How to fix 'Method's type is not PInvoke Compatible'

Question

Trying to return a struct with the exact layout as in C++, the problem is when TCHAR rMsg[256] is included in the C++ struct and [MarshalAs(UnmanagedType.ByValTStr, SizeConst = 256)] public char rMsg;is included in the C# struct (both will be shown below) I get the error 'Method's type is not PInvoke compatible'.

I should also point out the C++ isn't mine and I can't change it as other functions require the layout of the struct given below.

I know the issue is the TCHAR as when I comment out the relevant lines in C++ and C# I don't get the error.

I'm calling the .dll I created (which I will also include in the code below) just to return a struct so that I know I have the correct syntax.

C++:

extern "C" {
    __declspec(dllexport) tVDACQ_CallBackRecVal (  _stdcall TestAcq2())
    {   
                //Just adding random values so I know it works.
        tVDACQ_CallBackRecVal AR;
        AR.rFlags = 1;
        AR.rFrameHeight = 10;
        AR.rFrameWidth = 10;
        return AR;
    }
}

//Struct
typedef struct {
    int    rFlags,           
        rType,             
        rEvent,           
        rSocket;          
    TCHAR  rMsg[256]; //Has to stay as TCHAR       
    int    rFrameWidth,      
        rFrameHeight;      
    short* rFrameBuffer;      
    union {
        int    rCaptureRows;      
        int    rCaptureFrames;    
    };
    int    rCapturePercent;   
    void* rUserCallBackProc, 
        * rUserParam;       
    int    rAborted;          
    void* rPacketData;       
    int    rFGControl;        
} tVDACQ_CallBackRecVal;

C#:

//Simply calling the .dll
[DllImport("C:\\Users\\jch\\source\\repos\\FlatPanelSensor\\x64\\Debug\\VADAV_AcqS.dll", EntryPoint = "TestAcq2", CallingConvention = CallingConvention.Cdecl)]
        public unsafe static extern tVDACQ_CallBackRec TestAcq2();

 [StructLayout(LayoutKind.Sequential)]
        public struct tVDACQ_CallBackRec
        {
            public int rFlags;
            public int rType;
            public int rEvent;
            public int rSocket;

            [MarshalAs(UnmanagedType.ByValTStr, SizeConst = 256)]
            public string rMsg; //I believe this is the issue

            public int rFrameWidth;
            public int rFrameHeight;

            public IntPtr rFrameBuffer;

            public int rCaptureRows;
            public int rCaptureFrames;
            public int rCapturePercent;

            public IntPtr rUserCallBackProc;
            public IntPtr rUserParam;

            public int rAborted;

            public IntPtr rPacketData;

            public int rFGControl;
        }

//Then I just call the function
tVDACQ_CallBackRec testStruct= TestAcq2();

I would expect a struct that has all the components as defined, but instead I get the error as stated in the title.

Again, if I were to not include the TCHAR (c++ struct) and string (c# struct) then I wouldn't get the error so I know that is the underlying issue and I don't know how to solve that part.

EDIT: For solution see first comment from @David Heffernan

Answer 1

The easiest way to solve this is to change the prototype so that the struct is passed as an argument to the function by ref:

void _stdcall TestAcq2(tVDACQ_CallBackRecVal *retval)