Statically Initialized Singleton

public class Singleton
{
    private readonly static Singleton instance = new Singleton();
    private Singleton() { }
    public static Singleton Instance => instance;
}

This implementation is thread-safe because in this case instance object is initialized in the static constructor. The CLR already ensures that all static constructors are executed thread-safe.

Mutating instance is not a thread-safe operation, therefore the readonly attribute guarantees immutability after initialization.