Question

I have just started with the .NET framework. Today, I was taught about the IDisposable interface and the dispose() method. I was taught a few things regarding it:

dispose() should contain the cleanup code corresponding to an object(like closing any resources occupied by any objects – files or database connections,etc.)

I was also told that in case we don’t do it in the dispose() method, the same could be done in the destructor, but that doesn’t ensure immediate execution, and we are left to the mercy of GC.

And if at all we don’t provide any cleanup code at all, the GC will forcefully terminate all connections to resources that our objects were holding. Hence, we should handle the cleanup code ourselves.

But I was curious as to why doesn’t CLR handle this on it’s own? It takes care of Memory Management, it takes care of Garbage Collection. So, it should very well know which Object holds onto which resource(s) and when that Object dies off. So, it should be capable of de-allocating those resources as well?

I asked a few people about it. The answer I was given was that it is because we need to close it gracefully, where as GC closes it forcefully. Is it actually the reason?

Answer 1

In .NET there’s much more than managed code that the GC knows about. There’s like a huge volume of unmanaged code involved: all the file handles, database connections, network sockets, … all this is plain ol’ unmanaged Win32 code. You can’t even believe that in almost every single BCL function you are calling from your pretty C# application, you will be hitting like tons of unmanaged functions written in C++ (and may God forbid VB6) and buried deep into the internals of the OS itself. All those functions are allocating unmanaged memory, handles, … The managed world doesn’t know what happens there.

For example every single time you open a file (FileStream) you are basically calling (behind the scenes of course) the CreateFile unmanaged Win32 function. This function allocates an unmanaged file handle directly from the file system. .NET and the GC has strictly no way of tracking this unmanaged code and everything it does. That’s why those classes implement the IDisposable interface. So that you could always wrap their instances in using statements and ensure that the Dispose method is always called, even in the event of an exception, and this as soon as possible. The Dispose method will take care of calling another unmanaged function to clean the mess it created.

So basically the way you could think about the IDisposable interface is the following:

The day when we have an operating system written in a fully managed language (something like Midori for example from Microsoft Research) we will probably no longer need IDisposable as the GC will be able to completely replace it as it will have knowledge of everything that happens within this system.