Yingcai Xiao

1. C# Yingcai Xiao

2. Part I Moving from Java to C#

3. Common Type System (CTS) • .NET Framework’s Data Types: CTS • Six categories of data types in CTS: • system-defined: • Primitives (int, float, …) • user-defined: • Classes • Structs • Interfaces • Enumerations • Delegates

4. Common Type System (CTS) • Named Space: grouped code, used to resolve naming conflicts. • namespace mine • { • int i=10; • } • namespace his • { • int i=20; • } • mine.i = his.i;

5. Named Space Example namespace WP1.CS.UA { class Hello { public Hello() { System.Console.WriteLine("Hello, world."); } } } namespace WP2.CS { class Hello { public Hello() { System.Console.WriteLine("Hello, again!"); } } }

6. Named Space Example namespace WP { class Test { public static void Main() { WP1.CS.UA.Hello mc = new WP1.CS.UA.Hello(); WP2.CS.Hello mc2 = new WP2.CS.Hello(); } } }

7. Classes Class: a group of code and data to be instantiated to form objects. Four categories of class members: Fields: member variables Methods: member functions Properties: fields exposed using accessor (get and set) methods Events: notifications a class is capable of firing

8. Example: How to define a class (user-defined data type) class Rectangle { // Fields protected int width = 1; protected int height = 1; // Methods public Rectangle () { } public Rectangle (int cx, int cy) { width = cx; height = cy; }

9. Example: How to define a class (user-defined data type) // Accessor Methods public void setWidth(int w) { width = w; } public int getWidth() { return width; } public void setHeight(int h) { height = h; } public int getHeight() { return height; } } // End of Rectangle class

10. Example: How to use a class (user-defined data type) Rectangle rect = new Rectangle(2,4); rect.setHeight(8); rect.setWidth(rect.getWidth() * 2); double darea = (double) (rect.getWidth() * rect.getHeight() ); We protected member fields: width and height. (Encapsulation) (1) Securer code. Methods not belonging to the class hierarchy can’t access protected members. If we don’t want anyone change the value of “width”, we just don’t provide the setWidth() method. (2) Easier to maintain the code. We can rename “width” to “w” without impacting the users of the “Rectangle” class. (3) Tedious to implement and use. If we define the member fields as public, the usage would be much easier. rect.width *= 2;

11. Property Can we make the member fields secure and easy to use at the same time?

12. Example: How to define properties // Properties defined as grouped accessor methods // in the Rectangle class public int Width // group name { get { return width; } set // the input parameter is implicit: value { if (value > 0) width = value; else throw new ArgumentOutOfRangeException ( "Width must be 1 or higher"); } }

13. public int Height // a field defined by type and accessor code { get { return height; } set { if (value > 0) height = value; else throw new ArgumentOutOfRangeException ( "Height must be 1 or higher"); } } Example: How to define properties

14. Example: How to define properties public int Area // a property of only get method { get { return width * height; } }

15. Defining Properties • Properties are defined in the following format: • protected type field-name; • public type property-name • { • get { /* return the field value */ } • set { /* reset the field value */ } • } • A property definition is composed of • a protected or private field • a property to expose the field • which in turn consists of at least one accessor (get()/set()).

16. Using Properties • Properties are used the same way as public fields. Rectangle rect = new Rectangle(2,4); rect.Width = 7; rect.Width *= 2; // Double the rectangle's width int area = rect.Area; // Get the rectangle's new area //Typecast a property “value” from int to double double darea = (double) rect.Area; • Advantage of Properties: allow users to access private/protected fields as if they were public fields.

17. Notes on Properties • Properties are public methods (set and get) used like fields. Data is secured (encapsulated) and access is simplified. • (2) The set and get methods are called accessors. A property may not have the set (read-only properties) or the get (write-only properties), but can not miss both. • (3) The implicit input argument, value, for the set method has the same type as the property. • (4) The type of a property must be the same as the type of the field member it protects. • (5) A property can’t be overloaded, e.g., we can’t define a “public double Area { … }” after defining “public int Area { … }”. You have to use a different property name, e.g. doubleArea, to define the area property of type double.

18. Signature of a method: name, number of arguments, types of the arguments. Return type is not part of the signature. • Overloading: two or more methods have the same name but different arguments. • Name Mangling encodes the name of an overloaded method with its signature (by the compiler). The internal names of the methods are unique (no internal overloading). Why we have to name properties of different types differently? • Property do not have any arguments, so the only way to differentiate properties of different type is by their names.

19. Interfaces • An interface is a group of zero or more abstract methods • Abstract methods have no default implementation. • Abstract methods are to be implemented in a child class or child struct. • Subclassing an interface by a class or struct is called implementation of the interface. • An interface can be implemented but not instantiated. • You can’t use an interface class to create an object. • Interfaces can also include properties and events, but no data. • An interface defines a contract between a type and users of that type. Used to define software interface standards. • All interface methods are public, no specifiers needed. • A class can implement multiple interfaces. Interfaces

20. interface ISecret { void Encrypt (byte[] inbuf, byte[] outbuf, Key key); void Unencrypt (byte[] inbuf, byte[] outbuf, Key key); } //no implementation, just prototyping. class Message : ISecret { public void Encrypt (byte[] inbuf, byte[] outbuf, Key key) { /* implementation here */ } public void Unencrypt(byte[] inbuf, byte[] outbuf, Key key) { /* implementation here */ } } Message msg = new Message(); // e.g. check if object msg implements interface ISecret if (msg is ISecret) { // type checking, // an object of a child type is also an object of the parent type, but not the other way around ISecret secret = (ISecret) msg; // from child to parent, explicit cast secret.Encrypt (...); } Interface Example

21. An abstract class is a class that can’t be instantiated, i.e., one can’t use an abstract class to create an object. • The definition of an abstract class looks like a regular class except the preceding keyword“abstract”. • It can have member fields and methods. • It can only be used as a base class for subclassing. • Its subclasses can inherit its methods as default implementation. (They can overwrite those methods too.) • It is not allowed to inherit from multiple abstract classes. Abstract Class

22. Both can’t be instantiated. • Both defines standards for their subclass to implement. • An abstract class defines the minimal implementation of its subclasses. • An interface has no implementation at all. • A child class can’t subclass from more than one abstract classes. No multiple inheritance for abstract classes. • A child class can implement more than one interfaces. Multiple inheritance allowed for interfaces. • Abstract classes and interfaces can be used together. Abstract Class vs. Interface Classes

23. abstract class DefaultTokenImpl { private readonly string name; public string ToString() { return name; } protected DefaultTokenImpl(string name) { this.name = name; } } interface IToken { string ToString(); } interface IVisitable { void Accept(ITokenVisitor visitor); } interface IVisitableToken : IVisitable, IToken { } class KeywordToken : DefaultTokenImpl, IVisitableToken { public KeywordToken(string name) : base(name) { } void IVisitable.Accept(ITokenVisitor visitor) { visitor.VisitKeyword(ToString());} } Abstract Class & Interface Examples

24. KeywordToken subclasses the abstract class DefaultTokenImpl • It also implements the interface IVisitableToken (which implements interfaces IVisitable and IToken) • It implements the Accept abstract method specified in interface IVisitable (a parent of IVisitableToken) • It inherits the default implementation of ToString from the abstract class DefaultTokenImpl to implement the ToString abstract method specified in interface IToken (the other parent of IVisitableToken). Abstract Class & Interface Examples

25. Exception

26. Exception Handling (object-oriented event-driven runtime error handling) • An exception is thrown when a run-time error occurs. • The CLR defines how exceptions are thrown and how they’re handled. (How exception “events” are generated and how they’re handled by exception “event” handlers). • You can throw an exception in any language and catch it in any other. • You can throw exceptions across machines Exception Handling

27. CLR’s Exception Handling Mechanism: try, catch, finally, and throw File file = null; // Do not define it in the try block. Why? try { file = new File ("Readme.txt"); if(file != null) { /* Do the work here. */} … } catch (FileNotFoundException e) { Console.WriteLine (e.Message); } catch ( … ) { … } catch ( … ) { … } } finally { // Always come here even if there were exceptions. if (file != null) file.Close (); } Exception Handling

28. • The CLR calls the handler that most closely matches the type of exception thrown. • All of the exception types defined in the FCL are derived directly or indirectly from System.Exception. • FCL exception classes contain a Message property, which holds an error message describing what went wrong, and a StackTrace property, which details the call chain leading up to the exception. • Exception handlers can be nested. • Code in a finally block is guaranteed to execute, whether an exception is thrown or not. • An exception that is best handled by the caller rather than the callee. • Programmers can throw exceptions defined in FCL or their own exceptions derived from System.ApplicationException. if (value > 0) width = value; else throw new ArgumentOutOfRangeException ( "Width can’t be negative."); Exception Handling

29. System.Object : root class for all other classes. • Every class inherits the Finalize( ) method from System.Object. • It is called just before an object is destroyed by the garbage collector of CLR. • The time of call is determined by CLR not by the program. • Use System.GC.Collect() to force a garbage collection (system wide, time consuming). • Destructor in C++ is called just before an object is destroyed by the program, when the object is freed (for heap objects) or out of scope (for stack objects). The Object Class: System.Object

30. C# Part II Beyond Java

31. Internal Memory Structures of Data Store

32. Class Name; In C++: “Rectangle rect” declares an object of class Rectangle. rect Instantiating a Class (in C++) “rect” is the name of a memory space that stores a Rectangle object.

33. Class Name; In CTS:“Rectangle rect” declares a reference of class Rectangle. rect “rect” is the name of a memory space that stores a reference. This is similar to Java. Instantiating a Class (in CTS) A “reference” is an internal pointer, it needs to “point” to an object before being dereferenced.

34. Rectangle rect;  int area = rect.Area; // will not compile in C# Rectangle rect = new Rectangle (3, 4); // Use the second constructor rect 0x12345678 References • Dereferencing is automatic for a reference. (No *rect or rect->) • int area = rect.Area; • Please note the notation difference between a “pointer/reference” and a “name” in this lecture.

35. int i; In CTS: is i a reference to an integer or just an integer? In CTS: i is an int (a system defined primitive type), not a reference to an integer. “i” is the name of a memory space that stores an integer value. int i = 8; i is a value type, for which we can directly store an integer value into the memory named as i. Compiler already allocated memory to store the value and we don’t need to “new” to allocate memory to store the value. i Value Types

36. Fields of value types have the object (not reference) memories allocated by the compiler and can be used as an object directly without “new”. Can we define user types behave like value types? Class is used to define user types, but need to be “new”ed before using. Memories are allocated at runtime. => Tedious and Slow. Value Types

37. Structs: user-defined value types, less overhead and easier to use than classes. struct Point { public int x; public int y; • public Point () {x = 0 ; y = 0; } public Point (int x, int y) { this.x = x; this.y = y; } } Point pnt1; // pnt1 is an object, not a reference. x = 0, y = 0 Point pnt2= new Point (); // pnt2 is an object, not a reference. x = 0, y = 0 Point pnt3 = new Point (3, 4); // pnt3 is an object, not a reference. x = 3, y = 4 // The compiler uses “new” to initialize an object. Point pnt4(3,4); is not allowed in CTS. Structs

38. In CTS, Value Types are Stack Objects: memory allocated at compile time on the stack auto destruction, no garbage collection needed less overhead, code runs faster less flexible, sizes need to be known at compile time • In CTS, Reference Types are Heap Objects: • memory allocated at run time on the heap • garbage collected • more flexible, sizes need not to be known at compile time • more overhead, code runs slower • Class defines reference types (heap objects) • Struct defines value types (stack objects), even though “new” is used to create struct objects. Value types can’t derive from other types except interfaces. Summary: Value and Reference Types in CTS

39. class Point { public int x; public int y; } Point p1 = new Point (); p1.x = 1; p1.y = 2; Point p2 = p1; // Copies the underlying pointer p2.x = 3; p2.y = 4; Console.WriteLine ("p1 = ({0}, {1})", p1.x, p1.y); Console.WriteLine ("p2 = ({0}, {1})", p2.x, p2.y); Point p3; p3.x = 5; p3.y = 6; Class Code

40. class Point { public int x; public int y; } Point p1 = new Point (); p1.x = 1; p1.y = 2; Point p2 = p1; // Copies the underlying pointer p2.x = 3; p2.y = 4; Console.WriteLine ("p1 = ({0}, {1})", p1.x, p1.y); // Writes "(3, 4)" Console.WriteLine ("p2 = ({0}, {1})", p2.x, p2.y); // Writes "(3, 4)" Point p3;//Creats a reference(pointer), no memory allocated p3.x = 5; // Will not compile p3.y = 6; // Will not compile Class Code

41. struct Point { public int x; public int y; } Point p1 = new Point(); //Creates a value object on stack. p1.x = 1; p1.y = 2; Point p2 = p1;//Makes a new copy of the object on the stack p2.x = 3; p2.y = 4; Console.WriteLine ("p1 = ({0}, {1})", p1.x, p1.y); // Writes "(1, 2)" Console.WriteLine ("p2 = ({0}, {1})", p2.x, p2.y); // Writes "(3, 4)" Point p3; //Creates a value object on the stack p3.x = 5; // It works. p3.y = 6; Console.WriteLine ("p3 = ({0}, {1})", p3.x, p3.y); // Writes "(5, 6)" Struct Code

42. Arrays in C++ int a[2]; a[0] = 5; a[1] = 10; // stack objects , size has to be known at compile time and can’t be changed at runtime. int size = 2; int *p; // a pointer p = new int[size]; p[0] = 5; p[1] = 10; // heap objects; dynamically allocated at runtime, “size” can be a variable delete p; // free the memory. a p

43. Array (same syntax for both class and struct): • Point[] pa = new Point[2]; • pa[0] = new Point(); • pa[1] = new Point(); • Console.WriteLine ("pa[0] = ({0}, {1})", pa[0].x, pa[0].y); • Console.WriteLine ("pa[1] = ({0}, {1})", pa[1].x, pa[1].y); • Challenges: • (1) Draw pictures to show the memory layout of pa for Point as a class and a struct. • (2) What will happen when lines 2 and 3 are removed. Explain what will happen for Point as a class and then as a struct. Arrays in CTS

44. Generics : parameterized types According to Microsoft (http://msdn2.microsoft.com/en-us/library/512aeb7t(VS.80).aspx) • Use generic types to maximize code reuse, type safety, and performance. • The most common use of generics is to create collection classes. • The .NET Framework class library contains several new generic collection classes in the System.Collections.Generic namespace. These should be used whenever possible in place of classes such as ArrayList in the System.Collections namespace. • You can create your own generic interfaces, classes, methods, events and delegates. • Generic classes may be constrained to enable access to methods on particular data types. • Information on the types used in a generic data type may be obtained at run-time by means of reflection. C# and CLR Generics

45. Basic CTS Types: Class, Struct, Enum, Property, Event, Delegate, Value Type, Reference Type, Basic .NET Concepts: Garbage Collection, DLL, Versioning, Strong Name, GAC, Exception Handling, Try, Catch, Finally, Throw. What is it? How to define and use it? Can you write a program? Can you trace a program? Do you know what going on inside? The Fundamentals C#