(const aFolderOrAlias: WideString): ILCCAMQM_Project;
begin
try
Result:=TLCCAMQM_Project.Create(aFolderOrAlias); // wrapper om TdmAMEditBase
except
SHowMessage(ExceptionToString(ExceptObject,ExceptAddr));
end;
end;
Where the code fails because the aFolderOrAlias parameter string is empty. I added the exception handler in order to debug outside the Delphi IDE. when debugging inside the IDE, the parameter string indeed appears as empty.
I have also tried to pass the parameter as a Variant, or const Variant (and adjusting the type library accordingly), but in that case I get a VT_RECORD variant type (0x0024) which does not make any sense to me.
Here is what the interface definition of the type library looks like.
....
[
uuid(EDD8E7FC-5D96-49F1-ADB7-F04EE9FED7B5),
helpstring("Dispatch interface for LCCAMQM_Application Object"),
dual,
oleautomation
]
interface ILCCAMQM_Application: IDispatch
{
[id(0x000000C9)]
int _stdcall Connect(void);
[id(0x000000CA)]
int _stdcall Disconnect(void);
[id(0x000000CB)]
ILCCAMQM_Project* _stdcall OpenProject([in] BSTR aFolderOrAlias);
[propget, id(0x000000CC)]
HRESULT _stdcall Connected([out, retval] VARIANT_BOOL* Value);
};
[
uuid(590DBF46-76C9-4877-8F47-5A926AFF389F),
helpstring("LCCAMQM_Application Object")
]
coclass LCCAMQM_Application
{
[default] interface ILCCAMQM_Application;
};
....
I am fairly sure there must be a way to pass strings from VBA to COM objects. But after fiddling around for several hours I am lost :s.

Answer 1

As it turned out, ComIntern and Remy were right. I had completely misunderstood the whole stdcall and safecall interfaces.
The .ridl file now looks like this:
....
interface ILCCAMQM_Application: IDispatch
{
[id(0x000000C9)]
int _stdcall Connect(void);
[id(0x000000CA)]
int _stdcall Disconnect(void);
[propget, id(0x000000CC)]
HRESULT _stdcall Connected([out, retval] VARIANT_BOOL* Value);
[id(0x000000CB)]
HRESULT _stdcall OpenProject([in] BSTR aFolderOrAlias, [out, retval] ILCCAMQM_Project** Value);
};
....
And the generated ...TLB.pas file looks like this:
....
// *********************************************************************//
// Interface: ILCCAMQM_Application
// Flags: (4416) Dual OleAutomation Dispatchable
// GUID: {EDD8E7FC-5D96-49F1-ADB7-F04EE9FED7B5}
// *********************************************************************//
ILCCAMQM_Application = interface(IDispatch)
['{EDD8E7FC-5D96-49F1-ADB7-F04EE9FED7B5}']
function Connect: SYSINT; stdcall;
function Disconnect: SYSINT; stdcall;
function Get_Connected: WordBool; safecall;
function OpenProject(const aFolderOrAlias: WideString): ILCCAMQM_Project; safecall;
property Connected: WordBool read Get_Connected;
end;
// *********************************************************************//
// DispIntf: ILCCAMQM_ApplicationDisp
// Flags: (4416) Dual OleAutomation Dispatchable
// GUID: {EDD8E7FC-5D96-49F1-ADB7-F04EE9FED7B5}
// *********************************************************************//
ILCCAMQM_ApplicationDisp = dispinterface
['{EDD8E7FC-5D96-49F1-ADB7-F04EE9FED7B5}']
function Connect: SYSINT; dispid 201;
function Disconnect: SYSINT; dispid 202;
property Connected: WordBool readonly dispid 204;
function OpenProject(const aFolderOrAlias: WideString): ILCCAMQM_Project; dispid 203;
end;
....
And the OpenProject now works from my internal unit test (written in delphi) as well as from Excel-VBA.
Now I am struggling with properties to set and get through Excel VBA as well as through an OleVariant in delphi. But I will have to put that in another Q.