C# to C++ process with WM_COPYDATA passing struct with strings

Question

From a c# program I want to use WM_COPYDATA with SendMessage to communicate with a legacy c++/cli MFC application.

I want to pass a managed struct containing string obje

Answer 1

From the documentation:

The data being passed must not contain pointers or other references to objects not accessible to the application receiving the data.

So you need to pack your string into COPYDATASTRUCT.lpData. If you have a max length for each string then you can embed it in a fixed length structure

typedef struct tagMYDATA
{
   char  s1[80];
   char  s2[120];
} MYDATA;

If you have only one variable length string you can put the string at the end and use a header followed by string data

typedef struct tagMYDATA
{
   int value1;
   float value2;
   int stringLen;
} MYDATAHEADER;

MyCDS.cbData = sizeof(MYDATAHEADER)+(int)stringData.size();
MyCDS.lpData = new BYTE[MyCDS.cbData];
memcpy(MyCDS.lpData,&dataHeader,sizeof*(MYDATAHEADER);
StringCbCopyA (
    ((BYTE*)MyCDS.lpData)+sizeof*(MYDATAHEADER)
    ,stringData.size()
    ,stringData.c_str());

If you have multiple variable length strings you can still use a header and allocate more spaces for every strings plus a double null terminator, or serialize everything into one XML string.