c# dllimport parameters shift

Question

I am currently developing an Excel add-in in C#, and I am using interop with a C++ library. Both are developed by me. The C++ library is not a COM component, but just a DLL exporting some functions.

I have one C# function, that I want to pass a number and two strings to the C++ function, and the C++ function returns a string. First I tried to return a string directly, so the prototypes look something like

//C#
[return: MarshalAs(UnmanagedType.LPStr)]
private static extern string Function([MarshalAs(UnmanagedType.I4)] int number,
                                      [MarshalAs(UnmanagedType.LPStr)]string str1,
                                      [MarshalAs(UnmanagedType.LPStr)] string str2);
//C++
extern "C" __declspec(dllexport) string Function(int, char*, char*);

(Btw, I set already the calling convention in C# for this function as Cdecl)

Then when I call the C# version, and stepped into the C++ (when debugging), I found that the parameters are shifted, so the integer is the value of the pointer to the first string, and the first string is the second string, and the second string points to somewhere else. I also checked the memory, and found that the parameters were not passed according to the _cdecl convention, the integer was the last one pushed into the stack (which is ok), but the positions of the two strings were switched. There are many other information that the compiler added, so I don't want to go further deep into this.

However, later I changed the return string as a parameter, and the prototypes look like

//C#
private static extern void Function([MarshalAs(UnmanagedType.I4)]int number,
                                    [MarshalAs(UnmanagedType.LPStr)] string str1,
                                    [MarshalAs(UnmanagedType.LPStr)] string str2,
                                    [MarshalAs(UnmanagedType.LPStr), Out] StringBuilder returnStr);
//C++
extern "C" __declspec(dllexport) void Function(int, char*, char*, string&);

And now the parameters are passed correctly.

So my first question is, how does this happen?

My second question is, even using the second method, I could not get the program run correctly, the program crashed when returning from the C++ function (actually in the C++ function, the only thing being done is calling another member function of a class, so the function is just like a wrapper of the member function, and the program crashed exactly when returning in the member function. Since the warpper function just calls the member function, so I cannot tell where the problem is).

Does anyone have an idea how to do this? So the main point of the function is to take a number and two strings and return another string.

Thanks in advanced.

Answer 1

look at this article about the usage of stringbuilder in unmanaged code

try something like this:

//C#
public string Function(int number, string str1, string str2)
{
    StringBuilder sbStr1 = new StringBuilder(str1);
    StringBuilder sbStr2 = new StringBuilder(str2);
    StringBuilder sbReturnStr = new StringBuilder(1024);
    Function(number, sbStr1 , sbStr2 , sbReturnStr , sbReturnStr.Capacity);
    return sbReturnStr.ToString();
}

//C# p/invoke
private static extern void Function(int number, StringBuilder str1, StringBuilder str2, StringBuilder returnStrBuffer, int size);

//C++
extern "C" __declspec (dllexport) void __stdcall Function(int number, const char* str1, const char* str2, char* returnStrBuffer, int size)
{
     //fill the returnStrBuffer
     strncpy(returnStrBuffer, "blablalb", size); //example!!
}