Advance programming 2

Advance programming 2

1. Enumerations and Bit Fields

   • enum keyword used to declare an enumeration
     • enum.GetValues() --> return the values of an enumeration as system array
   • a distinct type consisting of a set of named constants called the enumerator list
   • bit fields are enumeration that have a flags attribute that allows a bitwise combination of its member values
   • bit fields can be combined using a bitwise or operation, enumerated constants cannot
     • e.g. bit field with value stored in powers of 2 (1,2,4,8,16...)
       • stored in binary format and combine values into single number, we can see which code is used
         • i.e. 2 + 8 + 16 gives 011010, which is 000010 + 001000 + 010000
   • C# use codes like this when values can be true at the same time, e.g. bold, italic underline using 000001,000010 and 000100 to represent, 000011 means bold, italic but not underline
   • bit fields generally used for lists of elements that might occur in combination
     • so bit fields combined with a bitwise or operator to generate unnamed value
   • enumeration constants are used for lists of mutually exclusive elements

2. Bitwise operations

   • action that work at the level of single bits in a stored integer
   • integer in PCs is 32-bit with each bit hold 1 or 0, starting from the rightmost
   • computer system use 1s and 0s to hold sets of true/false data
     • more economical in storage and processing time than hold 32 true/false values separate number
     • need to be able to 1. turn a particular bit on/off. 2. check to see if a particular bit is on or off
     • cannot directly do to individual bits, need to work at level of integer , this is bitwise operations
   • bitwise operator
     • binary operator
       • combine bits in a pair of integers according to a particular set of rules
         • e.g &(and), |(or), ^(Xor)
     • unary operator
       • change all the bits in a single integer according to a particular set of rules
         • e.g. ~(not), <<(left shift), >>(right shift)
     • and
       • check turn on or not
     • or
       • turn some bits on
     • Xor
     • not
       • change bits current status
     • left shift
     • right shift

3. Multithreading

   • the carrying out serveral different processes so they appear to be running simultaneously
     • done by allocating each process a time slice in turn
   • if not done right, ill effect
     • value turn to utter grabage
     • counters fail to increment
     • application suddently freexe
     • resources such as database unexpectedly close or exhausted
   • types
     • main thread
     • worker thread
     • background thread
     • thread pool
       • a collection of thread objects created for use by an application
       • provide to each application that stays in existence, but recommended to left it to application and create own threads manually
   • thread safe
     • two threads use same object result in one thread affected to be meaningless or deadlock
     • objects prevent destructive interference between threads sharing the objects are thread safe
     • ways to nake sure only one thread at a time can access a resource
       • use lock keyword
         • when use by a thread, the session will remain locked until the thread leaves
           • deadlock may appear if threads use more than one shared section, as two threads remain in wait states and each is waiting another thread to unlock the locked session
       • callback method
         • method 1 passed the name of another method as a parameter and first method will call the second method at some point in its execution
           • advantage is programmer can make method run code that suits the programmer but not par of the method
       • synchronous and asynchronous calls and blocking
         • synchronous method call waits for the method to complete before continuing with program flow
         • asynchronous method call return immediately so the program can perform other operations wihle the called method completes its work
           • beginlnvoke method
             • initiates an asynchronous operation
           • endInvoke
             • call when operation completes, always call it after an asynchronous call completes
           • common ways to use
             • endInvoke will block until asynchronous operation completes
             • delegate begininvoke and endinvoke method are dynamically generated
   • threads with windows form
     • windows forms are not thread safe

4. serialization

   • process of writing objects on a persistent storage media such as a file on the disk
   • Three types
     • binary
       • useful when storing object when both storing and using program are c#
       • most compact and effecient
       • allow to store an exact copy of the state of an object at a given point in time
     • SOAP
       • simple object access protocol
       • store data in a specialized XML format
       • all data can be store
       • advantage: it allows the data to be passed across firewalls
     • XML
       • stores public data and properties in XML format
       • private data is not stored
       • useful for making data available in a format that can be access by other languages and platform
       • n be set through firewalls
   • de-serialization is we can use to restore objects from the stored form
   • programmer does not have to write extra code to store an object for each class being used
   • advantage is programmer only need to write a minimum of code

5. inheritance and polymorphism

   • inheritance is where one class is defined as an extension of another class
     • allowed provided the class we start with is not specified as sealed
     • class that is inherited from: parent class, base class, immediate ancestor and super class
     • class that built by inheriting from a parent class: child class, derived class, descendant class, sub class
     • child class has all data members and methods of the parent class and can add extra data members and extra methods
     • syntax for inheritance is the name of the child class followed by a colon and then the name of parent class, can have only one parent class
     • the child class constructor need to receive any parameters that are needed to intialize the base class as well as the child class