Introduction to C# John K. Bennett with thanks to: Anders - PowerPoint PPT Presentation

Introduction to C# John K. Bennett with thanks to: Anders Hejlsberg Distinguished Engineer Developer Division Microsoft Corporation

Key Language Features Unified object system  Everything is an object, even primitives  Single inheritance  Interfaces  Specify methods & interfaces, but no  implementation Structs  A restricted, lightweight (efficient) type  Delegates  Expressive typesafe function pointer  Useful for strategy and observer design  patterns Preprocessor directives 

What’s an Object? 1. A set of data, together with … 2. Methods that manipulate that data

Key Ideas about Objects Information Hiding  Code Reuse  Abstraction  Modularity  Encapsulation  Inheritance  Polymorphism 

How These Ideas are Implemented : Classes  Instances 

C# – The Big Ideas A component oriented language Component concepts in C#:   Properties, methods, events  Design-time and run-time attributes  Integrated documentation using XML Enables one-stop programming   No header files, IDL, etc.  Can be embedded in web pages

C# – The Big Ideas Everything really is an object  Traditional views  C++, Java: Primitive types are “magic” and do not interoperate with objects  Smalltalk, Lisp: Primitive types are objects, but at great performance cost  C# unifies with no performance cost  Deep simplicity throughout system  Improved extensibility and reusability  New primitive types: Decimal, SQL…  Collections, etc., work for all types

C# – The Big Ideas Robust and durable software Built-In Garbage collection   No memory leaks and stray pointers Exceptions   Error handling is not an afterthought Type-safety   No uninitialized variables, unsafe casts Versioning   Pervasive versioning considerations in all aspects of language design

C# Versioning Example

C# Basic Syntax Case-sensitive  Whitespace has no meaning  Sequences of space, tab, linefeed,  carriage return Semicolons used to terminate statements  (;) Curly braces { } enclose code blocks  Comments:  /* comment */  // comment  /// <comment_in_xml>  Automatic XML commenting facility 

Classes and Objects A class combines together  Data  Class variables  Behavior (code for manipulating these data)  Methods  This is a key feature of object-oriented languages  Class and Instances  Class defines a “template”  Instances of a class represent reified, well, instances  Each instance gets its own copy of the class variables;  except “static” methods and variables are unique to  the class. Example: Class Person, where each instance has a  unique age, height, name, etc.; the static class variable numPersons keeps track of how many instances of Person we have created.

Inheritance How it Works  If class B inherits from base class A, it gains all of  the variables and methods of A Class B can optionally add more variables and  methods Class B can optionally “override” (change) the  methods of A Uses  Reuse of class by specializing it for a specific  context Extending a general class for more specific uses  Interfaces ( a language feature)  Allow reuse of method definitions of interface  Subclass must implement method definitions 

Inheritance Example class A { public void display_one() { System.Console.WriteLine("I come from A"); } } class B : A { public void display_two() { System.Console.WriteLine("I come from B, child of A"); } } class App { static void Main() { A a = new A(); // Create instance of A B b = new B(); // Create instance of B a.display_one(); // I come from A b.display_one(); // I come from A b.display_two(); // I come from B, child of A } }

Inheritance Qualifiers Abstract  indicates an incomplete implementation.  can be used with classes, methods, properties, indexers, and events.  When used with classes, indicates a class intended only to be a base class of other  classes. Members marked as abstract, or included in an abstract class, must be  implemented (overridden) by classes that derive from the abstract class. Virtual  used to modify a method, property, indexer, or event declaration  may be overridden in a derived class.  public abstract class myBase { // If you derive from this class you must implement this method. notice we have no method body here either public abstract void YouMustImplement(); // If you derive from this class you can change the behavior of this method, but are not required to public virtual void YouMayOverride() { } } public class MyBase { // This will not compile because you cannot have an abstract method in a non-abstract class public abstract void YouMustImplement(); }

Visibility A class is a container for data and behavior  Often want to control over which code:  Can read & write data 1. Can call methods 2. Access modifiers:  public  No restrictions. Members visible to any method of any class  private  Members in class A marked private only accessible to  methods of class A Default visibility of class variables  protected  Members in class A marked protected accessible to methods  of class A and subclasses of A.

Visibility Example class A Class A can see:  { public int num_slugs; num_slugs: is public  protected int num_trees; num_trees: is protected, but is  … defined in A } Class B can see:  class B : A { num_slugs: is public in A  private int num_tree_sitters; num_trees: is protected in  … parent A } num_tree_sitters: is private, but  class C is defined in B { … Class C can see:  } num_slugs: is public in A  Can’t see:  num_trees: protected in A  num_tree_sitters: private in B 

Constructors Use “new” to create a new object instance  This causes the “constructor” to be called  A constructor is a method called when an  object is created C# provides a default constructor for every  class Creates object but takes no other action  Typically classes have explicitly provided  constructor Constructor  Has same name as the class  Can take arguments  Usually public, though not always  Constructor may be private to ensure that only one  object instance is created

Unusual Types in C# Bool  Holds a boolean value, “true” or “false”  Integer values are not equivalent to boolean  values 0 does not equal false  There is no built-in conversion from integer to  boolean Decimal  A fixed precision number up to 28 digits plus  decimal point Useful for money calculations  Example: 300.5m  Suffix “m” or “M” indicates decimal 

Variable Examples int number_of_slugs = 0; string name = “Bob”; // string is a built-in Class float myfloat = 0.5f; bool onFire = true; const int freezingPoint = 32; // const = fixed Variables must be initialized or  assigned before first use Class members require a visibility def  (defaults to private) Constants cannot be changed 

Enumeration Example public enum TokenType { KEYWORD, SYMBOL, IDENTIFIER, INT_CONST, STRING_CONST, UNKNOWN}; public TokenType tokenType = TokenType.UNKNOWN; Base type can be any integral type (int,  ushort, long), except for char Defaults to int, for example, in the  above example “KEYWORD” = 0

If-Else Example using System; if (i < 5) { Console.WriteLine(“i is < 5”); } else { Console.WriteLine(“i is >= to 5”); } C# supports C/C++/Java syntax for “if” statement  Expression must evaluate to a bool value  No integer expressions here  == means “equal to” for boolean comparison  if (i == 5) // if i equals 5  if (i = 5) // error, since i = 5 is not a boolean expression 

For Statement Example for (int i= 0; i < numShips; i++) { ship[i] = “armed”; } For statement is used to iterate  Three parts: (init; test; increment) 

While Statement Example // go till we find a quote character while (line[charInLine] != '"') { charInLine++; } charInLine++; // put index on the next character If boolean expression in while  statement is false, code in loop is never executed.

Switch Statement Example const int raining = 1; const int snowing = 0; int weather = snowing; switch (weather) { case snowing: System.Console.Writeln(“It is snowing!”); goto case raining; case raining; System.Console.Writeln(“I am wet!”); break; default: System.Console.Writeln(“Weather OK”); break; } Alternative to if-else chains  Typically use break; use goto to continue to another  case (fall-thru is illegal, except for empty cases)

C# Program Structure  Namespaces  Contain types and other namespaces  Type declarations  Classes, structs, interfaces, enums, and delegates  Members  Constants, fields, methods, properties, indexers, events, operators, constructors, destructors  Organization  No header files, code written “in - line”  No declaration order dependence