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Chapter 2 Types & Exceptions Yingcai Xiao. Part I Moving from C++/Java to C#

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1 Chapter 2 Types & Exceptions Yingcai Xiao

2 Part I Moving from C++/Java to C#

3  What you should do to design a language? How can you design a language?  Computer: a device for data processing storing and processing data  Programming = Data Structures + Algorithms  Computer Languages: tools for users to define data structures to store the data and to develop algorithms to process the data.  Data Types: System-defined Types & User-defined Types Data Structures + Algorithms

4 .NET Framework ’ s Data Types: CTS Categories of data types in CTS: system-defined: Primitives (int, float, …) user-defined: Classes Properties Structs Interfaces Enumerations Events Delegates Generics Templates Common Type System (CTS)

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

6 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!"); } Named Space Example

7 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(); } Named Space Example

8 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

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

10 // 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 Example: How to define a class (user-defined data type)

11 Rectangle rect = new Rectangle(2,4); rect.setHeight(8); rect.setWidth(rect.getWidth() * 2); double darea = (double) (rect.getWidth() * rect.getHeight() ); Example: How to use a class (user-defined data type)

12 DLL  What is it?  How to create one?  How to use one?  What ’ s inside?

13 DLL  Dynamic Linking: Write a Library for Others to Use/Link DLL (Dynamically Linked Libraries) => Shared by multiple applications + Dynamically loaded as needed => Save disk and memory space Understanding by Examples (C2\Weak).  Creating a DLL in.NET as a Multi-file Assembly (Math.dll) The example assembly = two classes (SimpleMath in Visual Basic + ComplexMath in C#) + three files (Simple.netmodule + Complex.netmodule + Math.dll (assembly manifest))

14 Steps for DLL (1) Create source Complex.cs. and compile it: csc /target:module Complex.cs => Complex.netmodule (2) Create source Simple.vb and compile it: vbc /target:module Simple.vb => Simple.netmodule (3) Create the assembly (Math.DLL) using AL (Assembly Linker) al /target:library /out:Math.dll Simple.netmodule Complex.netmodule

15 Linking Assembly  Dynamically Linking of an Assembly to an Application Create the source to use the dll: MathDemo.cs Compile it to create the exe: csc /target:exe /reference:math.dll MathDemo.cs (csc /t:exe /r:math.dll MathDemo.cs)  Installation and Uninstallation of the Application (1) To install: copy all needed files to the installation directory. MathDemo.exe, math.dll, Simple.netmodule, Complex.netmodule (2) To uninstall: remove the installed directory.

16 An Example Assembly: Math.dll Simple.netmoduleComplex.netmodule Math.dll

17 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; Encapsulation and convenience

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

19 // 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"); } Example: How to define properties

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

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

22  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 */ } } Defining Properties  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()).

23  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. Using Properties

24 (1)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. Notes on Properties

25  Signature of a method: name, number of arguments, types of the arguments. Return type is not part of the signature. Why we have to name properties of different types differently?  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).  Property do not have any arguments, so the only way to differentiate properties of different type is by their names.

26 Exception

27 Exception Handling What happens if a run-time error occurs before the PC reaches the return statement in a method? And you don’t have a computed value (say, the square root of the input, or the file handle of a disk file) to return yet. Where do you place PC after returning? How can you tell the caller don’t continue on the normal execution path since there is no returning value? How can you tell the caller what went wrong?

28 Exception Handling Exception Handling (object-oriented event-driven runtime error handling) 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). An exception is thrown when a run-time error occurs. You can thrown an exception any where in the code. You can throw an exception in any language and catch it in any other. You can throw exceptions across machines

29 Exception Handling  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 ( … ) { … } } finally { // Always come here even if there were exceptions. if (file != null) file.Close (); }

30 Exception Handling 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.");

31 Interfaces  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.

32 Interface Example 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 (...); }

33 Abstract Class 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.

34 Abstract Class vs. Interface Classes 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.

35 Abstract Class & Interface Examples 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());} }

36 Abstract Class & Interface Examples 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).

37  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

38 C# Part II Beyond Java

39 Internal Memory Structures of Data Store

40 Instantiating a Class (in C++) Class Name; In C++: “ Rectangle rect ” declares an object of class Rectangle. int width; int height; Rectangle () Rectangle (int w, int h) … rect “ rect ” is the name of a memory space that stores a Rectangle object.

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

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

43 Value Types 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. 8 i

44 Value Types 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.

45 Structs 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.

46 Summary: Value and Reference Types in CTS  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.

47 Class Code 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;

48 Class Code 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

49 Struct Code 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)"

50 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 x a p Arrays in C++

51 Arrays in CTS  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.

52 Boxing and Unboxing  Boxing creates a copy of a value type on the managed heap (converts from value type to reference type)  Unboxing duplicates a reference type on the stack (converts from reference type to value type) int val = 1; // Declare an instance of a value type object Object obj = val; // Box it, inexplicit cast, an object containing value “ 1 ” is created on the heap and pointed to by reference “ obj ”. int val3 = (int) obj; // This will work, explicit cast. int val2 = obj; // This won't compile. Why ?

53 Boxing & Unboxing Typecast: converting data from one type to another type. Widening: copying from a memory space of smaller value range to a memory space of larger value range. Narrowing: copying from a memory space of larger value range to a memory space of smaller value range. Boxing is widening and can be implicitly casted. Unboxing is narrowing and has to be explicitly casted.

54 Typecast References class Parent { int i; setParent(int k) {i=k;) } class Child: Parent{ int j; public setChild(int m, int n) {i=m; j=n;} } Parent p1 = new Parent (); p1.setParent(1); Child c1 = new Child(); c1.setChild(2,3); Parent p2 = (Parent) c1; p2.setParent(4); Child c2 = (Child) p1; c2.setChild(5,6);

55 Typecast References class Parent { int i; Parent(int k) {i=k;) } class Child: Parent{ int j; public set(int m, int n) {i=m; j=n;} } Parent p1 = new Parent (1); Child c1 = new Child(); C1.set(2,3); Unboxing is narrowing and has to be explicitly casted. You can typecast a child reference to parent reference, but never typecast a parent reference to a child reference. When we typecast the reference, we do not alter the underlying object (memory for the data.) An object of a child type is an object of the parent type, but not the other way around.

56 Interface Example 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 (...); }

57 Enumerations Enumerations a set of named constants similar to enum in C++ (not defined in Java until JDK 1.5) implicitly derives from System.Enum enum Color { Red, Green, Blue } Color.Red // Red Color.Green // Green Color.Blue // Blue Color mycolor; mycolor = Color.Green; Console.WriteLine(mycolor); // Green

58 Enumerations Inside Enumerations  Defining a set of named constants.  By default, it starts with 0; can be changed to any number. enum Color { Red = 10, Green, Blue }  By default, it increments by 1 for each subsequent name.  You can sign a value to each name. enum Color { Red = 10, Green = 20, Blue = 21 }  You can increment a enum variable. Color mycolor; mycolor = Color.Green; mycolor += 1; Console.WriteLine(mycolor); // Blue

59 Events A menu in C++: char c; bool done = false; while(!done) { cout << “ Please make your selection, q to end: ” cin >> c; switch(c) { case “ + ” : add( ); break; case “ - ” : sub( ); break; case “ q ” : done = true; break; } Event Loop Event Mapping & Dispatching Event Event Handler

60 Events & Delegates.NET supports the development of GUI (Graphical User Interface) based EDPs (event-driven programs).  Event loop and event mapping for such applications can be very complicate. .NET internally implemented the event loop and event mapping mechanism for such applications.  The development of GUI-based applications in.NET needs only to design the GUI and write event handlers.

61 Events & Delegates How can we register event handlers written by us to the event loop written by Microsoft? The system event loop was implemented before the application event handlers and the names of the handlers are not pre- specified. Event handlers are also called “ callback methods ”. Events are also called messages..NET implement this by using event & delegate constructs.

62 Events & Delegates A delegate is a type-safe wrapper around a callback method. Callback methods are used to respond to events.  Three steps to understand events & delegates using an example 1.Define a callback method to use a predefined Timer class 2.Exam the Timer class which uses event & delegate 3.Analyze the internal code of delegate

63 Usage of a Predefined Timer Class // Call this callback method every 6 seconds. void UpdateData (Object sender, ElapsedEventArgs e) { // Update Data every 6 seconds. } // A Timer class has been implemented for us to use. // It has an event: Elapsed. Timer timer = new Timer (6000); timer.Elapsed += new ElapsedEventHandler (UpdateData); We registered a function pointer, UpdateData, as a callback, created a new instance of ElapsedEventHandler (a delegate) that wraps around (delegates to) UpdateData (a callback method) to respond to the Elapsed event.

64 Defining Events & Delegates Inside the Timer class: public delegate void ElapsedEventHandler (Object sender, ElapsedEventArgs e); // specifies arguments for the callback public class Timer { public event ElapsedEventHandler Elapsed; … } // Calling the callback methods when the event occurs if (Elapsed != null) // Make sure somebody's listening, use Elapsed as a reference. Elapsed (this, new ElapsedEventArgs (...)); // Fire! Use Elapsed as a method, its arguments are sent to the callback methods.

65 Events & Delegates cont. A delegate is defined by placing the key word, delegate, in front of a global method. The arguments of the method in the declaration are those of the callback functions. The “ real ” argument of a delegate is always a pointer (to the callback function). In the declaration of an event object, the type specifier between the “ event ” key word and the object name specifies the delegate to be used by the event to invoke event handlers. An event object can be used as a reference and a method. When an event object is used as a method, its arguments are sent to the callback methods, which are specified by the related delegate.

66 Events & Delegates? What ’ s Inside Events & Delegates? public delegate void ElapsedEventHandler (Object sender, ElapsedEventArgs e); Compiles to: public class ElapsedEventHandler : MulticastDelegate // System.MulticastDelegate { public ElapsedEventHandler (object target, int method) {... } public virtual void Invoke(object sender,ElapsedEventArgs e) {... }... } Use an event ’ s instance name as a method, actually call the Invoke method. It is a better way (a type safe way) to implement function pointers. (http://www.cs.uakron.edu/~xiao/ics/fun-ptrs.html).

67 NDD  Nondeterministic Destruction (NDD) due to garbage collection. No destructors in C# and Java (programmers don ’ t have to free memory). Finalize() method (inherited from Object class) is called during garbage collection. System decides when to perform garbage collection. File file = new File ("Readme.txt"); // file locked // Can ’ t delete “ file ” to release the file. file.Close (); // use close() to release the file in the Finalize().

68 NDD  The Problem: we don ’ t know when file.Close() will not be called.  Solution One: Call Close() whenever you know the file is not needed, instead of in the Finalize().  Solution Two: Force a garbage collection by calling GC.Collect (). Very expansive. Finalize() for all not used objects in all applications will be called.  Solution Three: Implement a Dispose method and call it when needed. (Read)

69 DLL Miss-matching  The problem: Many applications may share the same DLL, but may use different versions of the same DLL. A new application may not run correctly when linked into an older version of the DLL. The problem is solved in.NET. In.NET, one can use CLR ’ s Versioning, Strong Naming (SN), to avoid DLL miss-matching.

70 An Example Assembly: Math.dll Simple.netmoduleComplex.netmodule Math.dll

71 Strong Naming  Replacing GUID (Globally Unique Identifier)  Including the following data  name of the assembly  version of the assembly  culture of the assembly (internationalization/localization)  a public key  a digital signature (= a hash of the assembly + the private key)  Strong Naming info are added into the manifest of the assembly.

72 SN Assembly To Create a Strongly Named Assembly (1) Create a key file for strong naming using the sn command in SDK. sn /k Keyfile.snk (2) Create the modules csc /target:module /out:Complex.netmodule Complex.cs vbc /target:module /out:Simple.netmodule Simple.vb (3) Use AL to create a strongly named assembly (version ) that uses the keys found in Keyfile.snk (version and key are written into the manifest). al /keyfile:Keyfile.snk /target:library /out:Math.dll /version: simple.netmodule complex.netmodule (typo in the book: space missing between /target:library and /out:Math.dll) (4) Create the application bound to the version Math.dll. csc /target:exe /reference:Math.dll Mathdemo.cs

73 SN Assembly To install the application: Copy all needed files to the installation directory. MathDemo.exe, math.dll, Simple.netmodule, and Complex.netmodule run MathDemo.exe To verify that versioning is working, Create another version of Math.dll: al /keyfile:keyfile.snk /target:library /out:Math.dll /version: simple.netmodule complex.netmodule Copy the new Math.dll to the installation directory and run MathDemo.exe again. Will get System.IO.FileLoadException error.

74 Inside.NET Versioning (1)Versions are dynamically checked at runtime before loading DLLs. (2)Versions are strictly enforced. (3)To “ update ” a DLL you need use binding redirect. (5)Use “ binding redirect ” to link with different versions of DLLs and use “ sn /T math.dll ” to find the hash value of the dlls to be linked to. (all specified in MathDemo.exe.config)

75 Versioning in.NET // Versioning info can be specified in the config file for an application // MathDemo.exe.config

76 GAC  The Global Assembly Cache (GAC) is a repository for sharing (strong named) assemblies on an individual system. It is located at C:/Windows/Assembly. (1)Move math.dll and related files to any directory, say, Shared. (2)MathDemo will not run anymore. (3)Go to Shared: gacutil /i math.dll (4)MathDemo will run now. (5)To un-share (note: no dll extension): gacutil /u math

77 Applying Strong Names Using Attributes in the Source Code (An easier way to produce a strongly named assembly). Add to Complex.cs Using System.Reflection [assembly:AssemblyKeyFile ("Keyfile.snk")] [assembly:AssemblyVersion (" ")] Add to Simple.vb Imports System.Reflection The Version Number = major-version.minor- version.build.revision Coding with Strong Naming

78  Delayed Signing: to install and use a strongly named assembly without the private key. (for development use only, not for release). al /keyfile:public.snk /delaysign /target:library /out:Math.dll /version: simple.netmodule complex.netmodule or [assembly:AssemblyKeyFile ("Public.snk")] [assembly:AssemblyVersion (" ")] [assembly:DelaySign (true)]

79  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

80 The Fundamentals in Chapter Two 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?


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