public sealed class Singleton
{
    private static readonly Lazy<Singleton> lazy =
        new Lazy<Singleton>(() => new Singleton());
    
    public static Singleton Instance { get { return lazy.Value; } }

    private Singleton()
    {
    }
}

无法实现Generic Singleton

The problem with a generic singleton factory is that since it is generic you do not control the "singleton" type that is instantiated so you can never guarantee that the instance you create will be the only instance in the application.

If a user can provide a type to as a generic type argument then they can also create instances of that type. In other words, you cannot create a generic singleton factory - it undermines the pattern itself.

public class Singleton
{
    private Singleton() {}
    static Singleton() {}
    private static Singleton _instance = new Singleton();
    public static Singleton Instance { get { return _instance; }}
}

The

private static Singleton _instance = new Singleton();

line removes the need for locking, as a static constructor is thread safe.