Did the compiler set the disposed objects to null?

Question

I have the following code:

public class Program
{
 private static Task task;
 private static int counter = 0;

 private static void Main(string[] args)
 {
   for (int i = 0; i < 10; i++)
   {
    if (task == null)
    {
      Console.WriteLine(++counter);
    }

     using (task = new Task(Method))
     {
       task.Start();
       task.Wait();
     }

     //  task = null;

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

   Console.ReadKey();
 }

 public static void Method() { }
 }

My excepted output: 1 2 3 4 5 6... but the real output from this method is 1!

If I remove the comment from the code line Task = null; then I will became the expected result.

Why the disposed tasks are not null!? I thought, if the objects are disposed then they can be set to null from the GC (I have forced the GC to collect) in other words the GC will collect the disposed(orphan) objects and put them to null?!

I have found a very useful link for the problem: http://stackoverflow.com/questions/2926869/do-you-need-to-dispose-of-objects-and-set-them-to-null — Bassam Alugili, Nov 21 '13 at 16:52

score 6 · Accepted Answer · answered Nov 21 '13 at 14:42

Why would you expect it to be set to null? using/Dispose() never says that it will do that. It promises to dispose it. The best practice here is to use a separate variable in the using statement to avoid confusion:

using(var t = new Task(Method))
{
    t.Start();
    t.Wait();
}

But if you want it set to null; set it to null:

task = null;

As for your other point:

in other words the GC will collect the disposed(orphan) objects and put them to null?!

The GC never sets anything to null either; it collects things that are unreachable: that is all

Sriram Sakthivel · Answer 2 · 2013-11-21T14:48:41.693

Am not sure about what you mean by compiler. If you mean "c# compiler" it has no business while the code is executing. It just compiles the c# code to IL. That's it, rest all taken care by CLR and JIT.

And for your question: No, nobody sets reference to null when disposed. It's you, your responsibility to set to null when you don't need it.

GC will collect only unreachable objects. In other words when you have no managed references to your instance it is eligible for garbage collection.

score 2 · Answer 3 · answered Nov 21 '13 at 14:43

2

All static fields, like your task field, are GC roots. Objects reachable from GC roots will not be collected.

The GC also does not null-out references, and would not need to, as an unreachable object would not have any live references in scope of any threads.

answered Nov 21 '13 at 14:43

Mike Strobel

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Thanks for this information and for the correct answer. This is just a prototype I have just make it to describe the problem. I have marked Marc answer because the answer was very clear for me. – Bassam Alugili Nov 21 '13 at 15:25

Did the compiler set the disposed objects to null?

3 Answers3