(1)单例模式：

using System;
using System.Collections.Generic;
using System.Linq;
using System.Net.Http.Headers;
using System.Runtime.InteropServices.WindowsRuntime;
using System.Text;
using System.Threading.Tasks;

namespace Singleton
{
    public class Counter
    {
        public Counter()
        {
            Console.WriteLine("创建了Couter对像");
        }

        public static Counter _counter;
        private static readonly object lockobj = new object();
        /// <summary>
        /// 止方法只能用于非多线程的情况
        /// </summary>
        //public static Counter Instance
        //{
        //    get 
        //    { 
        //        if (_counter == null)
        //        {
        //            _counter = new Counter();
        //        }
        //        return _counter;
        //    }
        //}


        /// <summary>
        /// 加锁，但是多线程中会带来性能问题，每一个都要先获取锁
        /// </summary>
        //public static Counter Instance
        //{
        //    get
        //    {
        //        lock (lockobj)
        //        {
        //            if (_counter == null)
        //            {
        //                _counter = new Counter();
        //            }
        //            return _counter;
        //        }               
        //    }
        //}

        /// <summary>
        ///(1).加锁，在外面先判断再加锁，是正确的方式
        /// </summary>
        //public static Counter Instance
        //{
        //    get
        //    {
        //        if (_counter == null)
        //        {
        //            lock (lockobj)
        //            {
        //                if (_counter == null)
        //                {
        //                    _counter = new Counter();
        //                }
        //                return _counter;
        //            }
        //        }
        //        return _counter;
        //    }
        //}

        /// <summary>
        ///(2).使用静态对象单例特性
        ///// </summary>
        //public static Counter Instance=new Counter();

        /// <summary>
        ///(3).使用Lazy 懒加载
        /// </summary>
        public static Lazy<Counter> LazyInstance = new Lazy<Counter>(() => new Counter());

        public static readonly Counter Instance = LazyInstance.Value;

        /// <summary>
        ///(4).容器注册单例 如prism
        /// </summary>
     

    }
}

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace Singleton
{
    internal class Program
    {
        static void Main(string[] args)
        {

            var counter1 = Counter.Instance;
            var counter2 = Counter.Instance;

            for (int i = 0; i < 10; i++)
            {
                Task.Run(() => 
                {
                    var counter3 = Counter.Instance;
                    var counter4 = Counter.Instance;
                });             

            }

            Console.Read();
        }
    }
}