How to implement the disposable pattern in a class that inherits from another disposable class?

Question

Hi,

I often used the disposable pattern in simple classes that referenced small amount of resources, but I never had to implement this pattern on a class that inherits from another disposable class and I am starting to be a bit confused in how to free the whole resources.

I start with a little sample code:

public class Tracer : IDisposable
{
    bool disposed;
    FileStream fileStream;

    public Tracer()
    {
        //Some fileStream initialization
    }

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

    protected virtual void Dispose(bool disposing)
    {
        if (!disposed)
        {
            if (disposing)
            {
                if (fileStream != null)
                {
                    fileStream.Dispose();
                } 
            }

            disposed = true; 
        }
    }
}

public class ServiceWrapper : Tracer
{
    bool disposed;
    ServiceHost serviceHost;

    //Some properties

    public ServiceWrapper ()
    {
        //Some serviceHost initialization
    }

    //protected override void Dispose(bool disposing)
    //{
    //    if (!disposed)
    //    {
    //        if (disposing)
    //        {
    //            if (serviceHost != null)
    //            {
    //                serviceHost.Close();
    //            } 
    //        }

    //        disposed = true;
    //    }
    //}
}

My real question is: how to implement the disposable pattern inside my ServiceWrapper class to be sure that when I will dispose an instance of it, it will dispose resources in both inherited and base class?

Thanks.

Answer 1

The inherited class, ServiceWrapper, inherits from a class that implements IDisposable, so SericeWrapper also implements IDisposable. For proof, new up an instance of ServiceWrapper. You'll find (with intellisense) that it too has a Dispose() method.

Answer 2

I have seen this done a couple of ways:

1. In the derived class override Dispose(bool disposing), cleanup your stuff and then call the base class Dispose(bool disposing). The problem here is the base class already has the disposed gate variable and checks that would would need to be replicated.

2. Create a protected Cleanup method that Dispose(bool disposing) in the base class calls to actually do the cleanup. The derived class can override it, do it's cleanup, and call the base class Cleanup method. This leaves all of the disposed checks in the base class that don't have to be replicated.

Answer 3

Have a look at this blog post by Davy Brion: http://davybrion.com/blog/2008/06/disposing-of-the-idisposable-implementation/

It adds a little extra code to the parent class, but doing appropriate cleanup in the subclass becomes rather simple.