public static class Helper
    {
        // Property 1
        public static readonly MyRepository Instance1 = new MyRepository();

        // Property 2
        [ThreadStatic]
        private static MyRepository _instance2;
                
        public static MyRepository Instance2
        {
            get
            {
                if (_instance2 == null) _instance2 = new MyRepository();
                    return _instance2;
            }
        }

        // Error: NullReferenceException     //Instance1 = null  	
        public static object Method1() { return Instance1.GetData(); }

        // Error: NullReferenceException     //Instance2 = null
        public static object Method2() { return Instance2.GetData(); }

        // OK
        public static object Method3() { return "abc"; }
    }