.NET System.Threading - 說明範例 2

System.Threading 裡面提供了很多用來控制執行緒的工具

層次比較高，用起來效率高很多

來一個一個介紹起

走起~~

1. CountdownEvent 跟 Interlocked (範例就放一起了)

CountdownEvent 用 AddCount 跟 Signal 來 增加跟減少計數，當計數歸零就可以通過

所以在用的時候，預設會放個1開始，避免連算第一次都沒有

Interlocked 則是用來處理一些非執行緒安全的計算

例如:   i += 1

	var count = 10000;
	var value = 0;
	// 由於 value += 1 並不是 thread-safe，所以當 count 夠大的時候 value 最後的加總，很難剛好等於 4*count
	// 因為使用了 ThreadPool 碰不到直接的 Thread 來 Join，改用 CountdownEvent Wait
	using(var cdEvent = new CountdownEvent(1))
	{
	cdEvent.AddCount();
	ThreadPool.QueueUserWorkItem((s) => { for (int i = 0; i < count; i++) value += 1; cdEvent.Signal();});
	cdEvent.AddCount();
	ThreadPool.QueueUserWorkItem((s) => { for (int i = 0; i < count; i++) value += 1; cdEvent.Signal();});
	cdEvent.AddCount();
	ThreadPool.QueueUserWorkItem((s) => { for (int i = 0; i < count; i++) value += 1; cdEvent.Signal();});
	cdEvent.AddCount();
	ThreadPool.QueueUserWorkItem((s) => { for (int i = 0; i < count; i++) value += 1; cdEvent.Signal();});
	cdEvent.Signal();
	cdEvent.Wait();
	}
	Console.WriteLine($"{value} != {4 * count}");
	/*
	19989 != 40000
	*/
	// value += 1 改用 Interlocked.Add 取代 (thread safe!!)
	value = 0;
	using(var cdEvent = new CountdownEvent(1))
	{
	cdEvent.AddCount();
	ThreadPool.QueueUserWorkItem((s) => { for (int i = 0; i < count; i++) Interlocked.Add(ref value, 1); cdEvent.Signal();});
	cdEvent.AddCount();
	ThreadPool.QueueUserWorkItem((s) => { for (int i = 0; i < count; i++) Interlocked.Add(ref value, 1); cdEvent.Signal();});
	cdEvent.AddCount();
	ThreadPool.QueueUserWorkItem((s) => { for (int i = 0; i < count; i++) Interlocked.Add(ref value, 1); cdEvent.Signal();});
	cdEvent.AddCount();
	ThreadPool.QueueUserWorkItem((s) => { for (int i = 0; i < count; i++) Interlocked.Add(ref value, 1); cdEvent.Signal();});
	cdEvent.Signal();
	cdEvent.Wait();
	}
	Console.WriteLine($"{value} == {4 * count}");
	/*
	40000 == 40000
	*/

view raw blog-dotnet-thread-tool-Interlocked.cs hosted with ❤ by GitHub

2. Monitor 跟 lock 陳述式

value += 1;
也可以用 lock(obj) { value += 1; } 來鎖定只有一個執行續通過

lock 其實 C# 的語法糖，下面兩個 IL code 是相同的

	var obj = new object();
	lock (obj)
	{
	}
	// -----
	var obj2 = new object();
	var islocken = false;
	try
	{
	Monitor.Enter(obj2, ref islocken);
	}
	finally
	{
	if(islocken)
	Monitor.Exit(obj2);
	}

view raw blog-dotnet-thread-tool-Monitor.cs hosted with ❤ by GitHub

3. Mutex

把 Mutex 當作資源本身模擬下用餐的範例

	var knife = new Mutex();
	var fork = new Mutex();
	var mealSize = 10;
	var p1 = new Thread(EatType1);
	p1.Name = "小明";
	var p2 = new Thread(EatType1);
	p2.Name = "大強";
	p1.Start();
	p2.Start();
	p1.Join();
	p2.Join();
	// 雖然共用餐具有點噁心，但是可以順利用完餐的
	p1 = new Thread(EatType1);
	p1.Name = "小明";
	p2 = new Thread(EatType2);
	p2.Name = "大強";
	p1.Start();
	p2.Start();
	p1.Join();
	p2.Join();
	// 最後大致會停在互相等待對方手上的餐具，導致無法順利用完餐
	// 但是有機會可以用完餐的
	/*
	...
	小明 - 拿起刀子
	小明 - 等待叉子
	大強 - 拿起叉子
	大強 - 等待刀子
	*/
	void EatType1()
	{
	for(var i = 0 ; i < mealSize; i++)
	{
	Console.WriteLine($"{Thread.CurrentThread.Name} - 等待刀子");
	knife.WaitOne();
	Console.WriteLine($"{Thread.CurrentThread.Name} - 拿起刀子");
	Console.WriteLine($"{Thread.CurrentThread.Name} - 等待叉子");
	fork.WaitOne();
	Console.WriteLine($"{Thread.CurrentThread.Name} - 拿起叉子");
	Console.WriteLine($"{Thread.CurrentThread.Name} - 用餐進度 {i+1}/{mealSize}");
	knife.ReleaseMutex();
	Console.WriteLine($"{Thread.CurrentThread.Name} - 放下刀子");
	fork.ReleaseMutex();
	Console.WriteLine($"{Thread.CurrentThread.Name} - 放下叉子");
	}
	}
	void EatType2()
	{
	for(var i = 0 ; i < mealSize; i++)
	{
	Console.WriteLine($"{Thread.CurrentThread.Name} - 等待叉子");
	fork.WaitOne();
	Console.WriteLine($"{Thread.CurrentThread.Name} - 拿起叉子");
	Console.WriteLine($"{Thread.CurrentThread.Name} - 等待刀子");
	knife.WaitOne();
	Console.WriteLine($"{Thread.CurrentThread.Name} - 拿起刀子");
	Console.WriteLine($"{Thread.CurrentThread.Name} - 用餐進度 {i+1}/{mealSize}");
	knife.ReleaseMutex();
	Console.WriteLine($"{Thread.CurrentThread.Name} - 放下刀子");
	fork.ReleaseMutex();
	Console.WriteLine($"{Thread.CurrentThread.Name} - 放下叉子");
	}
	}

view raw blog-dotnet-thread-tool-Mutex.cs hosted with ❤ by GitHub

4. Semaphore

(待補充)

5. Barrier

(待補充)

6. Timer

(待補充)

7. CancellationToken & CancellationTokenSource

(待補充)

8. Volatile (很特殊的情況)

(待補充)