is there any open source or use Environment Picker in dotnet that allows me to simply create a set of environments (whatever that may mean to my application) and then have a picker on startup.
it wouldn't be too hard to build a simple custom one but i would imagine that this is done over and over in every company so wanted to see if there was one put together.
Have a look at app.config files. They are XML files which contain user or application settings. You'll be able to specify different application (read-only) settings at compile-time. Additionally, you'll be able to specify user settings at runtime.
I use app.config files to save and load things like window positions and other user preferences. I know many people store things like database connection strings, file paths, etc, in app.config.
Here's a simple example.
It is highly unlikely that you will find such prebuilt functionality, or that it would satisfy you if you did find it.
You may find this answer relevant.
Bottom-line: you will probably be happier and find it to be quicker to build your own.
However, someone just mentioned Spring for .NET, which you should investigate.