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);
}