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Using CompositeDisposable in base classes

To help make an object eligible for collection by the GC (garbage collector) one would implement the IDisposable interface. Executing the dispose methods on muliple implmentations at the correct same time is often done using a custom DisposeWith extension method:

   1:  public static class DisposableExtensions
   2:  {
   3:      public static T DisposeWith<T>(this T disposable, CompositeDisposable disposables) where T : IDisposable
   4:      {
   5:          disposables.Add(disposable);
   6:   
   7:          return disposable;
   8:      }
   9:  }

Typically used as follows - explicitly creating a CompositeDisposable instance and then disposing the child view model with the instance:
Firstly can I get away without having to explicitly create an instance?

Also can I get to a point where I can just do something like this:
Inheritance seems the obvious choice but as you might already know, all the disposable types in the System.Reactive.Disposables namespace are sealed by design.

So inheriting directly from CompositeDisposable is a none starter, the next option would to create an intermediate class, @leeoades has a post showing the type of implementation we're using at the moment.

   1:  public interface IDisposableObject: IDisposable
   2:  {
   3:      void AddDisposable(IDisposable disposableObject);
   4:  }
   5:   
   6:  public class DisposableObject : IDisposableObject
   7:  {
   8:      CompositeDisposable _compositeDisposable = new CompositeDisposable();
   9:   
  10:      public void AddDisposable(IDisposable disposableObject)
  11:      {
  12:          _compositeDisposable.Add(disposableObject);
  13:      }
  14:   
  15:      public virtual void Dispose()
  16:      {
  17:          _compositeDisposable.Dispose();
  18:      }
  19:  }

This works well and does the job, but it does introduce another class into the inheritance hierarchy,e.g:

   1:  public abstract class ViewModel :  DisposableObject
   2:  {
   3:  }
   4:   
   5:  public class ParentViewModel : ViewModel
   6:  {
   7:  }

I always want to reduce the levels of inheritance as much as possible, especially when you're working on a framework that's going to be used by a wide variety of developers - keeping inheritance to a minimum helps new devs grok your code base quicker. It also introduces another line into the intellisense when using any derived classes, again this can be confusing when you're not au fait with the classes.
The other downside is having to change the DisposeWith extension method to have an instance of DisposableObject as the second parameter:

   1:  public static class DisposableExtensions
   2:  {
   3:      public static T DisposeWith<T>(this T disposable, IDisposableObject disposableObject) where T : IDisposable
   4:      {
   5:          disposableObject.AddDisposable(disposable);
   6:   
   7:          return disposable;
   8:      }
   9:  }

Is there away I can achieve the same functionality without introducing the DisposableObject class and not expose class methods that aren't relevant?

Yes - use a conversion operator, specifically use an implicit operator to convert from the source type to the destination type without having to specify the type using a dynamic cast. Below is taken from the MSDN example:
So now I'm able to remove the need for the DisposableObject class and prevent the need for any extra methods in my base class ViewModel. You'll notice I've also implemented the ICancelable interface from the  system.Reactive.Disposables namepsace, this exposes an IsDisposed method:

   1:  public abstract class ViewModel : ICancelable
   2:  {
   3:      private CompositeDisposable _disposable;
   4:   
   5:      protected ViewModel()
   6:      {
   7:          _disposable = new CompositeDisposable();
   8:      }
   9:   
  10:      public bool IsDisposed { get; private set; }
  11:   
  12:      public void Dispose()
  13:      {
  14:          if (!IsDisposed)
  15:          {
  16:              IsDisposed = true;
  17:   
  18:              _disposable.Dispose();
  19:              _disposable = null;
  20:          }
  21:      }
  22:   
  23:      public static implicit operator CompositeDisposable(ViewModel viewModel) 
  24:      {
  25:          return viewModel._disposable;
  26:      }
  27:  }

The parent & child view models haven't changed in declaration:

   1:  public class ParentViewModel : ViewModel
   2:  {
   3:  }
   4:   
   5:  public class ChildViewModel : ViewModel
   6:  {
   7:  }

 The intellisense has subtly changed, no longer exposing method declared on the DisposableObject class:
Testing produces the expected output - all view models disposed...

Comments

  1. Nice example. I like the code where you remove the implementation of a class, but provide an operator to do the conversion, and then return a backing field. I have never seen that before, or I never realized that it was doable like that. It explains some of the things I've seen in the past that seemed like 'magic' or the compiler being clever.

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