C# how to implement Dispose method

Question

I need some advice on the implementation of the Dispose method.

In our application the user designs their own UI. I have a preview window that shows what the UI is going to look like. All object drawn in this UI ultimately derive from a common base class ScreenObject. My preview manager contain a single object reference to a ScreenGrid which is the grid object for the entire preview area.

Question #1

Some of my derived screen classes hold onto unmanaged resources, such as a database connection, bitmap image and a WebBrowser control. These classes need to dispose of these objects. I created a virtual Dispose method in the base ScreenObject base class and then implemented an override Dispose method in each of the derived classes that hold onto unmanaged resources. However, right now I just created a method called Dispose, I am not implementing IDisposable. Should I implement IDisposable? If so how do I implement it?

• Just on the derived classes that have unmanaged resources
• The base class and derived classes that have unmanaged resources OR
• The base class and all derived classes including those that do not have unmanaged resources

Is it wrong to put a virtual Dispose method in a base class that doesn't have unmanaged resources so that you can take advantage of polymorphism?

Question #2

In reading about the Dispose method and the IDisposable interface Microsoft states that the disposing object should only call the Dispose method for its parent. The parent will call it for its parent and so on. To me this seems backwards. I may want to dispose of a child but keep its parent around.

I would think it should be the other way around, an object being disposed should dispose of its children. The children should then dispose of their children and so on.

Am I wrong here or am I missing something?

Answer 1

Question 1: Implement IDisposable as well, using the following pattern:

public class MyClass : IDisposable {     bool disposed;      protected virtual void Dispose(bool disposing)     {         if (!disposed)         {             if (disposing)             {                 //dispose managed resources             }         }         //dispose unmanaged resources         disposed = true;     }      public void Dispose()     {         Dispose(true);         GC.SuppressFinalize(this);     } } 

Question 2: What Microsoft means is that a derived class calls dispose on it's parent class. The owner of the instance only calls Dispose on the most derived type.

An (shortened) example:

class Parent : IDisposable  {     protected virtual void Dispose(bool disposing)     {         if (!disposed)         {             if (disposing)             {                 //dispose managed resources             }         }         //dispose unmanaged resources         disposed = true;     }  } class Child : Parent, IDisposable  {      protected override void Dispose(bool disposing)     {         if (!disposed)         {             if (disposing)             {                 //dispose managed resources             }             base.Dispose(disposing);         }         //dispose unmanaged resources         disposed = true;     }  } class Owner:IDisposable {     Child child = new Child();     protected virtual void Dispose(bool disposing)     {         if (!disposed)         {             if (disposing)             {                 if(child!=null)                 {                     child.Dispose();                 }             }         }         //dispose unmanaged ressources         disposed = true;     } } 

The owner only calls Dispose on the Child, but not on the Parent. The Child is responsible for calling Dispose on the Parent.

Answer 2

Question 1:

Based on the types of objects that you list (i.e. Database, WebBrowser, Bitmap, etc.) these are only managed resources as far as .NET is concerned. Thus, you should implement IDisposable on any class that has disposable types as members. If they are locally declared instances, you just call 'using()' on them. While these instances you mention do have unmanaged resources under them, this is abstracted away from you by .NET thru the types you are using. Since you are only using managed types, you should implement IDisposable but without a finalizer. You only need to implement a finalizer if you truly have unmanaged resources as class members.

Question 2:

It seems that you are confusing inheritance (is a) with aggregation/containment (has a). For example, if "Container" contains a disposable resource as a class member, it is called aggregation/containment. So, calling base.Dispose() in the IDisposable implementation of Container has nothing to do with disposing of the disposable resource inside of Container. You should remember that if a class derives from Container, say "DerivedContainer", that it is an instance of Container albeit with additional members and/or functionality. So any instance of "DerivedContainer" has all of the members that its base class "Container" does. If you never called base.Dispose(), the disposable resource in "Container" would not be released properly (it would actually by the GC, but it's bad practice for many reasons to just 'let .NET take care of it') - please refer to my posted answer at Is it bad practice to depend on the .NET automated garbage collector?.

If you didn't call the base class Dispose(), you'd end up with a partially disposed object (disposed in the derived class but not in the base class) - a very bad scenario. So it is very important to call the base class Dispose().

I have a best practices pattern I've developed (with lots of experience and debugging memory dumps) written on my blog as an example. It shows how to implement IDisposable on a base class as well as a derived class:

http://dave-black.blogspot.com/2011/03/how-do-you-properly-implement.html