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How to Call Delphi DLL Method from C++ with Var WideString parameter


I have a Delphi DLL that works when called by delphi apps and exports a method declared as:

Procedure ProduceOutput(request,inputs:widestring; var ResultString:widestring);stdcall;

On the C++ side I have tried:

[DllImport( "ArgumentLab.dll", CallingConvention = CallingConvention.StdCall, CharSet=CharSet.WideString )];
 extern void ProduceOutput(WideString request, WideString inputs, WideString ResultString);

WideString arequest = WideString(ComboBox1->Text);
WideString ainput = "<xml> Input Text Goes Here </XML>";
WideString  aresultstring;
WideString &aresultstringpointer = aresultstring;
aresultstring = " ";
ProduceOutput(arequest, ainput, &aresultstringpointer);

Memo1->Lines->Text = &aresultstringpointer;

My console error reads:

  Unit1.cpp(13): candidate function not viable: no known conversion from 'BSTR *' (aka 'wchar_t **') to 'System::WideString' for 3rd argument;

I have built the DLL and the c++ test app using Rad Studio XE4 - it is a 64 bit DLL and APP

How should I have gone about doing this?  

Best regards,

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1 Solution
I am not using C++, so i am just suggesting here.
Would it be the pointer that makes the road bumpy?
Have you tried to use:
ProduceOutput(arequest, ainput, aresultstring);

Memo1->Lines->Text = aresultstring;

Open in new window

Or use function instead to return the ResultString:
function ProduceOutput(request,inputs:widestring; ResultString:widestring):widestring; stdcall;

Open in new window

So your call would be:
Memo1->Lines->Text = ProduceOutput(arequest, ainput, aresultstring);

Open in new window

garrymottAuthor Commented:
JimyX - Thank you for your response - no I believe that C++ cannot simply pass a widestring as (an in/out) parameter as can be done from Delphi.  Also I do not have the option to use a string as a result.

The function was originally provided in a DLL to be used with a COM server. The callers used Visual Basic and BSTRs to operate the DLL.  Now the same peeps want to skip using COM and handle the DLL directly from C++ or C#.  

I need to give them some guidance on how to proceed.

again thanks for your response
garrymottAuthor Commented:
Ok - solved it myself.  Here is the code for anybody who needs to walk this path. - It is also embedded here:       CPPDllLabMain.cpp

typedef void* (*VoidReturnFunc)(WideString request, WideString inputs, WideString &ResultString);

extern "C" __declspec(dllimport) void* __stdcall GetClassInstance();

 HINSTANCE load = LoadLibrary(library);

WideString arequest = WideString("EchoInputs");//(ComboBox1->Text);
WideString ainput = WideString("<xml> Input Text Goes Here </XML>");
WideString aresultstring = WideString( "hello") ;

 if (load) {
                        VoidReturnFunc myFunc;
                        myFunc = (VoidReturnFunc)GetProcAddress(load, "ProduceOutput");
                        myFunc(arequest, ainput, aresultstring);
                        Memo1->Lines->Text = aresultstring;

            else {
                        ShowMessage("Library not loaded!");
garrymottAuthor Commented:
The use of Widestrings as var parameters provides a safe method of returning large string data from DLL procedures.  We do this in our COM servers but the same process is suitable for direct calls from C++.  There is a lot of info out there but the specifics on implementation on the C++ side are rare or non-existent.  I tested this before I posted it and trust the info is good - enjoy - Peace and Good to you
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