Lambda Expression cause weakreference's target cannot be GC?

Question

namespace Test
{

    class Test
    {
        delegate void HandleMessage(string message);

        public void handleMessage(string message){}

        static void Main(string[] args)
        {
            HandleMessage listener1 = new Test().handleMessage;
            WeakReference w1 = new WeakReference(listener1);

            HandleMessage listener2 = (message) => { };
            WeakReference w2 = new WeakReference(listener2);

            Console.WriteLine("w1.Target:\t[" + w1.Target + "]");
            Console.WriteLine("w2.Target:\t[" + w2.Target + "]");

            listener1 = null;
            listener2 = null;
            GC.Collect();
            Console.WriteLine("after GC");

            Console.WriteLine("w1.Target:\t[" + w1.Target + "]");
            Console.WriteLine("w2.Target:\t[" + w2.Target + "]");

            Console.ReadLine();
        }
    }
}

why w2.Target is not null after GC.
---------------------------------------------------------
w1.Target:      [Test.Test+HandleMessage]
w2.Target:      [Test.Test+HandleMessage]
after GC
w1.Target:      []
w2.Target:      [Test.Test+HandleMessage]

Answer 1

Hi,

The Lambda Expression is defined as a private static delegate,and then pushed onto stack . when listener2 is set to null , the Lambda is still referenced , and won't be collected.

Please check that in the Il code using IL Disassembler.

If we the code of Main is :

static void Main(string[] args)

{

HandleMessage listener1 = new Test().handleMessage;

HandleMessage listener2 = (message) => { };

}

The il code would be:

.method private hidebysig static void  Main(string[] args) cil managed

{

.entrypoint

// Code size       51 (0x33)

.maxstack  3

.locals init ([0] class linq_relating_to_weak_reference.Test/HandleMessage listener1,

[1] class linq_relating_to_weak_reference.Test/HandleMessage listener2)

IL_0000:  nop

IL_0001:  newobj     instance void linq_relating_to_weak_reference.Test::.ctor()

IL_0006:  ldftn      instance void linq_relating_to_weak_reference.Test::handleMessage(string)

IL_000c:  newobj     instance void linq_relating_to_weak_reference.Test/HandleMessage::.ctor(object,

native int)

IL_0011:  stloc.0

IL_0012:  ldsfld     class linq_relating_to_weak_reference.Test/HandleMessage linq_relating_to_weak_reference.Test::'CS$<>9__CachedAnonymousMethodDelegate1'

IL_0017:  brtrue.s   IL_002c

IL_0019:  ldnull

IL_001a:  ldftn      void linq_relating_to_weak_reference.Test::'<Main>b__0'(string)

IL_0020:  newobj     instance void linq_relating_to_weak_reference.Test/HandleMessage::.ctor(object,

native int)

IL_0025:  stsfld     class linq_relating_to_weak_reference.Test/HandleMessage linq_relating_to_weak_reference.Test::'CS$<>9__CachedAnonymousMethodDelegate1'

IL_002a:  br.s       IL_002c

IL_002c:  ldsfld     class linq_relating_to_weak_reference.Test/HandleMessage linq_relating_to_weak_reference.Test::'CS$<>9__CachedAnonymousMethodDelegate1'

IL_0031:  stloc.1

IL_0032:  ret

} // end of method Test::Main

Code on line 12 shows:

ldsfld pushes the value of a static field onto the evaluation stack.

'CS$<>9__CachedAnonymousMethodDelegate1'  is the static field generated by compiler.

then on line 25 ,

listener2 is set to the compiler generated delegate.


Best regards,
Harry

Answer 2

Thank Harry for your correct reply.

http://stackoverflow.com/questions/535972/lambda-expression-cause-weakreferences-target-cannot-be-gc
I also got the answer from above。

The following example show that :

if Test#create() method don't reference to any Test instance object's properties or methods, then "private static HandleMessage CS$<>9__CachedAnonymousMethodDelegate1" will be created by compiler, like what Jon Skeet has said - That makes it more efficient when you use the same lambda expression multiple times.

if Test#create() method reference to any Test instance object's properties or methods, like the example below calling this.toString(); then compiler can not create static method to replace the intstance's method logic, so after GC the HandleMessage instance can be collected.

namespace Test {

class Test
{
    public delegate void HandleMessage(string message);

    public void handleMessage(string message)
    {
    }

    public HandleMessage create()
    {
        return (message) => { 
            this.ToString(); 
        };
    }       

    static void Main(string[] args)
    {
        HandleMessage listener1 = new Test().handleMessage;
        WeakReference w1 = new WeakReference(listener1);

        HandleMessage listener2 = new Test().create();//(message) => { };
        WeakReference w2 = new WeakReference(listener2);

        Console.WriteLine("w1.Target:\t[" + w1.Target + "]");
        Console.WriteLine("w2.Target:\t[" + w2.Target + "]");

        listener1 = null;
        listener2 = null;
        GC.Collect();
        Console.WriteLine("after GC");

        Console.WriteLine("w1.Target:\t[" + w1.Target + "]");
        Console.WriteLine("w2.Target:\t[" + w2.Target + "]");

        Console.ReadLine();
    }
}

}

Best regards,
Eric