.net 为什么 System.Timers.Timer 能在 GC 中存活，而 System.Threading.Timer 不能

似乎有System.Timers.Timer个实例通过某种机制保持了活动状态，但有System.Threading.Timer个实例没有.

具有周期性System.Threading.Timer和自动复位System.Timers.Timer的示 routine 序:

class Program
{
  static void Main(string[] args)
  {
    var timer1 = new System.Threading.Timer(
      _ => Console.WriteLine("Stayin alive (1)..."),
      null,
      0,
      400);

    var timer2 = new System.Timers.Timer
    {
      Interval = 400,
      AutoReset = true
    };
    timer2.Elapsed += (_, __) => Console.WriteLine("Stayin alive (2)...");
    timer2.Enabled = true;

    System.Threading.Thread.Sleep(2000);

    Console.WriteLine("Invoking GC.Collect...");
    GC.Collect();

    Console.ReadKey();
  }
}

当我运行这个程序(.NET 4.0 Client，Release，在调试器之外)时，只有System.Threading.Timer个是GC:

Stayin alive (1)...
Stayin alive (1)...
Stayin alive (2)...
Stayin alive (1)...
Stayin alive (2)...
Stayin alive (1)...
Stayin alive (2)...
Stayin alive (1)...
Stayin alive (2)...
Invoking GC.Collect...
Stayin alive (2)...
Stayin alive (2)...
Stayin alive (2)...
Stayin alive (2)...
Stayin alive (2)...
Stayin alive (2)...
Stayin alive (2)...
Stayin alive (2)...
Stayin alive (2)...

EDIT:我已经接受了下面约翰的回答，但我想解释一下.

当运行上面的示 routine 序(断点为Sleep)时，下面是相关对象和GCHandle表的状态:

!dso
OS Thread Id: 0x838 (2104)
ESP/REG  Object   Name
0012F03C 00c2bee4 System.Object[]    (System.String[])
0012F040 00c2bfb0 System.Timers.Timer
0012F17C 00c2bee4 System.Object[]    (System.String[])
0012F184 00c2c034 System.Threading.Timer
0012F3A8 00c2bf30 System.Threading.TimerCallback
0012F3AC 00c2c008 System.Timers.ElapsedEventHandler
0012F3BC 00c2bfb0 System.Timers.Timer
0012F3C0 00c2bfb0 System.Timers.Timer
0012F3C4 00c2bfb0 System.Timers.Timer
0012F3C8 00c2bf50 System.Threading.Timer
0012F3CC 00c2bfb0 System.Timers.Timer
0012F3D0 00c2bfb0 System.Timers.Timer
0012F3D4 00c2bf50 System.Threading.Timer
0012F3D8 00c2bee4 System.Object[]    (System.String[])
0012F4C4 00c2bee4 System.Object[]    (System.String[])
0012F66C 00c2bee4 System.Object[]    (System.String[])
0012F6A0 00c2bee4 System.Object[]    (System.String[])

!gcroot -nostacks 00c2bf50

!gcroot -nostacks 00c2c034
DOMAIN(0015DC38):HANDLE(Strong):9911c0:Root:  00c2c05c(System.Threading._TimerCallback)->
  00c2bfe8(System.Threading.TimerCallback)->
  00c2bfb0(System.Timers.Timer)->
  00c2c034(System.Threading.Timer)

!gchandles
GC Handle Statistics:
Strong Handles:       22
Pinned Handles:       5
Async Pinned Handles: 0
Ref Count Handles:    0
Weak Long Handles:    0
Weak Short Handles:   0
Other Handles:        0
Statistics:
      MT    Count    TotalSize Class Name
7aa132b4        1           12 System.Diagnostics.TraceListenerCollection
79b9f720        1           12 System.Object
79ba1c50        1           28 System.SharedStatics
79ba37a8        1           36 System.Security.PermissionSet
79baa940        2           40 System.Threading._TimerCallback
79b9ff20        1           84 System.ExecutionEngineException
79b9fed4        1           84 System.StackOverflowException
79b9fe88        1           84 System.OutOfMemoryException
79b9fd44        1           84 System.Exception
7aa131b0        2           96 System.Diagnostics.DefaultTraceListener
79ba1000        1          112 System.AppDomain
79ba0104        3          144 System.Threading.Thread
79b9ff6c        2          168 System.Threading.ThreadAbortException
79b56d60        9        17128 System.Object[]
Total 27 objects

正如John在他的回答中指出的那样，两个定时器都在GCHandle表中注册了它们的回调(System.Threading._TimerCallback).正如Hans在他的 comments 中指出的那样，完成此操作后，state参数也保持有效.

正如John指出的，System.Timers.Timer之所以保持活动状态是因为回调引用了它(它作为state参数传递给内部System.Threading.Timer)；同样，我们的System.Threading.Timer被GC’ed的原因是因为它的回调引用了not.

向timer1的回调(例如Console.WriteLine("Stayin alive (" + timer1.GetType().FullName + ")"))添加显式引用足以防止GC.

在System.Threading.Timer上使用单参数构造函数也可以，因为计时器随后会将自身引用为state参数.以下代码使两个计时器在GC之后保持活动状态，因为它们都被GCHandle表中的回调引用:

class Program
{
  static void Main(string[] args)
  {
    System.Threading.Timer timer1 = null;
    timer1 = new System.Threading.Timer(_ => Console.WriteLine("Stayin alive (1)..."));
    timer1.Change(0, 400);

    var timer2 = new System.Timers.Timer
    {
      Interval = 400,
      AutoReset = true
    };
    timer2.Elapsed += (_, __) => Console.WriteLine("Stayin alive (2)...");
    timer2.Enabled = true;

    System.Threading.Thread.Sleep(2000);

    Console.WriteLine("Invoking GC.Collect...");
    GC.Collect();

    Console.ReadKey();
  }
}