Self-study C#13 from 0--Lock object problem of subclass and parent class methods

调用父类方法时，如何与子类方法，在锁对象不是同一个实例下，能线程安全，请见下面三种情况。

case1：

如下代码，在调用父类的方法时，和子类的方法，发生线程安全问题。原因的锁对象的实例不是同一个。

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

namespace ForTest//check the lock object for thread safety{
    class Program
    {        static void Main(string[] args)
        {
            son myson = new son();
            Parallel.For(0, 5, (int i) =>
             {
                 myson.methodA();
                 myson.methodC();
             });
            Console.ReadKey();
        }
    }    public class grandfather
    {        //protected static object syncRoot = new object();
    }    public class father:grandfather
    {        private static object syncRoot = new object();        
protected int cont = 0;        public virtual bool methodA()
        {            lock (syncRoot)
            {
                cont++;
                Thread.Sleep(1000);
                Console.WriteLine("cout++ is " + cont);                
return true;
            }
        }        public virtual bool methodB()
        {            lock (syncRoot)
            {                return true;
            }
        }
    }    public class son:father
    {        private static object syncRoot = new object();        
public bool methodC()
        {            lock (syncRoot)
            {
                cont += 2;
                Thread.Sleep(2000);
                Console.WriteLine("cont += 2 is " + cont);                
return true;
            }
        }
    }
}

输出：

cout++ is 1
cout++ is 4
cont += 2 is 5
cout++ is 5
cout++ is 8
cont += 2 is 9
cout++ is 11
cont += 2 is 11
cont += 2 is 13
cont += 2 is 15

case2：

case1的解决方法是，在父类初始化锁对象，让子类继承。这样就线程安全了。如下。

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

namespace ForTest//check the lock object for thread safety{
    class Program
    {        static void Main(string[] args)
        {
            son myson = new son();
            Parallel.For(0, 5, (int i) =>
             {
                 myson.methodA();
                 myson.methodC();
             });
            Console.ReadKey();
        }
    }    public class grandfather
    {        protected static object syncRoot = new object();
    }    public class father:grandfather
    {        protected int cont = 0;        public virtual bool methodA()
        {            lock (syncRoot)
            {
                cont++;
                Thread.Sleep(1000);
                Console.WriteLine("cout++ is " + cont);                return true;
            }
        }        public virtual bool methodB()
        {            lock (syncRoot)
            {                return true;
            }
        }
    }    public class son:father
    {        public bool methodC()
        {            lock (syncRoot)
            {
                cont += 2;
                Thread.Sleep(2000);
                Console.WriteLine("cont += 2 is " + cont);                return true;
            }
        }
    }
}

输出：

cout++ is 1
cout++ is 2
cont += 2 is 4
cout++ is 5
cout++ is 6
cout++ is 7
cont += 2 is 9
cont += 2 is 11
cont += 2 is 13
cont += 2 is 15

case3：

当然有些特殊情况下，子类硬要重新实例化一个锁对象。如何避免上面第一种线程安全问题发生？需要：

子类加锁重写父类分方法（如果父类methodA是虚方法）

或者

new一下（如果父类methodA是实例方法）。

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

namespace ForTest//check the lock object for thread safety{
    class Program
    {        static void Main(string[] args)
        {
            son myson = new son();
            Parallel.For(0, 5, (int i) =>
             {
                 myson.methodA();
                 myson.methodC();
             });
            Console.ReadKey();
        }
    }    public class grandfather
    {        protected static object syncRoot = new object();
    }    public class father:grandfather
    {        protected int cont = 0;        public virtual bool methodA()
        {            lock (syncRoot)
            {
                cont++;
                Thread.Sleep(1000);
                Console.WriteLine("cout++ is " + cont);                return true;
            }
        }        public virtual bool methodB()
        {            lock (syncRoot)
            {                return true;
            }
        }
    }    public class son:father
    {        private static object sync = new object();        public override bool methodA()//重写
        {            lock (sync)
            {                return base.methodA();
            }
        }        public bool methodC()
        {            lock (sync)
            {
                cont += 2;
                Thread.Sleep(2000);
                Console.WriteLine("cont += 2 is " + cont);                return true;
            }
        }
    }
}

或

 public new bool methodA()\\new一下
        {            lock (sync)
            {                return base.methodA();
            }
        }

输出：

cout++ is 1
cout++ is 2
cont += 2 is 4
cout++ is 5
cout++ is 6
cout++ is 7
cont += 2 is 9
cont += 2 is 11
cont += 2 is 13
cont += 2 is 15

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