Pass pointer to DWORD from c# to c++ DLL

Question

I'm trying to call a unmanaged function from a DLL written in C++ from a C# console application. I have managed to do so for a couple of simple method calls however one of the functions takes a parameter of void* and I'm not sure what to pass it to get it to work.

C++ method signature

BOOL,   SetData,      (INT iDevice, BYTE iCmd, VOID* pData, DWORD nBytes)

C# method signature

[DllImport("NvUSB.dll", CallingConvention = CallingConvention.Cdecl, CharSet = CharSet.Unicode)]
public static unsafe extern bool SetData(int iDevice, byte iCmd, void* pData, uint nBytes);

Working C++ Call

DWORD pDatatoHi[] = { 'R','E','B','O','O','T' };
SetData(0, 0, pDatatoHi, sizeof(pDatatoHi))

Not working C#

uint* cmd = stackalloc uint[6];

cmd[0] = 'R';
cmd[1] = 'E';
cmd[2] = 'B';
cmd[3] = 'O';
cmd[4] = 'O';
cmd[5] = 'T';

SetData(address, 0, cmd, 6);

The method I am trying to call from the unmanaged DLL should reboot a USB device, when the function is called as per the C++ example above the device reboots correctly. When I call the C# version above the code executes and returns true, however the device does not reboot. I suspect this is due to the way in which I am passing the pData parameter?

Answer 1

When you call SetData(address, 0, cmd, 6) the number of bytes is 24, not 6. 6 is the number of items, not the size of the array, which is what you're passing in the C++ example you've given