Class constructor after dllimport

[iipee]

I am trying to upgrage an old VB3 & RDM4.5 application with fairly big database into VS.net and RMD7.1. As RDM 7.1 has been created with unmanaged C, I have instead of VB made a test program as a VC++ forms application.

Now I can transfer the RDM functions with for instance:
[DllImport("rdm7.dll", EntryPoint = "d_opentask",
CharSet = Ansi)]
extern "C" int d_opentask(void *);

and even the unmanaged structures with:
[StructLayout( LayoutKind::Sequential )]
public __gc class mVETS
{
public:
int mvetno;
String* mvetnimi;
String* mprof;
String* merik;
}

VC++ creates a __gc class FORM1 with an event button1_click. As Windows demands that the designers of the Form are as the first member of the class FORM1, I have created a separate namespace WinSys32 for the above marshalling. Everything works fine with the test routine in button1_click till I try to read some real data from the old (but converted)data base. After that I receive a NullReferenceError and I have no luck either after trying to place a constructor for the data in many different places.

I am sure I can't write or place the constructor correctly and I wonder if someone could help me. I have tried the following contsructor:

mVETS::mVETS( int eno, String *ell, String *amm, String
*spe )
{
mvetno = eno;
mvetnimi = ell;
mprof = amm;
merik = spe;
}

If I put the constructor in the class WinSys32::mVETS and declare the class in Form1 button1_click with

SysWin32::mVETS *pvet = new SysWin32::mVETS;

I get an error message telling that there is no default constructor available.

Thankful for help!
Ilkka Pitkänen

 

[cpjust]

Since you gave it a constructor that takes some parameters, I don't think the compiler automatically creates a default constructor for you. If you really intended to create a new default mVETS object, add a default constructor:

mVETS::mVETS()
{
    // Give mvetno, mvetnimi, mprof & merik some default values here.
};

 

[iipee]

Hi!
Thank you very much for your message. I allready once tried a default constructor, but I thougth it was not correct or in a wrong place. Now after I added it to the class, compiler and linker accept it, but executing the code, I get again the awkward:
'System.NullReferenceException',
which I thought was a result of bad constructor definition and which I also received earlier without a constructor.

Did I do correctly what you suggested, could you still look at the shortened code of the test program:

#pragma once
#include "stdafx.h"
#using <mscorlib.dll>
using namespace System;
using namespace System::Runtime::InteropServices;

#using <system.windows.forms.dll>
#include "rdm.h"
- - - - -
namespace SysWin32
{
- - - - - -
[DllImport("rdm7.dll", EntryPoint = "d_recread", CharSet = Ansi)]
extern "C" int d_recread(void*, DB_TASK*, int);
- - - - -
[StructLayout( LayoutKind::Sequential )]
public __gc class mVETS
{
public:
int mvetno;
String* mvetnimi;
String* mprof;
String* merik;

mVETS::mVETS( int eno, String *ell, String *amm, String *spe )
{
mvetno = eno;
mvetnimi = ell;
mprof = amm;
merik = spe;
}

// here I added your suggested default constructor

mVETS::mVETS()
{
// Give mvetno, mvetnimi, mprof & merik some default values here.
mvetno = 1;
mvetnimi = "XXXXX YYYYYYYYYYY ";
mprof = "ZZZZZZZZZZZ ";
mvetnimi = "

http://WWWWWWWWWWWWWWWW

";
} // end dft cnstrt
}; //end class
} // end namespace SysWin32

namespace MC9
{
using namespace System;
using namespace System::ComponentModel;
using namespace System::Collections;
using namespace System::Windows::Forms;
using namespace System:ata;
using namespace System:rawing;
using namespace System::Text;

public __gc class Form1 : public System::Windows::Forms::Form
{
public:
Form1(void)
{
InitializeComponent();
}

protected:
void Dispose(Boolean disposing)
{
if (disposing && components)
{
components->Dispose();
}
__super:ispose(disposing);
}
private: System::Windows::Forms::Button * button2;
private:
System::ComponentModel::Container * components;
void InitializeComponent(void)
{
this->MC9 = new System::Windows::Forms::Label();
- - - - -
(VC++ designer code)
- - - - -

private: System::Void button1_Click(System::Object * sender, System::EventArgs * e)
{
int stat;
- - - - -
DB_TASK *Currtask;
SysWin32::mVETS *pvet = new SysWin32::mVETS;
- - - - - -
if stat = (SysWin32::d_recread(&pvet, Currtask, CURR_DB))
// This row now gives the NullReferenceException
{
- - - - -

DB_TASK and CURR_DB have in rdm.h definitions:
typedef void DB_TASK;
#define CURR_DB -1


I tried to include everything essential.

 

[cpjust]

String* mvetnimi;
String* mprof;
String* merik;

Since these are pointers, you shouldn't assign literal values to them. You should allocate memory to them

mvetnimi = new String;
...

and then assign the text into that new String.
I guess you can do this:

*mvetnimi = "some text";

I've never used Managed C++, so I don't know if you have to explicitely delete your pointers when you're done or just set them to NULL?

Is there a reason why those Strings have to be pointers? If they were just String then you could assign values to them much easier without having to worry about new & delete.

 

[iipee]

Thank you again for your reply!

The reason why I used pointers String* in the class came from some example and it was the only way to make the compiler accept those strings. Using the method you suggested:
mvetnimi = new String;
gives in compiling the error:
C2501: 'SysWin32::mVETS::mvetnimi' : missing storage-
class or type specifiers

I tried in many ways to define the storage class or type but I could not do it so that the compiler would accept it.

Also, I tried in button1_click() the definition
SysWin32::mVETS *pvet = new SysWin32::mVETS();
compiler and linker accept, but the test program executes with the same NullReferenceException result with additional explanation:
'Object reference not set to an instance of an object'.

I also tried to change the class to an unmanaged class with __nogc definition, again with the same results.

About managed classes like __gc class mVETS the books tell that:
"Managed types are garbage collected (i.e. memory no
longer in use is freed) and managed by the CLR part of
the .NET Framework that executes MC++ programs"

I also found out that in the class MC9 I can define
String elk;
end then after *pvet definition in the click routine
elk = pvet->mvetno;
and the compiler accepts even this and the same with other members in the class, but the function d_recread leads to the same NullReferenceException.

So, something is still wrong what I don't seem to be able to solve.
Thank you for your patience!