Is this really a singleton? [closed]

问题:

I've got a singleton class like this.

 class UserInteraction
    {
        private CustomerInformation _loginDetails;
        private MusicDetails[] _musicFilesDownloaded;

        private static volatile UserInteraction _interactionObject = null;
        private static object syncRoot = new Object();
        private UserInteraction()
        {

        }
        public static UserInteraction Instance
        {
           get 
            {
                if (_interactionObject == null) 
                {
                 lock (syncRoot) 
                 {
                     if (_interactionObject == null)
                    _interactionObject = new UserInteraction();
                 }
                }

                return _interactionObject;
            }
        }

        public CustomerInformation UserInfo
        {
            get
            {
                return _loginDetails;
            }
            set
            {

                _loginDetails = value;

            }
        }



        public MusicDetails[] FilesDownloaded
        {
            get
            {
                return _musicFilesDownloaded;
            }

            set
            {
                _musicFilesDownloaded= value;

            }
        }

        public void PurgeContents()
        {

        }
        public void SerializeValues()
        {
        }

    }

I can use the following code to access this singleton class

UserInteraction.Instance.UserInfo.CardData = "";

My questions are as follows

1) As you can see I can read/write into UserInfo structure, that means i can any number of copy of UserInfo. Is this against the laws of SingleTon?

2) If this is against the law of Singleton, how can i prohibit the duplication of UserInfo at the same time assign values into it?

Thanks.

回答1:

No, this is not against the "laws" of Singleton. A singleton guarantees, that you have at most one instance of a certain class. That you can change the members of that class doesn't matter.
BTW: There are better ways to implement a singleton. See here: http://csharpindepth.com/Articles/General/Singleton.aspx

回答2:

UserInteraction is a singleton. UserInfo is probably no singleton. There can be only one instance of UserInteraction per AppDomain, so it is a singleton.

You have global mutable state which is evil. So I wouldn't write code like that.

I'm not sure if the check-lock-check pattern is guaranteed to work in .net. But why do you use something complicated like this when you can simply use Lazy<T> in .net 4? http://csharpindepth.com/Articles/General/Singleton.aspx

回答3:

This is a singleton on UserInteraction, not on UserInfo. If you need UserInfo to be a singleton, then you need to refactor.

回答4:

I think these will guide you to answer your own questions:

1. Singleton Pattern (Wikipedia);
2. The Singleton Design Pattern (Fully explanatory PDF);
3. Exploring the Singleton Design Pattern.

In short, the Singleton guarantees that you have only one instance of a given class shared among your system modules. Perhaps should you consider make UserInfo a singleton too? That is only a matter of design, since two different developers will have multiple ways to achieve the same end-behaviour.

回答5:

This is a singleton. One and only one instance. Thus its not against the laws, I dont think you have a valid question.

1. You can read/write the UserInfo section, but you are doing it through the singleton.
2. Not valid.