Question

Okay, newbie multi-threading question:

I have a Singleton class. The class has a Static List and essentially works like this:

class MyClass {
    private static MyClass _instance;
    private static List<string> _list;
    private static bool IsRecording;

    public static void StartRecording() {
        _list = new List<string>();
        IsRecording = true;
    }

    public static IEnumerable<string> StopRecording() {
        IsRecording = false;
        return new List<string>(_list).AsReadOnly();
    }

    public MyClass GetInstance(){

    }

    public void DoSomething(){
        if(IsRecording) _list.Add("Something");
    }
}

Basically a user can call StartRecording() to initialize a List and then all calls to an instance-method may add stuff to the list. However, multiple threads may hold an instance to MyClass, so multiple threads may add entries to the list.

However, both list creation and reading are single operations, so the usual Reader-Writer Problem in multi-threading situations does not apply. The only problem I could see is the insertion order being weird, but that is not a problem.

Can I leave the code as-is, or do I need to take any precautions for multi-threading? I should add that in the real application this is not a List of strings but a List of Custom Objects (so the code is _list.Add(new Object(somedata))), but these objects only hold data, no code besides a call to DateTime.Now.

Edit: Clarifications following some answers: DoSomething cannot be static (the class here is abbreviated, there is a lot of stuff going on that is using instance-variables, but these created by the constructor and then only read).
Is it good enough to do

lock(_list){
    _list.Add(something);
}

and

lock(_list){
    return new List<string>(_list).AsReadOnly();
 }

or do I need some deeper magic?

Answer 1

You certainly must lock the _list. And since you are creating multiple instances for _list you can not lock on _list itself but you should use something like:

private static object _listLock = new object();

As an aside, to follow a few best practices:

• DoSomething(), as shown, can be static and so it should be.

• for Library classes the recommended pattern is to make static members thread-safe, that would apply to StartRecording(), StopRecording() and DoSomething().

I would also make StopRecording() set _list = null and check it for null in DoSomething().

And before you ask, all this takes so little time that there really are no performance reasons not to do it.