Question

In the course of a discussion in chat, I wrote this console application.

Code:

using System;

class Program
{
    static void Main(string[] args)
    {
        CreateClass();
        Console.Write("Collecting... ");
        GC.Collect();
        Console.WriteLine("Done");
    }

    static void CreateClass()
    {
        SomeClass c = new SomeClass();
    }
}

class SomeClass
{
    ~SomeClass()
    {
        throw new Exception();
    }
}

Result:

Collecting... Done

Unhandled Exception: System.Exception: Exception of type 'System.Exception' was
thrown.
   at SomeClass.Finalize()

I would have expected the app to crash before Done was printed.

I don’t care much about how to make it. My question is, why doesn’t it?

Answer 1

Objects with finalizers cannot be collected within a single garbage collection procedure. Such objects are moved to f-reachable queue, and remain there until finalizers are called. Only after that they can be garbage-collected.

Following code is better, but you should not rely on it anyway:

GC.Collect();
GC.WaitForPendingFinalizers();
GC.Collect();

Also, throwing exceptions in finalizer seems too brutal for me, even for testing purposes.

Also, interesting side-effect of finalizers: an object with finalizer can still ‘resurrect’ itself (effectively prevent garbage collection of itself), if stores this reference in finalizer (assigns it to some static variable).