单实例和IEnumerable<T>的重载方法

public class Singleton
{
    private static Singleton instance;
    private static readonly object lockObj = new object();

    private Singleton() { }

    public static Singleton Instance
    {
        get
        {
            if (instance == null)
            {
                lock (lockObj)
                {
                    if (instance == null)
                    {
                        instance = new Singleton();
                    }
                }
            }
            return instance;
        }
    }
}

public interface IEnumerable<out T> : IEnumerable
{
    IEnumerator<T> GetEnumerator();
}

List<int> numbers = new List<int> { 1, 2, 3, 4, 5 };

foreach (int number in numbers)
{
    Console.WriteLine(number);
}