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Basically, it looks like functionally thread-safe, but there is one strategic mistake in your code. You should not use the field buses as a lock object. The idiomatic technique is this:

public class EventBusManager {       
    //...
    public EventBus GetIncrementUsage(string roomId) {
        lock (lockObject) {
            //...
            return busUsage.Bus;
        }
    }
    public void DecrementUsage(string roomId) {
        lock (lockObject) {
            //...
        }
    }
    object lockObject = new();
    // System.Threading.Lock lockObject = new(); // for .NET 9 and up
}

Note that the field lockObject should always be private.

You can find some rationale behind that here. The specifically stated point is that a lock object should be an instance not used for any other purpose.

Thank you.
—SA

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