https://www.dotnetzone.gr:443/cs/blogs/birbilis/archive/2009/10/23/removing-old-style-inclusion-guards-from-visual-c-header-files.aspxRecent versions of Microsoft Visual C++ compiler support the following directive in C++ header files (.h): #pragma once This is used to prevent their multiple inclusion (prevents multiple definition of symbols and also spurious inclusion loops that couldelCommunityServer 2.1 SP3 (Build: 20423.1)