Question

The .NET garbage collector will eventually free up memory, but what if you want that memory back immediately? What code do you need to use in a class MyClass to call

MyClass.Dispose() 

and free up all the used space by variables and objects in MyClass?

Answer 1

IDisposable has nothing to do with freeing memory. IDisposable is a pattern for freeing unmanaged resources — and memory is quite definitely a managed resource.

The links pointing to GC.Collect() are the correct answer, though use of this function is generally discouraged by the Microsoft .NET documentation.

Edit: Having earned a substantial amount of karma for this answer, I feel a certain responsibility to elaborate on it, lest a newcomer to .NET resource management get the wrong impression.

Inside a .NET process, there are two kinds of resource — managed and unmanaged. ‘Managed’ means that the runtime is in control of the resource, while ‘unmanaged’ means that it’s the programmer’s responsibility. And there really is only one kind of managed resource that we care about in .NET today — memory. The programmer tells the runtime to allocate memory and after that it’s up to the runtime to figure out when the memory can freed. The mechanism that .NET uses for this purpose is called garbage collection and you can find plenty of information about GC on the internet simply by using Google.

For the other kinds of resources, .NET doesn’t know anything about cleaning them up so it has to rely on the programmer to do the right thing. To this end, the platform gives the programmer three tools:

1. The IDisposable interface and the ‘using’ statement in VB and C#
2. Finalizers
3. The IDisposable pattern as implemented by many BCL classes

The first of these allows the programmer to efficiently acquire a resource, use it and then release it all within the same method.

using (DisposableObject tmp = DisposableObject.AcquireResource()) {     // Do something with tmp } // At this point, tmp.Dispose() will automatically have been called // BUT, tmp may still a perfectly valid object that still takes up memory 

If ‘AcquireResource’ is a factory method that (for instance) opens a file and ‘Dispose’ automatically closes the file, then this code cannot leak a file resource. But the memory for the ‘tmp’ object itself may well still be allocated. That’s because the IDisposable interface has absolutely no connection to the garbage collector. If you did want to ensure that the memory was freed, your only option would be to call GC.Collect() to force a garbage collection.

However, it cannot be stressed enough that this is probably not a good idea. It’s generally much better to let the garbage collector do what it was designed to do, which is to manage memory.

What happens if the resource is being used for a longer period of time, such that its lifespan crosses several methods? Clearly, the ‘using’ statement is no longer applicable, so the programmer would have to manually call ‘Dispose’ when he or she is done with the resource. And what happens if the programmer forgets? If there’s no fallback, then the process or computer may eventually run out of whichever resource isn’t being properly freed.

That’s where finalizers come in. A finalizer is a method on your class that has a special relationship with the garbage collector. The GC promises that — before freeing the memory for any object of that type — it will first give the finalizer a chance to do some kind of cleanup.

So in the case of a file, we theoretically don’t need to close the file manually at all. We can just wait until the garbage collector gets to it and then let the finalizer do the work. Unfortunately, this doesn’t work well in practice because the garbage collector runs non-deterministically. The file may stay open considerably longer than the programmer expects. And if enough files are kept open, the system may fail when trying to open an additional file.

For most resources, we want both of these things. We want a convention to be able to say ‘we’re done with this resource now’ and we want to make sure that there’s at least some chance for the cleanup to happen automatically if we forget to do it manually. That’s where the ‘IDisposable’ pattern comes into play. This is a convention that allows IDispose and a finalizer to play nicely together. You can see how the pattern works by looking at the official documentation for IDisposable.

Bottom line: If what you really want to do is to just make sure that memory is freed, then IDisposable and finalizers will not help you. But the IDisposable interface is part of an extremely important pattern that all .NET programmers should understand.