1. Automatic Memory Management
GARBAGE COLLECTION

3.  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

4. Memory Allocation
 Use the resources by accessing the class members
student.Name = txtName.Text
student.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

5. 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

6. 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 Generation Generation
Zero One Two

7. 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 themanged heap
 Add bytes required for an object’s
overhead
 Each object has two overhead fields:
1. Method Table Pointer
2. SyncBlockIndex

8. 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

9. 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

10. Cont’d
 As Garbage Collector starts it cleans up any objects
that have been marked for cleaning
AB C
 Three Objects
 B has lost its scope
 B is marked for collection
 Finally it is removed
A C

11.  System only allocates memory at the end
 Job of garbage collector to compact the memory
structure
A C
 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!!

12. 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

13. 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

14. 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

15. Sharing Classes
CLASS LIBRARIES

16. Classes Outside Applications
 We have spent the last couple of weeks
creating classes within our application
 How useful is that?
 A bit useful…

17. 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

18. Creating a Class Library

20. Sample Class Library Code

21. Change some properties

22. Compile…

23. Create New Project

24. Add Form

25. Add Reference

31. Using our Class Library