>>Why is the application class holding database properties? What you're wanting to do is called tight-coupling is not a good idea as it makes one class rely too much on the other.
>>
>I see. Do you suggest to not have database a property of the application class but rather main class use both classes independently? I still want to have a reference to either one or 2 classes, so I can pass invoke information around.

I think what you are looking for is something like this:

    public class Application
    {
        private DataBase dataBase;

        public Application()
        {
            dataBase = new DataBase(this);
        }
    }

    public class DataBase
    {
        private Application application;
        public DataBase(Application app)
        {
            application = app;
        }
    }

But, in general I agree with Craig. For this type of thing you should be using something like Prism or MEF....