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C# destructor not called even after code scope + GC? [duplicate]

This question already has answers here:
Understanding garbage collection in .NET
(2 answers)
Closed 2 years ago.
I've got a simple test code in Nunit+.net core 2.0 under VS2019:
public class Tests
{
public static int i = 0;
class First
{
~First()
{
i += 1;
Console.WriteLine("First's destructor is called.");
}
}
class Second : First
{
~Second() { i += 10; Console.WriteLine("Second's destructor is called."); }
}
class Third : Second
{
~Third() { i += 100; Console.WriteLine("Third's destructor is called."); }
}
[Test]
public static void Test()
{
{
Third t = new Third();
}
Thread.Sleep(1000);
System.GC.Collect();
Assert.AreEqual(111, i);
}
}
It always fails, while I found finally i=0, and the destructors are called right after Test(). But you can see the "t" is in a code block, and is invalid after the code block, and I also called
System.GC.Collect();
before my "Assert".
Why destructor is not called even after GC? How to fix my code to make the test pass?
Thanks a lot.

Use using statement and the IDisposable interface instead destructor;
public class Tests
{
public static int i = 0;
class First : IDisposable
{
public virtual void Dispose()
{
i += 1; Console.WriteLine("First's Dispose is called.");
}
}
class Second : First
{
public override void Dispose()
{
i += 10; Console.WriteLine("Second's Dispose is called.");
base.Dispose();
}
}
class Third : Second
{
public override void Dispose()
{
i += 100; Console.WriteLine("Third's Dispose is called.");
base.Dispose();
}
}
public static void Test()
{
using (Third t = new Third()) {
Console.WriteLine("Now everything will be ok, after leaving this block");
Console.WriteLine("t object will be dispose");
}
Thread.Sleep(1000);
System.GC.Collect();
Assert.AreEqual(111, i);
}
}
You can call System.GC.Collect() but it is not matter. The destructor will not be call immediately as you expect. (In above code you don't need System.GC.Collect())
It is strange to me why Microsoft does not make this clearer, instead of the old definition "Forces an immediate garbage collection of all generations." which leads developers to think they should expect an immediate result and GC.Collect() should wait for garbage collection to be complete before it returns?!
GC.Collect Method(System) - Microsoft Docs
Even more strange to me is that the code they give as an example for using of GC.Collect() don't work as they claim, for the same reason. You can try yourself by commenting GC.Collect(); line. I don't know, maybe they just don't update their documents as often. Maybe the situation was different before with GC.Collect()?! GC.CollectRequest() would be a more appropriate name for this function, if you ask me.
P.s. I'm new to C# so please, excuse me if I say things that do not fully understand or something I'm wrong in the code I write.

Call GC.WaitForPendingFinalizers() after GC.Collect()
The finalizers (not destructors) run as background tasks asynchronously, so after you call GC.Collect() and reach the assertion statement the finalizers are not guaranteed to have finished.

Unclear behavior by Garbage Collector while collecting instance properties or fields of reachable object

Till today I was thinking that members of reachable objects are also considered to be reachable.
But, today I found one behavior which creates a problem for us either when Optimize Code is checked or application is executed in Release Mode. It is clear that, release mode comes down to the code optimization as well. So, it seems code optimization is reason for this behavior.
Let's take a look to that code:
public class Demo
{
public Action myDelWithMethod = null;
public Demo()
{
myDelWithMethod = new Action(Method);
// ... Pass it to unmanaged library, which will save that delegate and execute during some lifetime
// Check whether object is alive or not after GC
var reference = new WeakReference(myDelWithMethod, false);
GC.Collect(GC.MaxGeneration, GCCollectionMode.Forced, true);
GC.WaitForPendingFinalizers();
GC.Collect(GC.MaxGeneration, GCCollectionMode.Forced, true);
Console.WriteLine(reference.IsAlive);
// end - Check whether object is alive or not after GC
}
private void Method() { }
}
I simplified code a bit. Actually, we are using our special delegate, not Action. But the behavior is same. This code is written in mind with "members of reachable objects are also considered to be reachable". But, that delegate will be collected by GC asap. And we have to pass it to some unmanaged library, which will use it for some time.
You can test demo by just adding that line to the Main method:
var p = new Demo();
I can understand the reason of that optimization, but what is the recommended way to prevent such case without creating another function which will use that variable myDelWithMethod which will be called from some place? One, option I found that, it will work if I will set myDelWithMethod in the constructor like so:
myDelWithMethod = () => { };
Then, it won't be collected until owning instance is collected. It seems it can't optimize code in the same way, if lambda expression is setted as a value.
So, will be happy to hear your thoughts. Here are my questions:
Is it right that, members of reachable objects are also considered to
be reachable?
Why it is not collected in case of lambda expression?
Any recommended ways to prevent collection in such cases?

However strange this would sound, JIT is able to treat an object as unreachable even if the object's instance method is being executed - including constructors.
An example would be the following code:
static void Main(string[] args)
{
SomeClass sc = new SomeClass() { Field = new Random().Next() };
sc.DoSomethingElse();
}
class SomeClass
{
public int Field;
public void DoSomethingElse()
{
Console.WriteLine(this.Field.ToString());
// LINE 2: further code, possibly triggering GC
Console.WriteLine("Am I dead?");
}
~SomeClass()
{
Console.WriteLine("Killing...");
}
}
that may print:
615323
Killing...
Am I dead?
This is because of inlining and Eager Root Collection technique - DoSomethingElse method do not use any SomeClass fields, so SomeClass instance is no longer needed after LINE 2.
This happens to code in your constructor. After // ... Pass it to unmanaged library line your Demo instance becomes unreachable, thus its field myDelWithMethod. This answers the first question.
The case of empty lamba expression is different because in such case this lambda is cached in a static field, always reachable:
public class Demo
{
[Serializable]
[CompilerGenerated]
private sealed class <>c
{
public static readonly <>c <>9 = new <>c();
public static Action <>9__1_0;
internal void <.ctor>b__1_0()
{
}
}
public Action myDelWithMethod;
public Demo()
{
myDelWithMethod = (<>c.<>9__1_0 ?? (<>c.<>9__1_0 = new Action(<>c.<>9.<.ctor>b__1_0)));
}
}
Regarding recommended ways in such scenarios, you need to make sure Demo has lifetime long enough to cover all unmanaged code execution. This really depends on your code architecture. You may make Demo static, or use it in a controlled scope related to the unmanaged code scope. It really depends.

Can .NET detect "irrelevant" code at runtime and garbage-collect objects in it?

Suppose I have a C# method like this
public void foo(){
Object very_large_object;
if (very_large_object.some_runtime_condition ()) {
do_something ();
return;
}
do_something_else (very_large_object);
}
In my case, do_something is an expensive method and may take some time to finish. When it's running, can .NET GC detects that do_something_else won't be called and thus very_larget_object is no longer relevant and garbage-collects it?

Yes it can. It's a common misconception that object lifetime is somehow related to the enclosing scope (for example that very_large_object can only be collected when foo scope ends). You can verify your particular case with simple application:
class Program {
static void Main() {
var o = new TestObject();
var test = Console.ReadKey();
if (test.Key == ConsoleKey.Enter) {
DoSomething();
Console.ReadKey();
return;
}
DoSomethingElse(o);
}
static void DoSomething() {
Console.WriteLine("doing something");
Thread.Sleep(500);
GC.Collect();
GC.WaitForPendingFinalizers();
Console.WriteLine("GC ran");
}
static void DoSomethingElse(TestObject o) {
Console.WriteLine(o);
}
}
public class TestObject {
public TestObject() {
}
~TestObject() {
Console.WriteLine("finalizer running");
}
}
If you compile that in Release mode with optimizations enabled, it will output the following (after you press Enter in console):
doing something
finalizer running
GC ran
So it's clear that o was considered garbage and was put into finalizer queue while we were still inside DoSomething method. Optimizer could not just throw away branch with DoSomethingElse because it depends on your console input whether it will run or not. If you remove return then of course it won't be collected.

Potential deadlock?

public class MyClass
{
public void DoSomething()
{
Lock(this)
{
//Do Something.
}
}
}
public class AnotherClass
{
MyClass myclass = new MyClass();
Public void DoAnotherThing()
{
lock(myclass)
{
Myclass.DoSomething();
}
}
}
Will this create a deadlock ?
As per my understanding and as per the articles I have read – It will. Why ? – Whenever DoSomething() is called it will try to attain a lock and wait for lock(myclass) to be released and hence a deadlock. Please confirm my understanding(a little explanation is also requested) and correct me if I am wrong.

I think what the articles you read were trying to tell you is that you shouldn't lock (this) because some other code might also try to lock the same object. This will only happen if two or more threads are involved.
Here's some sample code which demonstrates a deadlock problem. Try running it and look at the result.
Then make the edit suggested on the lock (this) line and try it again.
The deadlock occurs because some code OUTSIDE the class is locking on the same instance of the class that the code INSIDE the class is using for a lock - and without careful documentation and visibility, that's a real possibility.
The moral of this story is that in general you should not lock on anything that is visible outside the class (unless you document carefully how to use the locking for that class), and you should NEVER lock (this).
using System;
using System.Threading.Tasks;
namespace Demo
{
class MyClass
{
public void DoSomething()
{
Console.WriteLine("Attempting to enter the DoSomething lock.");
lock (this) // Change to lock(_locker) to prevent the deadlock.
{
Console.WriteLine("In the DoSomething lock.");
}
}
readonly object _locker = new object();
}
internal static class Program
{
static void Main(string[] args)
{
var myClass = new MyClass();
lock (myClass)
{
var task = Task.Run(() => myClass.DoSomething());
Console.WriteLine("Waiting for the task to complete.");
if (!task.Wait(1000))
Console.WriteLine("ERROR: The task did not complete.");
else
Console.WriteLine("Task completed.");
Console.WriteLine("Press any key to continue...");
Console.ReadKey();
}
}
}
}

This cannot create a deadlock, because only one thread is involved. True, a lock on your MyClass object is requested when a lock on it is already held. But C# locks are "recursive", meaning that a single thread can hold multiple locks on the same object, with the result being the same as if it only held the outermost lock: The inner locks immediately succeed, and the object is locked until the last lock is released. (You don't truly understand how useful this is until you're forced to use a language which doesn't have it.)
But I agree with everyone above: lock(this) is bad juju.

What's the use of the SyncRoot pattern?

I'm reading a c# book that describes the SyncRoot pattern. It shows
void doThis()
{
lock(this){ ... }
}
void doThat()
{
lock(this){ ... }
}
and compares to the SyncRoot pattern:
object syncRoot = new object();
void doThis()
{
lock(syncRoot ){ ... }
}
void doThat()
{
lock(syncRoot){ ... }
}
However, I don't really understand the difference here; it seems that in both cases both methods can only be accessed by one thread at a time.
The book describes ... because the object of the instance can also be used for synchronized access from the outside and you can't control this form the class itself, you can use the SyncRoot pattern Eh? 'object of the instance'?
Can anyone tell me the difference between the two approaches above?

If you have an internal data structure that you want to prevent simultaneous access to by multiple threads, you should always make sure the object you're locking on is not public.
The reasoning behind this is that a public object can be locked by anyone, and thus you can create deadlocks because you're not in total control of the locking pattern.
This means that locking on this is not an option, since anyone can lock on that object. Likewise, you should not lock on something you expose to the outside world.
Which means that the best solution is to use an internal object, and thus the tip is to just use Object.
Locking data structures is something you really need to have full control over, otherwise you risk setting up a scenario for deadlocking, which can be very problematic to handle.

The actual purpose of this pattern is implementing correct synchronization with wrappers hierarchy.
For example, if class WrapperA wraps an instance of ClassThanNeedsToBeSynced, and class WrapperB wraps the same instance of ClassThanNeedsToBeSynced, you can't lock on WrapperA or WrapperB, since if you lock on WrapperA, lock on WrappedB won't wait.
For this reason you must lock on wrapperAInst.SyncRoot and wrapperBInst.SyncRoot, which delegate lock to ClassThanNeedsToBeSynced's one.
Example:
public interface ISynchronized
{
object SyncRoot { get; }
}
public class SynchronizationCriticalClass : ISynchronized
{
public object SyncRoot
{
// you can return this, because this class wraps nothing.
get { return this; }
}
}
public class WrapperA : ISynchronized
{
ISynchronized subClass;
public WrapperA(ISynchronized subClass)
{
this.subClass = subClass;
}
public object SyncRoot
{
// you should return SyncRoot of underlying class.
get { return subClass.SyncRoot; }
}
}
public class WrapperB : ISynchronized
{
ISynchronized subClass;
public WrapperB(ISynchronized subClass)
{
this.subClass = subClass;
}
public object SyncRoot
{
// you should return SyncRoot of underlying class.
get { return subClass.SyncRoot; }
}
}
// Run
class MainClass
{
delegate void DoSomethingAsyncDelegate(ISynchronized obj);
public static void Main(string[] args)
{
SynchronizationCriticalClass rootClass = new SynchronizationCriticalClass();
WrapperA wrapperA = new WrapperA(rootClass);
WrapperB wrapperB = new WrapperB(rootClass);
// Do some async work with them to test synchronization.
//Works good.
DoSomethingAsyncDelegate work = new DoSomethingAsyncDelegate(DoSomethingAsyncCorrectly);
work.BeginInvoke(wrapperA, null, null);
work.BeginInvoke(wrapperB, null, null);
// Works wrong.
work = new DoSomethingAsyncDelegate(DoSomethingAsyncIncorrectly);
work.BeginInvoke(wrapperA, null, null);
work.BeginInvoke(wrapperB, null, null);
}
static void DoSomethingAsyncCorrectly(ISynchronized obj)
{
lock (obj.SyncRoot)
{
// Do something with obj
}
}
// This works wrong! obj is locked but not the underlaying object!
static void DoSomethingAsyncIncorrectly(ISynchronized obj)
{
lock (obj)
{
// Do something with obj
}
}
}

Here is an example :
class ILockMySelf
{
public void doThat()
{
lock (this)
{
// Don't actually need anything here.
// In this example this will never be reached.
}
}
}
class WeveGotAProblem
{
ILockMySelf anObjectIShouldntUseToLock = new ILockMySelf();
public void doThis()
{
lock (anObjectIShouldntUseToLock)
{
// doThat will wait for the lock to be released to finish the thread
var thread = new Thread(x => anObjectIShouldntUseToLock.doThat());
thread.Start();
// doThis will wait for the thread to finish to release the lock
thread.Join();
}
}
}
You see that the second class can use an instance of the first one in a lock statement. This leads to a deadlock in the example.
The correct SyncRoot implementation is:
object syncRoot = new object();
void doThis()
{
lock(syncRoot ){ ... }
}
void doThat()
{
lock(syncRoot ){ ... }
}
as syncRoot is a private field, you don't have to worry about external use of this object.

Here's one other interesting thing related to this topic:
Questionable value of SyncRoot on Collections (by Brad Adams):
You’ll notice a SyncRoot property on many of the Collections in System.Collections. In retrospeced (sic), I think this property was a mistake. Krzysztof Cwalina, a Program Manger on my team, just sent me some thoughts on why that is – I agree with him:
We found the SyncRoot-based synchronization APIs to be insufficiently flexible for most scenarios. The APIs allow for thread safe access to a single member of a collection. The problem is that there are numerous scenarios where you need to lock on multiple operations (for example remove one item and add another). In other words, it’s usually the code that uses a collection that wants to choose (and can actually implement) the right synchronization policy, not the collection itself. We found that SyncRoot is actually used very rarely and in cases where it is used, it actually does not add much value. In cases where it’s not used, it is just an annoyance to implementers of ICollection.
Rest assured we will not make the same mistake as we build the generic versions of these collections.

See this Jeff Richter's article. More specifically, this example which demonstrates that locking on "this" can cause a deadlock:
using System;
using System.Threading;
class App {
static void Main() {
// Construct an instance of the App object
App a = new App();
// This malicious code enters a lock on
// the object but never exits the lock
Monitor.Enter(a);
// For demonstration purposes, let's release the
// root to this object and force a garbage collection
a = null;
GC.Collect();
// For demonstration purposes, wait until all Finalize
// methods have completed their execution - deadlock!
GC.WaitForPendingFinalizers();
// We never get to the line of code below!
Console.WriteLine("Leaving Main");
}
// This is the App type's Finalize method
~App() {
// For demonstration purposes, have the CLR's
// Finalizer thread attempt to lock the object.
// NOTE: Since the Main thread owns the lock,
// the Finalizer thread is deadlocked!
lock (this) {
// Pretend to do something in here...
}
}
}

Another concrete example:
class Program
{
public class Test
{
public string DoThis()
{
lock (this)
{
return "got it!";
}
}
}
public delegate string Something();
static void Main(string[] args)
{
var test = new Test();
Something call = test.DoThis;
//Holding lock from _outside_ the class
IAsyncResult async;
lock (test)
{
//Calling method on another thread.
async = call.BeginInvoke(null, null);
}
async.AsyncWaitHandle.WaitOne();
string result = call.EndInvoke(async);
lock (test)
{
async = call.BeginInvoke(null, null);
async.AsyncWaitHandle.WaitOne();
}
result = call.EndInvoke(async);
}
}
In this example, the first call will succeed, but if you trace in the debugger the call to DoSomething will block until the lock is release. The second call will deadlock, since the Main thread is holding the monitor lock on test.
The issue is that Main can lock the object instance, which means that it can keep the instance from doing anything that the object thinks should be synchronized. The point being that the object itself knows what requires locking, and outside interference is just asking for trouble. That's why the pattern of having a private member variable that you can use exclusively for synchronization without having to worry about outside interference.
The same goes for the equivalent static pattern:
class Program
{
public static class Test
{
public static string DoThis()
{
lock (typeof(Test))
{
return "got it!";
}
}
}
public delegate string Something();
static void Main(string[] args)
{
Something call =Test.DoThis;
//Holding lock from _outside_ the class
IAsyncResult async;
lock (typeof(Test))
{
//Calling method on another thread.
async = call.BeginInvoke(null, null);
}
async.AsyncWaitHandle.WaitOne();
string result = call.EndInvoke(async);
lock (typeof(Test))
{
async = call.BeginInvoke(null, null);
async.AsyncWaitHandle.WaitOne();
}
result = call.EndInvoke(async);
}
}
Use a private static object to synchronize on, not the Type.