void DoTest()
        {
            Preference instance1 = Preference.GetInstance(); // initialize correctly

            // set value "123" with key "abc"
            instance1.setPreference("abc", "123");
            string abcValue = instance1.getPreference("abc"); // get with key "abc";

            Assert.Equal("123", abcValue);

            //Preference instance2 = Preference.GetInstance; // initialize correctly
            // set value "567" with key "abc" in instance2.
            //instance2.setPreference("abc", "567");--> This will get error because of same key.
            //That's actually prove that singleton constractor creates it's instance only once.
            // get value with key "abc" from instance1.
            //string abcValue2 = instance2.getPreference("abc");
            // assert
            //Assert.Equal("567", abcValue2);
        }