#if debug --> #if myOwnConfig?

Yes, this is possible.

Instructions:

1. Build -> Configuration manager -> Active solution configuration -> New...
2. Create a new configuration named: Offline.
3. Project -> Properties -> Build -> Configuration -> Offline.
4. Conditional compilation symbols, type: OFFLINE.
5. Save project.
6. Restart Visual Studio (reloading the project is not enough).

Yes, you can. But before you can, follow these steps:

1. In Visual Studio, go to Properties -> Build.
2. At the configuration dropdown, select "Offline"
3. Add "OFFLINE" to the "Conditional compilation symbols" text box

If you skip these steps, your #if OFFLINE won't work because in that case, OFFLINE isn't defined.

I followed exactly the same steps as above. But they are not working. They are missing one steps or they didn't explain this clearly.

When you are doing step 4. Conditional compilation symbols: type OFFLINE. You also need to uncheck "Define DEBUG constant". Otherwise the new symbol OFFLINE won't work.

I posted this here in case someone wasted lots of time as me when try to do the same thing.