Delegates, Events and Generics in C

1
Delegates, Events and Generics in C

• CS2335
• Spring 2007

2
Exceptions

• Dont use to find handle bugs fix the bug!!
• Dont use to detect user mistakes write
  validation code and use return codes
• Use for predictable but unpreventable problems
  (open non-existent file, run out of memory, etc.)

3
A lot like Java!!!

• try / catch block
• We can have simple catch that gets everything
• try catch
• Or specify the exception
• try catch (System.ArithmeticException)
• finally ensures completion
• try catch finally
• cannot have break, return, goto or continue in a
  finally block.

4
Throwing exceptions

• throw anException instance
• throw new System.Exception()
• execution immediately halted and CLR starts
  unwinding looking for handler.
• if no handler, exception is passed to the VM
  which shows the user an exception dialog.

5
Custom Exceptions

• Derive from System.ApplicationException class
• constructor takes a string and passes to base
  class
• public MyException(string msg) base (msg)

6
Built-in exceptions

• Never throw System.Exception or
  System.ApplicationException
• Dont just blindly catch all exceptions, know
  what you are looking for, Dont try to handle
  exceptions that you cannot.
• System.OutOfMemoryException
• System.NullReferenceException
• System.InvalidCastException
• System.IndexOutOfRangeException

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Delegates

• Loose coupling mechanism Buttons
• Delegates are first-class objects
• Syntactic sugar for function pointers
• typedef int (CMP_FUNC_PTR)(void , void )
• public delegate int CMP_FUNC (object x, object
  y)
• Like an interface with only one method
• Add delegate , Remove delegate -

8
Events

• exist as class members just like attributes or
  properties
• uses the publish/subscribe model
• publishing class raises event (invokes delegates)
• subscribing class is notified of event
• publishing uses delegates
• subscribers use event handler (, -)
• event keywords stops direct invocation

9
Creating an Event

• Create a class to handle the event data (or
  arguments) Subclass EventArgs
• In the class raising the event, declare the
  delegate method signature everyone must
  implement.
• In the class raising the event declare the
  delegate with the event keyword
• In the class listening for the event, implement
  the delegate method and register it with the
  class raising the event.

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Event Conventions

• Signature is always (object sender, EventArgs
  args)
• Name always begins with On
• If you dont need args, you still declare them,
  but pass EventArgs.Empty