#832 – The Sequence in Which Finalizers Are Called

When you implement a finalizer, using the destructor syntax, the finalizer needs to always call the finalizer of its base class.  The C# compiler will automatically take care of this, invoking the finalizer of the base class within a finally block to ensure that it gets called.

Because the call to the finalizer of the base class occurs within a finally block, it will get called after all of the code within the derived class’ finalizer has executed.  This means that the sequence in which the finalizers are called is from the most derived class up the inheritance chain.

For example, if Terrier inherits from Dog, code in the Terrier’s finalizer will execute before code in the Dog’s finalizer.

 

 

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#737 – When to Implement a Finalizer

In C#, you implement a finalizer using the destructor (~) syntax.  The finalizer is called by the system when the object is being destroyed.

In general, you rarely need to implement a finalizer.  Rules of thumb for implementing a finalizer include:

  • Implement a finalizer only when the object has unmanaged resources to clean up (e.g. file handles)
  • Do not implement a finalizer if you don’t have unmanaged resources to clean up
  • The finalizer should release all of the object’s unmanaged resources
  • Implement the finalizer as part of the dispose pattern, which allows for deterministic destruction
  • The finalizer should only concern itself with cleanup of objects owned within the class where it is defined
  • The finalizer should avoid side-effects and only include cleanup code
  • The finalizer should not add references to any objects, including a reference to the finalizer’s own object
  • The finalizer should not call methods in any other objects

#428 – A Finalizer Should Always Call the Finalizer of Its Base Class

An implementation of Object.Finalize for a particular class in .NET should always call the Finalize method of its base class.  In the case of C#, you can’t explicitly override Finalize, but you provide a finalizer using the destructor (~) syntax.  When you do this, the compiler will automatically add code to ensure that your destructor calls Finalize in the base class.

In the example below, I write a simple destructor for the Dog class.  We can see in the IL that the Finalize method has been overridden and that it calls System.Object.Finalize in the finally clause.

        ~Dog()
        {
            Trace.WriteLine("This dog is on the way out..");
        }

#427 – Finalizer Gotchas

The idea of the finalizer in .NET is that you can override the Object.Finalize method, called by the garbage collector when it’s releasing memory for an object, and you can do any required cleanup of unmanaged resources.

There are several things to keep in mind if you want to write a finalizer (done in C# using the destructor syntax):

  • You can’t predict when your finalizer is called
  • The finalizer might never get called at all
  • The order that finalizers are called in is unpredictable.  You therefore can’t assume that any other managed object that your object references still exists, or is in a known state.
  • Garbage collection will take longer if you have objects with finalizers

 

#426 – Use a Destructor to Free Unmanaged Resources

When an object is no longer referenced by other objects, its memory may be reclaimed by the garbage collector in the CLR.  If you have any unmanaged resources that you need to clean up explicitly, you can implement a finalizer for your class.

A finalizer is the implementation of the Object.Finalize method for your class, which the garbage collector calls when it’s ready to release memory for an object.  In C#, you implement a finalizer using the destructor syntax–a method defined as a tilde (~), followed by the name of the class.

    public class Dog
    {
        ~Dog()
        {
            Trace.WriteLine("This dog is on the way out..");
        }

When you look at the IL for this class, you’ll see that the compiler has overridden the Finalize method.

You’ll notice that this method is now called when the garbage collector is releasing the object.