[原创]C#编写的读者写者问题(公平竞争的读者写者)

// 读者写者问题

using System;
using System.Threading;

public class Book{

 private int[] pages;
 private bool aWriterWait = false;
 private int readerCount = 0;

 private object mutex = new Object(); // 控制对 readerCount 的访问
 private object db = new Object(); // 控制对 pages 的访问
 private object obj  = new Object(); // 控制对 aWriterWait 的访问
 

 public Book(int numOfPages){
  pages = new int[numOfPages];
 }

 public int NumOfPages{
  get{
   return pages.Length; //常量，无须同步
  }
 }

 public int this[int index]{
  get{
   lock(obj){
    while( aWriterWait ){
     Monitor.Wait(obj);
    }
   }

   lock(mutex){ 
    // readerCount的锁
    // 检查是否为第一个读者
    readerCount++;
    if(readerCount == 1) Monitor.Enter(db);
   }
   
   int valueCopy = pages[index]; // 读取一个数据

   Console.WriteLine( Thread.CurrentThread.Name + " 读了 第" + (index + 1) + "页，他读到的数据是" + valueCopy );

   DisplayContent();

   lock(mutex){
    readerCount--;
    if(readerCount == 0) Monitor.Exit(db);
   }

   return valueCopy;
  }
  set{
   lock (obj){
    aWriterWait = true;
   }

   lock(db){ // 阻止其他的写者去写书
    pages[index] = value;
    Console.WriteLine( Thread.CurrentThread.Name + "把" + value + "写到第" + (index + 1) + "页");
    DisplayContent();
   }

   lock(obj){
    aWriterWait = false;
    Monitor.PulseAll(obj); // 通知等待的读者
   }
  }
 }
 
 private void DisplayContent(){
  Console.WriteLine("{0,-35}","书的内容是:");
  for(int i = 0;i < pages.Length;i++){
   Console.Write("{0,-9}",pages[i]);
  }
  Console.WriteLine("\r\n");
 }
}

public class Reader{
 private Book book;
 private Random rand;
 
 public Reader(Book b,Random random){
  book = b;
  rand = random;
 }
 
 public void Read(){
  for (int i=0; i < 5; i++){
   int p = rand.Next(0,book.NumOfPages);
   int a = book[p];
   Thread.Sleep(rand.Next(0,100));
  }
 }
}

public class Writer{
 private Book book;
 private Random rand;
 
 public Writer(Book b,Random random){
  book = b;
  rand = random;
 }
 
 public void Write(){
  for(int i = 0;i < 5;i++){
   int p = rand.Next(0,book.NumOfPages);
   book[p] = rand.Next(50,100);
   Thread.Sleep(rand.Next(0,100));
  }
 }
}

public class Test{
 public static void Main(string[] args){
  Book book = new Book(5);
  Random random = new Random();
  
  string[] readerNames = {"猪八戒","唐僧","沙和尚"};
  string[] writerNames = {"观音菩萨","孙悟空","玉皇大帝"};

  Thread[] readerThreads = new Thread[3];
  Thread[] writerThreads = new Thread[3];

  for(int i = 0;i < 3;i++){
   Reader reader = new Reader(book,random);
   readerThreads[i] = new Thread(new ThreadStart(reader.Read));
   readerThreads[i].Name = readerNames[i];
   
   Writer writer = new Writer(book,random);
   writerThreads[i] = new Thread(new ThreadStart(writer.Write));
   writerThreads[i].Name = writerNames[i];

  }
  
  for (int i=0; i<3; i++){
   readerThreads[i].Start();
   writerThreads[i].Start();
  } 
 }
}

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