使用Dispose（）方法

Question

Why I see "Hello" words many times when I add timer.Dispose() to my code in release mode. Without timer.Dispose() I see "Hello" once. Thanks.

using System.Threading;

namespace ConsoleApplication1
{
    class Program
    {
        static void Method(object state)
        {
            Console.WriteLine(state);
            GC.Collect();
        }

        static void Main()
        {
            var timer = new Timer(Method, "Hello", 0, 200);

            Console.ReadLine();
            timer.Dispose();
        }
    }
}

Answer 1

You see it once because the garbage collector has collected the timer object.

Since there are no more references to it that is considered "live" the garbage collector is free to do so.

In release mode, and when no debugger is attached, the JITter builds up knowledge of where local variables are used in a method. Once a variable is no longer used, it is no longer considered to be a root if the method is currently executing below that point. As such, the garbage collector can collect the timer object. However, before it can do so it must finalize the object, which destroys the underlying timer object and stops it from executing.

In debug builds, the scope of all local variables are artificially extended to be the entire method, so that if you place a breakpoint in the method you can inspect variables, even if the program would no longer actually require that variable to exist.

When you added the call to dispose, you extended the lifetime of the variable, and thus prevented the garbage collector from collecting your timer object.

Since you provoke a garbage collection in the timer event handler method, you effectively destroy the timer yourself in this case. When you extended the lifetime of the variable by adding in the call to Dispose, the garbage collector still runs as part of your timer event handler, but it cannot yet collect the timer, so it keeps running.

The answer left here by Hans Passant describes this much better than my meager attempt above:

Understanding Garbage Collection in .NET .

Answer 2

I suspect it's because GC.Collect will not collect the timer object when it's still referenced below the current line of code.

timer.Dispose(); is acting like GC.KeepAlive(timer); in this instance.

If you remove both the Dispose and the GC.Collect() , you will get a few "Hello"s, and then the GC will decide to collect by itself, and you'll get no more.