GARBAGE COLLECTIONAutomatic Memory Management

In order to set aside a piece of memory for our resource we use the new keyword

Dim myStudent As New Student

This create a newobj instruction in the MSIL code generated from your application

The constructor of class will then set up the initial state of the object

Sub New() Course = 1

End Sub

Memory Allocation

Use the resources by accessing the class members

student.Name = txtName.Textstudent.studentID = txtID.Text

Traditionally the programmer had to manually free up system resources

For example in C++ you use a special Destructor Programmers often forget to kill objects, or try to

access a piece of memory that has already been wiped

myStudent = Nothing

Garbage Collection in .NET

Garbage Collection is operating as a separate thread in the background

GC itself requires resources to run It is assigned low priority Running out of memory?

Garbage Collection given REALTIME priority and collect all unwanted objects

How does it Locate Garbage?

When an application is loaded a portion of memory is assigned to that particular program.

This piece of memory is called the managed heap.

The memory is seperated into three parts:

Generation Zero

Generation One

Generation Two

Memory Allocation

When you create an object using the new contructor

Newobj called in MSIL When it executes:

Calculates the number of bytes for the object or type needs to be loaded into the

manged heap Add bytes required for an object’s

overhead Each object has two overhead fields:

1.Method Table Pointer2.SyncBlockIndex

Memory Allocation Cont’d

CLR checks to see if the bytes required to allocate the object are available

If it fits it is allocated at the address pointed to by

NextObjPtr The address of the object is

returned

NextObjPtr navigates past the object and finds where the next object will be placed in the heap

Cont’d All these processes happen in

Generation Zero level When generation zero does not have

enough space to allocate to other objects

Garbage Collector given real time priority

Garbage Collector will monitor level zero again to check objects scope

It will mark items that are no longer needed

Cont’d

As Garbage Collector starts it cleans up any objects that have been marked for cleaning

Three Objects B has lost its scope B is marked for collection Finally it is removed

A B C

A C

System only allocates memory at the end

Job of garbage collector to compact the memory structure

Garbage collection has not ended Looks at all the objects that have survived the

sweep

Those objects will be moved to Generation one

Generation Zero can be used to store new objects.

If Generation One doesn’t have enough space the process will be carried out there too!!

A C

Problems with Garbage Collection Garbage Collection is run non-

deterministically In VB 6.0 you could set a object =

Nothing and the Class Terminate Event was raised.

In VB .NET you can still set an object = Nothing, but this will not actually kill your object it will only mark it for cleaning

GC Cont’d

If you don’t know when GC is going to run how can you clear up extra resources associate with a class instance?

Extra Resources such as files, network connections, database connections are not handled very well by garbage collection

Invoke the Sub Finalize

Finilize

This is the method that the Garbage Collector must run on the object to clear any unmanaged resources, prior to reclaiming memory used by the object

By default the finilize method doesn’t do anything

We can override it and put in code to explicitly clean up unmanaged resources

CLASS LIBRARIESSharing Classes

Classes Outside Applications We have spent the last couple of

weeks creating classes within our application

How useful is that? A bit useful…

Seperating Classes from your Application You can place your project in a class

library This is a project that gets compiled

into a DLL You can then reference it from

another project and access its functionality

It allows you to reuse code

Creating a Class Library

Sample Class Library Code

Change some properties

Compile…

Create New Project

Add Form

Add Reference

Using our Class Library