It is convenient to set custom conditional preprocessing at project level when some actions need to be skipped lets say for tests.
Go to Solution Explorer
-> Click Right Mouse on project you want to set variable to -> Properties
-> Build
-> In General find field Conditional compilation symbols
and enter your conditional variable here
Code example that will skip some code:
public void Init()
{
#if !IGNOREREFRESHDB
// will skip code here
db.Initialize();
#endif
}