Question

What is

#define _WIN32_DCOM

in a visual c++ project ? If I want to target the application build to x64, do I need to change it to WIN64_DCOM object ?

Answer 1

From Microsoft support:

If your custom interface uses BSTR, VARIANT, or SAFEARRAY, and if you
want to use your custom interface marshaling code only on a DCOM
platform, you can use MIDL generated marshaling code. You will need to
use -D_WIN32_DCOM as a C compiler flag in the build process of the
MIDL generated code. Otherwise, a compiler error will be generated
that indicates that your code will compile only if you mark it as
runable only on a DCOM platform.

The symbol is a logic ones, you may use it unchanged under Windows 64 bits