Is `??` thread safe in C#?

Question

Simple question: Are "??" and "?." and "? :" thread-safe? And can I trust them? Or I should use other thread-safety solutions? For instance this code:

public static T Instance => _Instance ?? (_Instance = CreateInstance());

is thread-safe?

Answer 1

It is not threadsafe, due to a race condition. (e.g Thread A checks value, finds it null, thread B checks value, finds it null, thread B initialises value with call to CreateInstance(), thread A intialises value with call to CreateInstance().)

The correct approach to the threadsafe initialisation of a singleton is to use the Lazy<T> class, for example:

public T Instance => _instance.Value;

static Lazy<T> _instance = new Lazy<T>(CreateInstance);

where CreateInstance() returns an instance of type T.