Dispose Dos and Don'ts
Due to the positive response on my previous entry on Dispose, I thought I'd write another, this time on what one should and shouldn't do with a Dispose method.
- Dispose should contain code to release any unmanaged resources associated with the object's instance. For example, any handles should be released, database connections closed, etc. This simulates deterministic finalization by letting the developer do the resource cleanup when he/she is done with the object. You may want to use an IsDisposed bool and check it inside Dispose.
- Dispose should be safely callable multiple times. In my example above, if some user of your class called Dispose twice, we would crash with an unhandled InvalidOperationException trying to “double free” the GCHandle.
- If you inherit from an IDisposable object, call the base class' Dispose method. Unlike finalizers, which the GC calls, Dispose is only called by user code (sometimes automatically in C#, as I covered previously). This means any unmanaged resources held by your object's parent, must also be disposed of.
- Suppress finalization in Dispose. There has been some debate about whether GC.SuppressFinalization(this) should go before or after the cleanup code. Some argue after, in case something goes wrong before you've finished cleaning up. Others argue before, otherwise you may open up a race condition between your finalizer and your Dispose method. You should consider both strategies depending on your object's implementation.
- Don't throw exceptions in Dispose. Nothing should go wrong with your object calling Dispose. Mainly for reasons stated above.
- Throw ObjectDisposedException if a caller tries to use a released resource. You may be in a situation where you've disposed of your object because you no longer need the unmanaged resource, but you still need to have the object hanging around. If this is the case, any methods that would normally access the resource should throw an ObjectDisposedException, while other (possibly useful) methods should not.
- Don't call Dispose from one thread, while still using the object on another. Try to make the thread that created the object be the one that disposes of it. This is more general good multithreaded programming advice, then Dispose specific.
- Whenever possible, follow the Dispose Pattern. That link, one more time: http://msdn.microsoft.com/library/default.asp?url=/library/en-us/cpgenref/html/cpconfinalizedispose.asp.
If you have any other Dispose Do's and Don'ts, please post them in the comments.
