Passing byte array between C++ and C# ByRef raises AccessViolationException

Question

I am trying to create a Win32 DLL exposes some functions which are called in C# as follows

__declspec(dllexport) int GetData(unsigned char* *data, int* size)         


        

Answer 1

You are returning an array allocated by a call to C++ new and hoping that the marshaler will turn it into a C# byte[]. That won't happen.

You'll need to pass a pointer by reference and then marshal it by hand. Your p/invoke should look like this:

[DllImport("MyDll.dll")]
static extern int GetData(out IntPtr data, out int size);

When the function returns data will point to the array and you can read the contents using the Marshal class. I guess you would copy it to a new byte array.

var arr = new byte[size];
Marshal.Copy(data, arr, 0, size);

Some other points:

1. The calling conventions do not match. The native side is cdecl and the managed is stdcall.
2. You'll need to export a deallocator to delete the memory returned by the native function. Consider a re-design where the caller allocates the buffer.