how to pass a string from c++ to c# (aspx.cs page)

[peraonline]

I must get a string from a function included into a DLL, imported in a ASP.NET page.
The code:
----

Into a .aspx.cs page:

//def. function included into a DLL
[DllImport("WebUtility.dll", EntryPoint = "getDescr")]

public static extern string getDescr(int id);

//call the function:
string descr=getDescr(123);

---

into the c++ program, the function is definied as:

char* getDescr(int id) {
char* ret=new char[LUNGHEZZA_STRINGA_MEM];
strcpy(ret,oggetto[id].descr);
return (ret);
}

If I throw the application, the server give me the error: System.NullReferenceException: Object reference not set to an instance of an object.
But this error is generated not every time, but random!! some time I can view the string correctly! Why?

Thanks for the help!!!