Question

NOTE The code below is from asp.net.

---

If I have the (poorly written) code below

AmazonS3 s3Client = Amazon.AWSClientFactory.CreateAmazonS3Client();

// ...
// details elided
// ...

BackgroundWorker worker = new BackgroundWorker();

worker.DoWork += new DoWorkEventHandler((s, args) =>
{
    s3Client.PutObject(titledRequest);
});

new Thread(() => worker.RunWorkerAsync()).Start();

Will the garbage collector be smart enough to never, ever collect the s3Client object until the background worker is done with it?

---

Note, I’m kicking off the background worker inside of a thread only in order to fix an annoying error that gets raised within asp.net that happens when I fire off the background worker directly.

Answer 1

Yes, it will. The compiler will generate a new class for you that contains fields for each of the locals you reference in a closure. The closure body will be emitted into a method on that class, and all of the locals in the containing function will be rewritten by the compiler to reference fields on that closure object.

All of this magic happens at compile-time; the runtime does not need to know anything about it. Since the runtime is already smart enough not to collect an object that is the target of a delegate, the lifetimes of locals referenced by a closure are guaranteed to extend to the lifetime of the resulting delegate object.

To illustrate, the compiler is going to spit out something like this:

[System.Runtime.CompilerServices.CompilerGeneratedAttribute]
internal class ClosureImplementation // See note 1
{
    public AmazonS3 s3Client;

    public void Method(object s, EventArgs args)
    {
        s3Client.PutObject(titledRequest); // See note 2
    }
}

Then, in your method, this is emitted instead:

ClosureImplementation closure = new ClosureImplementation();
closure.s3Client = Amazon.AWSClientFactory.CreateAmazonS3Client();

// ...

worker.DoWork += closure.Method;

Notes:

1. The name of the generated class is chosen by the compiler; ClosureImplementation is just an example.
2. I don’t have enough context to know where titledRequest comes from, so I intentionally did not address how the compiler will handle that.