Show variables in a MessageBox function?

Show variables in a MessageBox function?

Is there any way I can show different types of variables in a MessageBox?
I'm working with the Windows API, and I have for example this function:
long R1 = RegOpenKeyEx(hKey, lpSubKey, ulOptions,
samDesired, &phkResult);
The phkResult returns the pointer to the Opened Registry Key, so I would
like to show that Registry Key path in a MessageBox.
Can I do that?
Thanks!