When would dispose method not get called?

问题:

I was reading this article the other day and was wondering why there was a Finalizer along with the Dispose method. I read here on SO as to why you might want to add Dispose to the Finalizer. My curiousity is, when would the Finalizer be called over the Dispose method itself? Is there a code example or is it based on something happening on the system the software is running? If so, what could happen to not have the Dispose method run by the GC.

回答1:

The purpose of the finaliser here is simply a safety precaution against memory leaks (if you happen not to call Dispose explicitly). It also means you don't have to dispose your objects if you want them to release resources when the program shutdowns, since the GC will be forced to finalise and collect all objects anyway.

As a related point, it is important to dispose the object slightly differently when doing so from the finaliser.

~MyClass()
{
    Dispose(false);
}

public void Dispose()
{
    Dispose(true);
    GC.SuppressFinalize(this);
}

protected void Dispose(disposing)
{
    if (!this.disposed)
    {
        if (disposing)
        {
            // Dispose managed resources here.
        }
        // Dispose unmanaged resources here.
    }
    this.disposed = true;
}

The reason you do not want to dispose managed resources in your finaliser is that you would actually be creating strong references to them in doing so, and this could prevent the GC from doing it's job properly and collecting them. Unmanaged resources (e.g. Win32 handles and such) should always be explicitly closed/disposed, of course, since the CLR has no knowledge of them.

回答2:

This is mostly there to protect yourself. You cannot dictate what the end user of your class will do. By providing a finalizer in addition to a Dispose method, the GC will "Dispose" of your object, freeing your resources appropriately, even if the user forgets to call Dispose() or mis-uses your class.

回答3:

The Finalizer is called when the object is garbage collected. Dispose needs to be explicitly called. In the following code the finalizer will be called but the Dispose method is not.

class Foo : IDisposable
{
  public void Dispose()
  {
    Console.WriteLine("Disposed");
  }

  ~Foo()
  {
    Console.WriteLine("Finalized");
  }
}

...

public void Go()
{
  Foo foo = new Foo();
}

回答4:

The dispose method must be explicitly called, either by calling Dispose() or by having the object in a using statement. The GC will always call the finalizer, so if there is something that needs to happen before the objects are disposed of the finalizer should at least check to make sure that everything in the object is cleaned up.

You want to avoid cleaning up objects in the finalizer if at all possible, because it causes extra work compared to disposing them before hand (like calling dispose), but you should always at least check in the finalizer if there are objects lying around that need to be removed.

回答5:

An important but subtle note not yet mentioned: a seldom-considered purpose of Dispose is to prevent an object from being cleaned up prematurely. Objects with finalizers must be written carefully, lest a finalizer run earlier than expected. A finalizer can't run before the start of the last method call that will be made on an object(*), but it might sometimes run during the last method call if the object will be abandoned once the method completes. Code which properly Dispose an object can't abandon the object before calling Dispose, so there's no danger of a finalizer wreaking havoc on code which properly uses Dispose. On the other hand, if the last method to use an object makes use of entities which will be cleaned up in the finalizer after its last use of the object reference itself, it's possible for the garbage-collector to call Finalize on the object and clean up entities that are still in use. The remedy is to ensure any call method which uses entities that are going to get cleaned up by a finalizer must be followed at some point by a method call which makes use of "this". GC.KeepAlive(this) is a good method to use for that.

(*) Non-virtual methods which are expanded to in-line code that doesn't do anything with the object may be exempt from this rule, but Dispose usually is, or invokes, a virtual method.