What is it?

When compiling a project, if we're a dedicated server, we can run code in if #SERVER blocks.

protected override void OnUpdate()
{
#if SERVER
  Log.Info( $"This is a server update!" );
#else
  Log.Info( $"This is a client update!" );
#endif
}

If we're running on a dedicated server, we'll get This is a server update! spamming our console.

This is especially useful for omitting potentitally sensitive code from client builds, like contacting an API server, or a database.

.Server.cs files

Say you have a GameManager.cs file, which has a partial class, you can accompany it with a GameManager.Server.cs file which is safely omitted from client builds and wraps the whole file in an '#if SERVER block - it's a nice bit of sugar that saves you having a bunch of preprocessor blocks in your code.

What if I publish my game?

All serverside code is stripped out of published games by default - so there's no worries there.