public class MainViewModel  BaseViewModel
    {
        private static MainViewModel instance = new MainViewModel();
        static MainViewModel()
      { }
        private MainViewModel(){}
        public static MainViewModel Instance
        {
            get = instance;
        }
        ...
    }
     public class ClassA
        {
            private static readonly ClassA instance = new ClassA();
            static ClassA()  { }
            private ClassA() { }
            public static ClassA Instance
            {
                get = instance;
            }
    ...
    }