Friday, September 10, 2010

Memory Allocation and Disposal

Memory Allocation

The Common Language Runtime allocates memory for objects in two places: the stack and the heap. The stack is a simple first-in last-out memory structure, and is highly efficient. When a method is invoked, the CLR bookmarks the top of the stack. The method then pushes data onto the stack as it executes. When the method completes, the CLR just resets the stack to its previous bookmark—“popping” all the method’s memory allocations is one simple operation!

In contrast, the heap can be pictured as a random jumble of objects. Its advantage is that it allows objects to be allocated or deallocated in a random order. As we’ll see later, the heap requires the overhead of a memory manager and garbage collector to keep things in order.

To illustrate how the stack and heap are used, consider the following method:

void CreateNewTextBox()
TextBox myTextBox = new TextBox(); // TextBox is a class

In this method, we create a local variable that references an object. The local variable is stored on the stack, while the object itself is stored on the heap:

The stack is always used to store the following two things:

* The reference portion of reference-typed local variables and parameters (such as the myTextBox reference)
* Value-typed local variables and method parameters (structs, as well as integers, bools, chars, DateTimes, etc.)

The following data is stored on the heap:

* The content of reference-type objects.
* Anything structured inside a reference-type object.

Memory Disposal

Once CreateNewTextBox has finished running, its local stack-allocated variable, myTextBox, will disappear from scope and be “popped” off the stack. However, what will happen to the now-orphaned object on the heap to which it was pointing? The answer is that we can ignore it—the Common Language Runtime’s garbage collector will catch up with it some time later and automatically deallocate it from the heap. The garbage collector will know to delete it, because the object has no valid referee (one whose chain of reference originates back to a stack-allocated object).[1] C++ programmers may be a bit uncomfortable with this and may want to delete the object anyway (just to be sure!) but in fact there is no way to delete the object explicitly. We have to rely on the CLR for memory disposal—and indeed, the whole .NET framework does just that!

However there is a caveat on automatic destruction. Objects that have allocated resources other than memory (in particular “handles”, such as Windows handles, file handles and SQL handles) need to be told explicitly to release those resources when the object is no longer required. This includes all Windows controls, since they all own Windows handles! You might ask, why not put the code to release those resources in the object’s finalizer? (A finalizer is a method that the CLR runs just prior to an object’s destruction). The main reason is that the garbage collector is concerned with memory issues and not resource issues. So on a PC with a few gigabytes of free memory, the garbage collector may wait an hour or two before even getting out of bed!

So how do we get our textbox to release that Windows handle and disappear off the screen when we’re done with it? Well, first, our example was pretty artificial. In reality, we would have put the textbox control on a form in order to make it visible it in the first place. Assuming myForm was created earlier on, and is still in scope, this is what we’d typically do:

myForm.Controls.Add (myTextBox);

As well as making the control visible, this would also give it another referee (myForm.Controls). This means that when the local reference variable myTextBox drops out of scope, there’s no danger of the textbox becoming eligible for garbage collection. The other effect of adding it to the Controls collection is that the .NET framework will deterministically call a method called Dispose on all of its members the instant they’re no longer needed. And in this Dispose method, the control can release its Windows handle, as well as dropping the textbox off the screen.

All classes that implement IDisposable (including all Windows Forms controls) have a Dispose method. This method must be called when an object is no longer needed in order to release resources other than memory. There are two ways this happens:
- manually (by calling Dispose explicitly)
- automatically: by adding the object to a .NET container, such as a Form, Panel, TabPage or UserControl. The container will ensure that when it’s disposed, so are all of its members. Of course, the container itself must be disposed (or in turn, be part of another container).
In the case of Windows Forms controls, we nearly always add them to a container – and hence rely on automatic disposal.

The same thing applies to classes such as FileStream—these need to be disposed too. Fortunately, C# provides a shortcut for calling Dispose on such objects, in a robust fashion: the using statement:

using (Stream s = File.Create ("myfile.txt"))




This translates to the following code:

Stream s = File.Create ("myfile.txt");







if (s != null) s.Dispose();


The finally block ensurse that Dispose still gets executed should an exception be thrown within the main code block.

What about in WPF?

Most of the elements in WPF don’t wrap unmanaged handles requiring explicit disposal. So you can mostly ignore the disposal with WPF!

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