How do I set a system-wide _ifdef?

How do I set a system-wide _ifdef?

This is using Microsoft Visual Stuedio version 2019 with C#.

If I want to add specialized code to a project shared by multiple software developers that is designed for my selfish tests and evaluation, how do I manage a # definition inside the C# code?

I imagine some soft of solution varable that is deined in either the project file or the solution file that I do not merge with the master branch of the code in the version contorl system. And then this "# define" is somehow blocked out in the source file.

Has anyone experienced or used such a thing?