Could a class instance that is not being assigned to a variable get garbage-collected too early?

Question

(I don't even know whether my question makes sense at all; it is just something that I do not understand and is spinning in my head for some time)

Consider having the following class:

public class MyClass
{
    private int _myVar;

    public void DoSomething()
    {
        // ...Do something...

        _myVar = 1;

        System.Console.WriteLine("Inside");
    }
}

And using this class like this:

public class Test
{
    public static void Main()
    {
        // ...Some code...
        System.Console.WriteLine("Before");

        // No assignment to a variable.
        new MyClass().DoSomething();

        // ...Some other code...
        System.Console.WriteLine("After");
    }
}

(Ideone)

Above, I'm creating an instance of a class without assigning it to a variable.

I fear that the garbage collector could delete my instance too early.

My naive understanding of garbage collection is:

"Delete an object as soon as no references point to it."

Since I create my instance without assigning it to a variable, this condition would be true. Obviously the code runs correct, so my asumption seems to be false.

Can someone give me the information I am missing?

To summarize, my question is:

(Why/why not) is it safe to instantiate a class without asigning it to a variable or returning it?

I.e. is

new MyClass().DoSomething();

and

var c = new MyClass();
c.DoSomething();

the same from a garbage collection point-of-view?

Answer 1

The other answers are all good but I want to emphasize a few points here.

The question essentially boils down to: when is the garbage collector allowed to deduce that a given object is dead? and the answer is the garbage collector has broad latitude to use any technique it chooses to determine when an object is dead, and this broad latitude can lead to some surprising results.

So let's start with:

My naive understanding of garbage collection is: "Delete an object as soon as no references point to it."

This understanding is wrong wrong wrong. Suppose we have

class C { C c; public C() { this.c = this; } }

Now every instance of C has a reference to it stored inside itself. If objects were only reclaimed when the reference count to them was zero then circularly referenced objects would never be cleaned up.

A correct understanding is:

Certain references are "known roots". When a collection happens the known roots are traced. That is, all known roots are alive, and everything that something alive refers to is also alive, transitively. Everything else is dead, and eligable for reclamation.

Dead objects that require finalization are not collected. Rather, they are kept alive on the finalization queue, which is a known root, until their finalizers run, after which they are marked as no longer requiring finalization. A future collection will identify them as dead a second time and they will be reclaimed.

Lots of things are known roots. Static fields, for example, are all known roots. Local variables might be known roots, but as we'll see below, they can be optimized away in surprising ways. Temporary values might be known roots.

I'm creating an instance of a class without assigning it to a variable.

Your question here is a good one but it is based on an incorrect assumption, namely that a local variable is always a known root. Assigning a reference to a local variable does not necessarily keep an object alive. The garbage collector is allowed to optimize away local variables at its whim.

Let's give an example:

void M()
{
    var resource = OpenAFile();
    int handle = resource.GetHandle();
    UnmanagedCode.MessWithFile(handle);
}

Suppose resource is an instance of a class that has a finalizer, and the finalizer closes the file. Can the finalizer run before MessWithFile? Yes! The fact that resource is a local variable with a lifetime of the entire body of M is irrelevant. The runtime can realize that this code could be optimized into:

void M()
{
    int handle;
    {
        var resource = OpenAFile();
        handle = resource.GetHandle();
    }
    UnmanagedCode.MessWithFile(handle);
}

and now resource is dead by the time MessWithFile is called. It is unlikely but legal for the finalizer to run between GetHandle and MessWithFile, and now we're messing with a file that has been closed.

The correct solution here is to use GC.KeepAlive on the resource after the call to MessWithFile.

To return to your question, your concern is basically "is the temporary location of a reference a known root?" and the answer is usually yes, with the caveat that again, if the runtime can determine that a reference is never dereferenced then it is allowed to tell the GC that the referenced object might be dead.

Put another way: you asked if

new MyClass().DoSomething();

and

var c = new MyClass();
c.DoSomething();

are the same from the point of view of the GC. Yes. In both cases the GC is allowed to kill the object the moment that it determines it can do so safely, regardless of the lifetime of local variable c.

The shorter answer to your question is: trust the garbage collector. It has been carefully written to do the right thing. The only times you need to worry about the GC doing the wrong thing are scenarios like the one I laid out, where timing of finalizers is important for the correctness of unmanaged code calls.

Answer 2

It's somewhat safe. Or rather, it's as safe as if you had a variable which isn't used after the method call anyway.

An object is eligible for garbage collection (which isn't the same as saying it will be garbage collected immediately) when the GC can prove that nothing is going to use any of its data any more.

This can occur even while an instance method is executing if the method isn't going to use any fields from the current execution point onwards. This can be quite surprising, but isn't normally an issue unless you have a finalizer, which is vanishingly rare these days.

When you're using the debugger, the garbage collector is much more conservative about what it will collect, by the way.

Here's a demo of this "early collection" - well, early finalization in this case, as that's easier to demonstrate, but I think it proves the point clearly enough:

using System;
using System.Threading;

class EarlyFinalizationDemo
{
    int x = Environment.TickCount;

    ~EarlyFinalizationDemo()
    {
        Test.Log("Finalizer called");
    }    

    public void SomeMethod()
    {
        Test.Log("Entered SomeMethod");
        GC.Collect();
        GC.WaitForPendingFinalizers();
        Thread.Sleep(1000);
        Test.Log("Collected once");
        Test.Log("Value of x: " + x);
        GC.Collect();
        GC.WaitForPendingFinalizers();
        Thread.Sleep(1000);
        Test.Log("Exiting SomeMethod");
    }

}

class Test
{
    static void Main()
    {
        var demo = new EarlyFinalizationDemo();
        demo.SomeMethod();
        Test.Log("SomeMethod finished");
        Thread.Sleep(1000);
        Test.Log("Main finished");
    }

    public static void Log(string message)
    {
        // Ensure all log entries are spaced out
        lock (typeof(Test))
        {
            Console.WriteLine("{0:HH:mm:ss.FFF}: {1}",
                              DateTime.Now, message);
            Thread.Sleep(50);
        }
    }
}

Output:

10:09:24.457: Entered SomeMethod
10:09:25.511: Collected once
10:09:25.562: Value of x: 73479281
10:09:25.616: Finalizer called
10:09:26.666: Exiting SomeMethod
10:09:26.717: SomeMethod finished
10:09:27.769: Main finished

Note how the object is finalized after the value of x has been printed (as we need the object in order to retrieve x) but before SomeMethod completes.

Answer 3

As long as you're talking about a single threaded environment, you're safe. Fun things only start to happen if you're starting a new thread inside the DoSomething method, and even more fun happens if your class has a finalizer. The key thing to understand here is that a lot of the contracts between you and the runtime / optimizer / etc. are valid only in a single thread. This is one of the things that has disastrous results when you start programming on multiple threads in a language that isn't primaririly multi-threading oriented (yes, C# is one of those languages).

In your case, you're even using the this instance, which makes unexpected collection even less likely while still inside that method; in any case, the contract is that on a single thread, you can't observe the difference between the optimized and unoptimized code (apart from memory usage, speed, etc., but those are the "free lunch").

Answer 4

Of course, GC is transparent to you and no early collection can ever happen. So I guess you want to know the implementation details:

An instance method is implemented like a static method with an additional this parameter. In your case the this value lives in registers and is passed like that into DoSomething. The GC is aware what registers contain live references and will treat them as roots.

As long as DoSomething might still use the this value it stays live. If DoSomething never uses instance state then indeed the instance can be collected while a method call is still running on it. This is unobservable, therefore safe.