C# Thread Synchronization Issue
Thread 看起來是同步，但事實上是在相互搶資源…
前置準備：讓三個執行緒可以很乾淨的遞增
寫一個範例，讓三個執行緒可以很乾淨的遞增…
現在總共有三個執行緒，一個是主執行緒Main()，另外兩個是新建立的thread1、thread2執行緒。
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
internal class Program
{
static int count = 0;
static void Main(string[] args)
{
//建立兩個執行緒，讓它們都進入IncrementCount方法
var thread1 = new Thread(IncrementCount);
var thread2 = new Thread(IncrementCount);
thread1.Start();
Thread.Sleep(500); //讓主執行緒不同步半秒
thread2.Start();
}
static void IncrementCount()
{
while (true)
{
count++;
//做一個有趣的輸出
Console.WriteLine($"Thread ID: {Thread.CurrentThread.ManagedThreadId} incremented count to: {count}");
Thread.Sleep(1000); //讓執行緒睡眠1000毫秒(讓程式暫停一秒)
}
}
}
執行結果：
控制台輸出 託管的 Thread ID 和很乾淨的 Count 遞增。
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
Thread ID: 10 incremented count to: 1
Thread ID: 11 incremented count to: 2
Thread ID: 11 incremented count to: 3
Thread ID: 10 incremented count to: 4
Thread ID: 10 incremented count to: 5
Thread ID: 11 incremented count to: 6
Thread ID: 11 incremented count to: 7
Thread ID: 10 incremented count to: 8
Thread ID: 11 incremented count to: 9
Thread ID: 10 incremented count to: 10
Thread ID: 11 incremented count to: 11
Thread ID: 10 incremented count to: 12
Thread ID: 11 incremented count to: 13
Thread ID: 10 incremented count to: 14
Thread ID: 11 incremented count to: 15
Thread ID: 10 incremented count to: 16
Thread ID: 11 incremented count to: 17
模擬互相爭奪搶資源的情境
再改一下程式，可以看出他們互相爭奪搶資源的情境…
count++看起來像是一個操作，但事實上不是，其實它背後做了很多事，會執行很多步驟。
count++….count=count+1
修改一下，再讓主執行緒大量的睡一下，把差距拉大…
(一秒和半秒對計算機來說，是很長的時間…)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
internal class Program
{
static int count = 0;
static void Main(string[] args)
{
//建立兩個執行緒，讓它們都進入IncrementCount方法
var thread1 = new Thread(IncrementCount);
var thread2 = new Thread(IncrementCount);
thread1.Start();
Thread.Sleep(500); //讓主執行緒不同步半秒
thread2.Start();
}
static void IncrementCount()
{
while (true)
{
int temp = count;
Thread.Sleep(500); //讓主執行緒睡半秒
count = temp + 1;
//做一個有趣的輸出
Console.WriteLine($"Thread ID: {Thread.CurrentThread.ManagedThreadId} incremented count to: {count}");
Thread.Sleep(1000); //讓執行緒睡眠1000毫秒(讓程式暫停一秒)
}
}
}
執行結果：
會發現控制台有非常糟糕的輸出，count不再井然有序，會出現重複的 count值。
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
Thread ID: 9 incremented count to: 1
Thread ID: 10 incremented count to: 2
Thread ID: 9 incremented count to: 3
Thread ID: 10 incremented count to: 4
Thread ID: 9 incremented count to: 5
Thread ID: 10 incremented count to: 5
Thread ID: 9 incremented count to: 6
Thread ID: 10 incremented count to: 6
Thread ID: 9 incremented count to: 7
Thread ID: 10 incremented count to: 8
Thread ID: 9 incremented count to: 9
Thread ID: 10 incremented count to: 9
Thread ID: 9 incremented count to: 10
Thread ID: 10 incremented count to: 10
Thread ID: 9 incremented count to: 11
Thread ID: 10 incremented count to: 11