#if DEBUG It’s a preprocessor definition.
It compiles when you define DEBUG constant. And yes, it’s default on Debug Build Configuration.
Visual Studio 2010 Project Properties:
If Define DEBUG constant is checked VS will compile:
private const string BASE_URL = "http://www.a.com/";
Else (not checked) VS will compile:
private const string BASE_URL = "http://www.b.com//";