system.access.violation exception while calling c++ function from a thread in c#

Question

i am importing c++ function from a dll to my winform c# app:

[DllImport(@"eyeWhere.dll", CallingConvention = CallingConvention.Cdecl, CharSet = CharSet.Unicode)]
public static extern int Eye_GetPositionS(string filename, [MarshalAs(UnmanagedType.LPArray, SizeConst = 9)] double[] sensors);

when i call the function from the constructor its working fine.

the problem is when i am calling it from a new thread opened within the
"fleck websocket server" Fleck , "onMessage" Action,
then it throws the "system.access.violation" exception.

i managed to narrow down the problem to the double array that i am passing, it seems like the pointer to it is corrupted.

i cant find the source of the problem one thing is sure the function from the dll is working as i tested it.

function call(two stages):

1. open new thread within "fleck":

socket.OnMessage = message => {Thread locationThread = new Thread( unused => processLocation(fileName,socket,sensorsList,sensors) ); locationThread.Start();}

2. the actual function:

private void processLocation(string fileName, IWebSocketConnection sock, List<Sensor> sensorsList, double[] sensors) { int map_position = Eye_GetPositionS(fileName,sensors);
string locationString = "floor:1,mx:" + (map_position / 10000) + ",my:" + (map_position % 10000); // send location string to user sock.Send(LOCATION_CODE + "-" + locationString);}

the interface is:

extern "C" __declspec(dllexport) int Eye_GetPositionS(const wchar_t *fname_mob, double sensors[9], int &map_x, int &map_y)

i am not passing the two last arguments (int&) as agreed with the man who wrote the dll.

Answer 1

I am not passing the two last arguments ( int& ) as agreed with the man who wrote the DLL.

Well, there's the problem. You cannot omit parameters. It should be:

[DllImport(@"eyeWhere.dll", CallingConvention = CallingConvention.Cdecl, 
    CharSet = CharSet.Unicode)]
public static extern int Eye_GetPositionS(
    string filename, 
    [In, MarshalAs(UnmanagedType.LPArray, SizeConst = 9)] 
    double[] sensors,
    ref int map_x,
    ref int map_y
);

You simply cannot omit these parameters, no matter what the guy who wrote the DLL says. Perhaps he means that the parameters are really int* and you can pass nullptr .

I've used ref for these parameters, but perhaps they should be out . You presumably know which is appropriate.

Likewise I'm guessing at the intent of the sensors parameter. If the data flows out rather than in, then use Out . If the data flows in both directions, use [In, Out, ...] .