Passing a string to a DLL Function

[Christian Stern]

Hey all,

maybe you could help me solving this problem. I have create a simple DLL, like as described here:

https://www.mql5.com/de/articles/18

The DLL work when i use as parameter a Integer but when i switch to string, MT5 come up with the message, he couldn't find the method:

2017.11.18 11:24:35.507    Test(EURUSD,M1)    Cannot find 'testWchar_Ref' in 'Test\variableTest.dll'

This is my import in MT5:

#import "Test\variableTest.dll"
   void testInt_Ref   (int    &intTest_ref);
   void testWchar_Ref (string &wchar_tTest_Ref);
#import  

Implementation in MT5:

   string stringTest = "Hallo you...";  
   int    myInt = 0;
   
   testInt_Ref(myInt);
   Print("New INT: ", myInt );
   
   Print("PRE: " + stringTest);
   testWchar_Ref(u_stringTest);
   Print("POST: " + stringTest);

Export in DLL:

#ifdef VARIABLETEST_EXPORTS  
#define QVAR_API __declspec(dllexport)   
#else  
#define QVAR_API __declspec(dllimport)   
#endif

extern QVAR_API void testInt_Ref(int &intTest_ref);
extern QVAR_API void testWchar_Ref(wchar_t &wchar_tTest_Ptr);

DLL Function:

// Include header files
#include "stdafx.h"

using namespace std;

// add new INT value
void testInt_Ref(int &intTest_ref) {
    intTest_ref = 222;
}

// add new WCHAR_T value
void testWchar_Ref(wchar_t &wchar_tTest) {

    wchar_t *msg = L"This is cool.....";
    wchar_t *wchar_tTest_Ref = &wchar_tTest;

    //--- replace it
    memcpy(wchar_tTest_Ref, msg, wcslen(msg) * sizeof(wchar_t));
}

Result:

2017.11.18 11:24:35.507    Test(EURUSD,M1)    New INT: 222
2017.11.18 11:24:35.507    Test(EURUSD,M1)    PRE: Hallo you...
2017.11.18 11:24:35.507    Test(EURUSD,M1)    Cannot find 'testWchar_Ref' in 'Test\variableTest.dll'
2017.11.18 11:24:35.507    Test(EURUSD,M1)    unresolved import function call

it's compiled correctly. In my c++ Testprogram it works. Even when i leave the DLL Method empty it appears. It must be the parameter.

How can i pass a string to a DLL, modify it and use it back in MT5 ?

Thanks in Advanced

[daybyter]

In your C function you pass a reference to a char (1 character), but in your metatrader code, you pass a reference to a string (an array of characters).