Question

How would you name an interface which provides 1 method inUse()?

Answer 1

I would actually reconsider the name ‘inUse()’ in the first place; A boolean value obviously has only two possible values, but you’re actually adding the ability to get a state. I’d consider declaring an enum

public enum UsageState
{
    Idle,
    InUse
}

and name your interface IHasUsageState. This gives you the flexibility of adding things like Starting, Finishing, WaitingToBeUsed or other options depending on precisely what is is you’re doing, for example if you have threading issues to deal with in the future.

Also, you eliminate the need for negative checks like if (!obj.InUse()) { } in favor of the more readable and intuitive if (obj.Usage == UsageState.Idle) { }, not to mention you may decide in the future that you might want it to specify WHY it’s idle.