Zero value as a handle means error. The user can call GetError function to retrieve error description.

C/C++
char* GetError(HANDLE handle)

C#/VBA/scripts
string GetError(int handle)

Notes:

1. zero value is acceptable as parameter in case last called function returns 0 as handle.
2. the function accepts DG_HANGLE, PRJ_HANDLE, RULE_HANDLE, etc as a handle parameter.