Advance Programming 1

Advance Programming 1

1. Windows Messaging

   • used to communicate between operating system and application
   • C programming requires programmers learn a lot about window messaging and the code it used
   • C# allow programmmer to ignore window messages most of the time
     • when detected window message, e.g. keydown, application run keypress event handler if object has the focus on the form
   • Adv: programmer can ignore that way in which windows messages are involced in this
     • when windows detects an event, it passes the app a message
     • the event delegate matching the message is used to run any event handler connected to the event delegate
   • windows message containing 6 integers that generated by windows operating system
     • HWnd: get/set the window handle og the message
     • Msg: get/set the ID number for the msg
     • LParam: specifies the LParam field
     • WParam: get/set the WParam of the msg
     • Result: the value returned to windows in response to handling the msg
   • C# programmer do with windows messages
     • Listening for messages
       • create an overridden version of the form or the control's WinProc()
       • using a MessageFilter
     • Creating and sending message
       • SendMessage() and PostMessage()
   • Windows message processing
     • windows detects event --> generate message to store needed data -- > app has a msg loop method to keep checking new msg, remove old msg, and pass to approprate form --> form's WinProc() receive msg and pass to control --> control's WinProc() method run according to msg --> decide which control event the msg belong to --> run code on event handler

2. Overloading operator

   • operator are pre-defined in C#, the default meaning of operator has been coded and stored in C#
     • in order to use operators with objects for special classes, programmer can overload the operator
       • by creating extra definitions of some operators for use with objects of a class
         • only apply to the class containing the code to make them work
   • operator group
     • Unary operator: change meaning of a single value can be overloaded
       • e.g  "-1" +,-,|,~,++,--,true,false
     • Binary operator: operator used with two values - can be overloaded
       • e.g "1+2" +,-,*,/,%,&,|,<<,>>
     • relational operator: can be overloaded but only as pairs
       • ==,|=,<,>,<=,>=
     • conversion operator: &&, ||, () cannot be overloaded
     • Compound assignment operators: can be overloaded, but are automatically overloaded when binary operator is overloaded
       • +=,-=,*=,/=,%=
     • =,.,?.,->,new, is, as, sizeof cannot be overloaded
   • use the operator method to overloading
     • must be public and static
     • can take only value parameters
     • Ref and out parameters are not allowed as arguments
     • e.g. public static return_type operator op (argument list)
     • should return a reference or value type and not void
     • same operator can be overloaded more than once
   • summary
     • operator overloading cannot modify the syntaz, precedence or associativity of an operator
     • user defined operator implementations are given preference over predefined implementations
     • can't return void
     • the operator overload methods can be overloaded

3. Static Keyword

   • static class cannot be instantiated. i.e. cannot use the "new" keyword to create a var of the class type
   • access the members of the static class by using the class name itself.
   • static class can be used as a convenient container for sets of methods only operate on input parameters and not have to get / set internal instance fields
     • e.g. System.Math
   • Main feature:
     • Contains only static members
     • Cannot be instantiated
     • Is sealed
   • non-static class can contain static methods, fields, properties or events.
     • static member is always accessed by class name, not instance name
     • static member is callable even when on instance has been created
   • static methods and properties cannot access non-static fields and events and cannot access instance var of any object unless its explicitly passed in a method parameter
   • more common to declare non-static class with some static members than declare an entire class as static
   • two common use
     • keep a count of the number of objects that have been instantiated
     • store a value that must be shared among all instances
   • a field cannot be declared as static const, a const field is essentially static in its behavior, so const fields can be accessed by classname.membername notation
   • C# does not support static local variables

4. interface

   • a way of defining a reference type that can point to objects
   • interface properties
     • contains the signatures of some methods
     • contain propseties
     • does not contain data or implementation code
   • can declare classes implement an interface
     • class MUST implement any methods and properties that the interface contains
     • the implementations must be public
   • class can implement more then one interface
     • can have any number of other methods and data member
     • PROVIDED it contains all methods and properties specified in the interface that it implement
   • interface is a contract to be met by the person writing a class
     • if a class implements an interface, need to provide the methods and properties specify in the interface
     • effect: if an object from a class that implements an interface, can point to it the interface reference and make use of methods and properties that the interface knows

5. Delegates and events

   • Delegates are reference type which allow indirect calls to methods
   • delegate instance holds reference to some methods and invoking the delegate one causes all methods to be called
   • the usefulness of delegates lies in the fact that the functions invoke them are blind to the underlying methods they cause to run
   • can be specified in a namespace on their own or else can be specified in another class
   • delegate is limited to referencing methods of a particular kind only
     • type is indicated by the delegate declaration
   • A common use of delegates: to allow a programmer to alter the behavior of an existing method by passing the method a delegate to another method
     • the existing method will invoke the passed delegate and run whatever code the delegate points to
     • possible to make a method very flexible so that it can use methods written by another programmer
   • event is often something like a button click that happens outside the application
   • event can be set up for any class
   • when sth happen to an object, it check a public delegate to see if it point to any methods
     • as the delegate is public, it allows code in other classes to link a method to the delegate
   • event delegate in C# is with signature based on a void return type and a set of arguments
   • e.g. of event: a user's selection of a button on a graphical user interface, action trigger a number of methods to 'handle' it
   • distinguishes event and other method: event are backed by a particular 'subscription-notification' model.
   • good practice for Events
     • when raise an event, don't trigger the class's event object directly, but call a 'protected, virtual' method to trigger it
     • when event are raised it pass two objects to the subscribers. 1. reference to the class 2. instance of the system.eventargs class
     • if event not interested in pass data to subscribers, the delegate will return null to eventArgs / if event pass data, it is standard to use a specific class derives from the eventargs class to old the data
     • event can be intercept by overriding the method used to raise it