How to get interface from C dll to Delphi? - delphi

Question

I have a library written in C and I have a header file with a description of the interface in C. The DLL has a function to get this interface. How to describe it correctly and get it in the DELPHI application?
using DllCallbackClassPtr = void*;
using DllCallbackFunction = void(*)(const char *, DllCallbackClassPtr);
#ifdef _WIN32
#include <Windows.h>
__interface IXeoma
{
public:
enum ConnectErrorCode {
OK = 0,
SERVER_NOT_FOUND,
WRONG_PASSWORD,
UNKNOWN
};
// return ConnectErrorCode
virtual int start(const char* connectionString) = 0;
virtual bool isConnected() = 0;
virtual void stop() = 0;
virtual void requestData(const char* request, const char* additionalData, DllCallbackClassPtr classPtr, DllCallbackFunction callbackFunc) = 0;
virtual const char* getRequestResult(const char* request) = 0;
virtual void setCameraRenderHandle(const char* previewId, HWND hWnd) = 0;
};
The library is loaded, but the function returns nil.
type
IXeoma = interface
function Start(connectionString: PChar): integer;
end;
type
TCreateXeomaInterface = function() : IXeoma; stdcall;
var
Form1: TForm1;
CreateXeomaInterface: TCreateXeomaInterface;
implementation
{$R *.dfm}
var
LibraryHandle: THandle;
procedure TForm1.Button1Click(Sender: TObject);
var
XeomaInt: IXeoma;
i: integer;
begin
LibraryHandle := LoadLibrary(PChar('D:\Projects\XeomaSDK\Win32\Debug\xeomaclientdll.dll'));
if LibraryHandle >= 32 then
begin
#CreateXeomaInterface := GetProcAddress(LibraryHandle, 'createXeomaInterface');
end;
XeomaInt := CreateXeomaInterface();
// Here XeomaInt = nil
end;

Answer 1

The __interface extension in Visual C++, and the interface keyword in Delphi, are not the same thing, and are not compatible with each other.
IXeoma in the C++ code is just an ordinary class type, not a COM interface. But in Delphi, all interfaces derive from IUnknown, and all classes derive from TObject, neither of which you want in this situation. So, you are going to have to use a plain record instead, and declare TCreateXeomaInterface as returning a pointer to that record.
Also, note that a Delphi record can't have virtual methods, but the C++ class does have them, so you are going to have to manually account for the C++ class's vtable in Delphi.
Try something like this:
type
DllCallbackClassPtr = Pointer;
DllCallbackFunction = procedure(Param1: PAnsiChar; Param2: DllCallbackClassPtr); cdecl;
IXeomaPtr = ^IXeoma;
IXeomaVTable = record
start: function(_Self: IXeomaPtr; connectionString: PAnsiChar): Integer; cdecl;
isConnected: function(_Self: IXeomaPtr): Boolean; cdecl;;
stop: procedure(_Self: IXeomaPtr); cdecl;
requestData: procedure(_Self: IXeomaPtr; request: PAnsiChar; additionalData: PAnsiChar; classPtr: DllCallbackClassPtr; callbackFunc: DllCallbackFunction); cdecl;
getRequestResult: function(_Self: IXeomaPtr; request: PAnsiChar): PAnsiChar; cdecl;
setCameraRenderHandle: procedure(_Self: IXeomaPtr; previewId: PAnsiChar; hWnd: HWND); cdecl;
end;
ConnectErrorCode = (
OK = 0,
SERVER_NOT_FOUND,
WRONG_PASSWORD,
UNKNOWN
);
IXeoma = record
vtable: ^IXeomaVTable:
end;
type
TCreateXeomaInterface = function() : IXeomaPtr; stdcall;
var
Form1: TForm1;
CreateXeomaInterface: TCreateXeomaInterface;
implementation
{$R *.dfm}
var
LibraryHandle: THandle;
procedure TForm1.Button1Click(Sender: TObject);
var
XeomaInt: IXeomaPtr;
i: integer;
begin
XeomaInt := nil;
LibraryHandle := LoadLibrary('D:\Projects\XeomaSDK\Win32\Debug\xeomaclientdll.dll');
if LibraryHandle >= 32 then
begin
#CreateXeomaInterface := GetProcAddress(LibraryHandle, 'createXeomaInterface');
XeomaInt := CreateXeomaInterface();
if XeomaInt <> nil then
XeomaInt^.vtable^.start(XeomaInt, '123:123#localhost:8090');
end;
...
end;