Question

In c++ the method that I am exporting is:

__declspec(dllexport) int __thiscall A::check(char *x,char *y,char *z)
{
  temp=new B(x,y,z);
}

In c# I am importing this method like this:

[DllImport("IDLL.dll", CallingConvention=CallingConvention.ThisCall, ExactSpelling = true, EntryPoint = "check")]
    public static extern int check(string x, string y, string z);

I am calling this method in c# like this and passing the values:

public int temp()
{
  string x="sdf";
  string y="dfggh";
  string z="vbnfg";
  int t;

  t=Class1.check(x,y,z);
  return t;
}

The problem is that when I debug in to the native code I see that the parameters x,y,z having values sdf,dfggh,vbnfg and being altered when they reach c++ dll like this even before it is entering the native c++ dll method.

x=dfggh,y=vbnfg,z=null value

and is giving me the error saying that null pointer value is passed to the function. Can any one help me out fixing this weird problem.

Answer 1

Looks like your native method is an instance(vs static) method. I guess your first parameter gets mapped to ‘this’ somehow.

Here is an example:

#include <fstream>
using namespace std;

class A
{
public:
__declspec(dllexport) static int __stdcall check(char *x,char *y,char *z)
{
    ofstream f;
    f.open("c:\\temp\\test.txt");
    f<<x<<endl;
    f<<y<<endl;
    f<<z<<endl;
    return 0;

    }

__declspec(dllexport) int __thiscall checkInst(char *x,char *y,char *z)
{
    ofstream f;
    f.open("c:\\temp\\testInst.txt");
    f<<x<<endl;
    f<<y<<endl;
    f<<z<<endl;
    return 0;

    }
};

see the static keyword on the first one?

Imports(i used the mangled names because I’m lazy):

[DllImport("TestDLL.dll", CallingConvention = CallingConvention.StdCall, ExactSpelling = true, EntryPoint = "?check@A@@SGHPAD00@Z")]
    public static extern int check(string x, string y, string z);

    [DllImport("TestDLL.dll", CallingConvention = CallingConvention.ThisCall, ExactSpelling = true, EntryPoint = "?checkInst@A@@QAEHPAD00@Z")]
    public static extern int checkInst(IntPtr theObject, string x, string y, string z);

That makes it work just like that:

check("x", "yy", "zzz");

the instance method requires an IntPtr

IntPtr obj = IntPtr.Zero;
checkInst(obj, "1", "12", "123");

and the contents of my test.txt are:

x
yy
zzz

and testInst.txt

1
12
123