我认为这个代码示例非常有趣。
我们有一个类——我们称之为Test——有一个Finalize方法。在Main方法中,有两个代码块,其中一个使用了lock语句,另一个使用了Monitor.Enter()方法调用。此外,我这里有两个Test类的实例。
这个实验非常简单:在锁定块内将Test变量置为null,然后尝试手动收集它并调用GC.Collect方法。
所以,为了看到Finalize调用,我调用GC.WaitForPendingFinalizers方法。你可以看到,一切都非常简单。
根据lock语句的定义,编译器将其打开到try{...}finally{...}块中,并在try块中使用Monitor.Enter调用和Monitor,然后在finally块中退出。我曾试图手动实现try-finally块。
我期望在使用lock和使用Monitor.Enter时会有相同的行为。但是,惊喜吧,它们是不同的,如下所示:
public class Test
{
private string name;
public Test(string name)
{
this.name = name;
}
~Test()
{
Console.WriteLine(string.Format("Finalizing class name {0}.", name));
}
}
class Program
{
static void Main(string[] args)
{
var test1 = new Test("Test1");
var test2 = new Test("Tesst2");
lock (test1)
{
test1 = null;
Console.WriteLine("Manual collect 1.");
GC.Collect();
GC.WaitForPendingFinalizers();
Console.WriteLine("Manual collect 2.");
GC.Collect();
}
var lockTaken = false;
System.Threading.Monitor.Enter(test2, ref lockTaken);
try {
test2 = null;
Console.WriteLine("Manual collect 3.");
GC.Collect();
GC.WaitForPendingFinalizers();
Console.WriteLine("Manual collect 4.");
GC.Collect();
}
finally {
System.Threading.Monitor.Exit(test2);
}
Console.ReadLine();
}
}
这个示例的输出结果是:
手动收集1。手动收集2。 手动收集3。正在完成类名为Test2的操作。 手动收集4。最后一个 finally 块中出现了空引用异常,因为 test2 是空引用。
我感到惊讶并分解了我的代码成为 IL(Intermediate Language)。因此,这里是 Main 方法的 IL 转储:
.entrypoint
.maxstack 2
.locals init (
[0] class ConsoleApplication2.Test test1,
[1] class ConsoleApplication2.Test test2,
[2] bool lockTaken,
[3] bool <>s__LockTaken0,
[4] class ConsoleApplication2.Test CS$2$0000,
[5] bool CS$4$0001)
L_0000: nop
L_0001: ldstr "Test1"
L_0006: newobj instance void ConsoleApplication2.Test::.ctor(string)
L_000b: stloc.0
L_000c: ldstr "Tesst2"
L_0011: newobj instance void ConsoleApplication2.Test::.ctor(string)
L_0016: stloc.1
L_0017: ldc.i4.0
L_0018: stloc.3
L_0019: ldloc.0
L_001a: dup
L_001b: stloc.s CS$2$0000
L_001d: ldloca.s <>s__LockTaken0
L_001f: call void [mscorlib]System.Threading.Monitor::Enter(object, bool&)
L_0024: nop
L_0025: nop
L_0026: ldnull
L_0027: stloc.0
L_0028: ldstr "Manual collect."
L_002d: call void [mscorlib]System.Console::WriteLine(string)
L_0032: nop
L_0033: call void [mscorlib]System.GC::Collect()
L_0038: nop
L_0039: call void [mscorlib]System.GC::WaitForPendingFinalizers()
L_003e: nop
L_003f: ldstr "Manual collect."
L_0044: call void [mscorlib]System.Console::WriteLine(string)
L_0049: nop
L_004a: call void [mscorlib]System.GC::Collect()
L_004f: nop
L_0050: nop
L_0051: leave.s L_0066
L_0053: ldloc.3
L_0054: ldc.i4.0
L_0055: ceq
L_0057: stloc.s CS$4$0001
L_0059: ldloc.s CS$4$0001
L_005b: brtrue.s L_0065
L_005d: ldloc.s CS$2$0000
L_005f: call void [mscorlib]System.Threading.Monitor::Exit(object)
L_0064: nop
L_0065: endfinally
L_0066: nop
L_0067: ldc.i4.0
L_0068: stloc.2
L_0069: ldloc.1
L_006a: ldloca.s lockTaken
L_006c: call void [mscorlib]System.Threading.Monitor::Enter(object, bool&)
L_0071: nop
L_0072: nop
L_0073: ldnull
L_0074: stloc.1
L_0075: ldstr "Manual collect."
L_007a: call void [mscorlib]System.Console::WriteLine(string)
L_007f: nop
L_0080: call void [mscorlib]System.GC::Collect()
L_0085: nop
L_0086: call void [mscorlib]System.GC::WaitForPendingFinalizers()
L_008b: nop
L_008c: ldstr "Manual collect."
L_0091: call void [mscorlib]System.Console::WriteLine(string)
L_0096: nop
L_0097: call void [mscorlib]System.GC::Collect()
L_009c: nop
L_009d: nop
L_009e: leave.s L_00aa
L_00a0: nop
L_00a1: ldloc.1
L_00a2: call void [mscorlib]System.Threading.Monitor::Exit(object)
L_00a7: nop
L_00a8: nop
L_00a9: endfinally
L_00aa: nop
L_00ab: call string [mscorlib]System.Console::ReadLine()
L_00b0: pop
L_00b1: ret
.try L_0019 to L_0053 finally handler L_0053 to L_0066
.try L_0072 to L_00a0 finally handler L_00a0 to L_00aa
我看不出lock语句和Monitor.Enter调用之间有任何区别。所以,为什么在使用lock的情况下,我仍然对test1实例有一个引用,而且对象没有被垃圾回收,但是在使用Monitor.Enter时,它会被回收和完成?