Advanced c#

Advanced C # Extracted from (with very minor modifications) Mark Sapossnek CS 594 Computer Science Department Metropolitan College Boston University Find the original set on WWW.GotDotNet.com Jim Fawcett CSE 681 – Software Modeling & Analysis

Learning Objectives Advanced features of the C# language Creating custom types with interfaces, classes and structs Delegates and events Miscellaneous topics

Agenda Review Object-Oriented Concepts Interfaces Classes and Structs Delegates Events Attributes Preprocessor Directives XML Comments Unsafe Code

Objects, instances and classes Identity Every instance has a unique identity, regardless of its data Encapsulation Data and function are packaged together Information hiding An object is an abstraction User should NOT know implementation details Review Key Object-Oriented Concepts

Review Key Object-Oriented Concepts Interfaces A well-defined contract A set of function members Types An object has a type, which specifies its interfaces and their implementations A variable also can have a type Inheritance Types are arranged in a hierarchy Base/derived, superclass/subclass Interface vs. implementation inheritance

Review Key Object-Oriented Concepts Polymorphism The ability to use an object without knowing its precise type Three main kinds of polymorphism Inheritance Interfaces Late binding Dependencies For reuse and to facilitate development, systems should be loosely coupled Dependencies should be minimized

Interfaces An interface defines a contract An interface is a type Includes methods, properties, indexers, events Any class or struct implementing an interface must support all parts of the contract Interfaces provide no implementation When a class or struct implements an interface it must provide the implementation Interfaces provide polymorphism Many classes and structs may implement a particular interface

Interfaces Example public interface IDelete { void Delete(); } public class TextBox : IDelete { public void Delete() { ... } } public class Car : IDelete { public void Delete() { ... } } TextBox tb = new TextBox(); IDelete iDel = tb; iDel.Delete(); Car c = new Car(); iDel = c; iDel.Delete();

Interfaces Multiple Inheritance Classes and structs can inherit from multiple interfaces Interfaces can inherit from multiple interfaces interface IControl { void Paint(); } interface IListBox: IControl { void SetItems(string[] items); } interface IComboBox: ITextBox, IListBox { }

Interfaces Explicit Interface Members If two interfaces have the same method name, you can explicitly specify interface + method name to disambiguate their implementations interface IControl { void Delete(); } interface IListBox: IControl { void Delete(); } interface IComboBox: ITextBox, IListBox { void IControl.Delete(); void IListBox.Delete(); }

Classes and Structs Similarities Both are user-defined types Both can implement multiple interfaces Both can contain Data Fields, constants, events, arrays Functions Methods, properties, indexers, operators, constructors Type definitions Classes, structs, enums, interfaces, delegates

Classes and Structs Differences No user-defined parameterless constructor Can have user-defined parameterless constructor No destructor Can have a destructor No inheritance (inherits only from System.ValueType ) Can inherit from any non-sealed reference type Value type Reference type Struct Class

Classes and Structs C# Structs vs. C++ Structs Very different from C++ struct Members can be public , internal or private Members are always public Always allocated on the stack or as a member Can be allocated on the heap, on the stack or as a member (can be used as value or reference) User-defined value type Same as C++ class, but all members are public C# Struct C++ Struct

Classes and Structs Class public class Car : Vehicle { public enum Make { GM, Honda, BMW } Make make; string vid; Point location; Car(Make m, string vid; Point loc) { this.make = m; this.vid = vid; this.location = loc; } public void Drive() { Console.WriteLine(“vroom”); } } Car c = new Car(Car.Make.BMW, “ JF3559QT98”, new Point(3,7)); c.Drive();

Classes and Structs Struct public struct Point { int x, y; public Point(int x, int y) { this.x = x; this.y = y; } public int X { get { return x; } set { x = value; } } public int Y { get { return y; } set { y = value; } } } Point p = new Point(2,5); p.X += 100; int px = p.X; // px = 102

Classes and Structs Static vs. Instance Members By default, members are per instance Each instance gets its own fields Methods apply to a specific instance Static members are per type Static methods can’t access instance data No this variable in static methods Don’t abuse static members They are essentially object-oriented global data and global functions

Classes and Structs Access Modifiers Access modifiers specify who can use a type or a member Access modifiers control encapsulation Top-level types (those directly in a namespace) can be public or internal Class members can be public , private , protected , internal , or protected internal Struct members can be public , private or internal

Classes and Structs Access Modifiers to T or types derived from T protected within T only (the default) private to T or types derived from T or to types within A protected internal to types within A internal to everyone public Then a member defined in type T and assembly A is accessible If the access modifier is

Classes and Structs Abstract Classes An abstract class is one that cannot be instantiated Intended to be used as a base class May contain abstract and non-abstract function members Similar to an interface Cannot be sealed

Classes and Structs Sealed Classes A sealed class is one that cannot be used as a base class Sealed classes can’t be abstract All structs are implicitly sealed Why seal a class? To prevent unintended derivation Code optimization Virtual function calls can be resolved at compile-time

Classes and Structs this The this keyword is a predefined variable available in non-static function members Used to access data and function members unambiguously class Person { string name; public Person(string name) { this.name = name; } public void Introduce(Person p) { if (p != this) Console.WriteLine(“Hi, I’m “ + name); } }

Classes and Structs base The base keyword is used to access class members that are hidden by similarly named members of the current class class Shape { int x, y; public override string ToString() { return "x=" + x + ",y=" + y; } } class Circle : Shape { int r; public override string ToString() { return base.ToString() + ",r=" + r; } }

Classes and Structs Constants A constant is a data member that is evaluated at compile-time and is implicitly static (per type) e.g. Math.PI public class MyClass { public const string version = “1.0.0”; public const string s1 = “abc” + “def”; public const int i3 = 1 + 2; public const double PI_I3 = i3 * Math.PI; public const double s = Math.Sin(Math.PI); //ERROR ... }

Classes and Structs Fields A field is a member variable Holds data for a class or struct Can hold: a class instance (a reference), a struct instance (actual data), or an array of class or struct instances (an array is actually a reference)

Classes and Structs Readonly Fields Similar to a const, but is initialized at run-time in its declaration or in a constructor Once initialized, it cannot be modified Differs from a constant Initialized at run-time (vs. compile-time) Don’t have to re-compile clients Can be static or per-instance public class MyClass { public static readonly double d1 = Math.Sin(Math.PI); public readonly string s1; public MyClass(string s) { s1 = s; } }

Classes and Structs Properties A property is a virtual field Looks like a field, but is implemented with code public class Button: Control { private string caption; public string Caption { get { return caption; } set { caption = value; Repaint(); } } } Button b = new Button(); b.Caption = "OK"; String s = b.Caption; Can be read-only, write-only, or read/write

Classes and Structs Indexers An indexer lets an instance behave as a virtual array Can be overloaded (e.g. index by int and by string ) public class ListBox: Control { private string[] items; public string this[int index] { get { return items[index]; } set { items[index] = value; Repaint(); } } } ListBox listBox = new ListBox(); listBox[0] = "hello"; Console.WriteLine(listBox[0]); Can be read-only, write-only, or read/write

Classes and Structs Methods All code executes in a method Constructors, destructors and operators are special types of methods Properties and indexers are implemented with get/set methods Methods have argument lists Methods contain statements Methods can return a value Only if return type is not void

Classes and Structs Method Argument Passing By default, data is passed by value A copy of the data is created and passed to the method For value types, variables cannot be modified by a method call For reference types, the instance can be modified by a method call, but the variable itself cannot be modified by a method call

Classes and Structs Method Argument Passing The ref modifier causes arguments to be passed by reference Allows a method call to modify a variable Have to use ref modifier in method definition and the code that calls it Variable has to have a value before call void RefFunction(ref int p) { p++; } int x = 10; RefFunction(ref x); // x is now 11

Classes and Structs Method Argument Passing The out modifier causes arguments to be passed out by reference Allows a method call to initialize a variable Have to use out modifier in method definition and the code that calls it Argument has to have a value before returning void OutFunction(out int p) { p = 22; } int x; OutFunction(out x); // x is now 22

Classes and Structs Overloaded Methods A type may overload methods, i.e. provide multiple methods with the same name Each must have a unique signature Signature is based upon arguments only, the return value is ignored void Print(int i); void Print(string s); void Print(char c); void Print(float f); int Print(float f); // Error: duplicate signature

Classes and Structs Parameter Arrays Methods can have a variable number of arguments, called a parameter array params keyword declares parameter array Must be last argument int Sum(params int[] intArr) { int sum = 0; foreach (int i in intArr) sum += i; return sum; } int sum = Sum(13,87,34);

Classes and Structs Virtual Methods Methods may be virtual or non-virtual (default) Non-virtual methods are not polymorphic They cannot be overridden Non-virtual methods cannot be abstract class Foo { public void DoSomething(int i) { ... } } Foo f = new Foo(); f.DoSomething();

Classes and Structs Virtual Methods Defined in a base class Can be overridden in derived classes Derived classes provide their own specialized implementation May contain a default implementation Use abstract method if no default implementation A form of polymorphism Properties, indexers and events can also be virtual

Classes and Structs Virtual Methods class Shape { public virtual void Draw() { ... } } class Box : Shape { public override void Draw() { ... } } class Sphere : Shape { public override void Draw() { ... } } void HandleShape(Shape s) { s.Draw(); ... } HandleShape(new Box()); HandleShape(new Sphere()); HandleShape(new Shape());

Classes and Structs Abstract Methods An abstract method is virtual and has no implementation Must belong to an abstract class Intended to be implemented in a derived class

Classes and Structs Abstract Methods abstract class Shape { public abstract void Draw(); } class Box : Shape { public override void Draw() { ... } } class Sphere : Shape { public override void Draw() { ... } } void HandleShape(Shape s) { s.Draw(); ... } HandleShape(new Box()); HandleShape(new Sphere()); HandleShape(new Shape()); // Error!

Classes and Structs Method Versioning Must explicitly use override or new keywords to specify versioning intent Avoids accidental overriding Methods are non-virtual by default C++ and Java product fragile base classes – cannot specify versioning intent

Classes and Structs Method Versioning class Derived: Base { // version 1 public virtual void Foo() { Console.WriteLine("Derived.Foo"); } } class Base { // version 1 } class Base { // version 2 public virtual void Foo() { Console.WriteLine("Base.Foo"); } } class Derived: Base { // version 2a new public virtual void Foo() { Console.WriteLine("Derived.Foo"); } } class Derived: Base { // version 2b public override void Foo() { base.Foo(); Console.WriteLine("Derived.Foo"); } }

Classes and Structs Constructors Instance constructors are special methods that are called when a class or struct is instantiated Performs custom initialization Can be overloaded If a class doesn’t define any constructors, an implicit parameterless constructor is created Cannot create a parameterless constructor for a struct All fields initialized to zero/null

Classes and Structs Constructor Initializers One constructor can call another with a constructor initializer Can call this(...) or base(...) Default constructor initializer is base() class B { private int h; public B() { } public B(int h) { this.h = h; } } class D : B { private int i; public D() : this(24) { } public D(int i) { this.i = i; } public D(int h, int i) : base(h) { this.i = i; } }

Classes and Structs Static Constructors A static constructor lets you create initialization code that is called once for the class Guaranteed to be executed before the first instance of a class or struct is created and before any static member of the class or struct is accessed No other guarantees on execution order Only one static constructor per type Must be parameterless

Classes and Structs Destructors A destructor is a method that is called before an instance is garbage collected Used to clean up any resources held by the instance, do bookkeeping, etc. Only classes, not structs can have destructors class Foo { ~Foo() { Console.WriteLine(“Destroyed {0}”, this); } }

Classes and Structs Destructors Unlike C++, C# destructors are non-deterministic They are not guaranteed to be called at a specific time They are guaranteed to be called before shutdown Use the using statement and the IDisposable interface to achieve deterministic finalization

Classes and Structs Operator Overloading User-defined operators Must be a static method class Car { string vid; public static bool operator ==(Car x, Car y) { return x.vid == y.vid; } }

Classes and Structs Operator Overloading Overloadable unary operators Overloadable binary operators -- ++ false true ~ ! - + >= != ~ <= > < >> << == ^ | & % ! / * - +

Classes and Structs Operator Overloading No overloading for member access, method invocation, assignment operators, nor these operators: sizeof , new , is , as , typeof , checked , unchecked , && , || , and ?: The && and || operators are automatically evaluated from & and | Overloading a binary operator (e.g. * ) implicitly overloads the corresponding assignment operator (e.g. *= )

Classes and Structs Operator Overloading struct Vector { int x, y; public Vector(x, y) { this.x = x; this.y = y; } public static Vector operator +(Vector a, Vector b) { return Vector(a.x + b.x, a.y + b.y); } ... }

Classes and Structs Conversion Operators User-defined explicit and implicit conversions class Note { int value; // Convert to hertz – no loss of precision public static implicit operator double(Note x) { return ...; } // Convert to nearest note public static explicit operator Note(double x) { return ...; } } Note n = (Note)442.578; double d = n;

Classes and Structs Implementing Interfaces Classes and structs can implement multiple interfaces A class or struct that inherits from an interface must implement all function members defined in that interface

Classes and Structs Implementing Interfaces public interface IDelete { void Delete(); } public class TextBox : IDelete { public void Delete() { ... } } public class Car : IDelete { public void Delete() { ... } } TextBox tb = new TextBox(); IDelete iDel = tb; iDel.Delete(); Car c = new Car(); iDel = c; iDel.Delete();

Classes and Structs Implementing Interfaces Explicit interface implementation Handles name collisions public interface IDelete { void Delete(); } public interface IFoo { void Delete(); } public class TextBox : IDelete, IFoo { public void IDelete.Delete() { ... } public void IFoo.Delete() { ... } }

Classes and Structs Nested Types Declared within the scope of another type Nesting a type provides three benefits: Nested type can access all the members of its enclosing type, regardless of access modifer Nested type can be hidden from other types Accessing a nested type from outside the enclosing type requires specifying the type name Nested types can be declared new to hide inherited types Unlike Java inner classes, nested types imply no relationship between instances

Classes and Structs is Operator The is operator is used to dynamically test if the run-time type of an object is compatible with a given type static void DoSomething(object o) { if (o is Car) ((Car)o).Drive(); } Don’t abuse the is operator: it is preferable to design an appropriate type hierarchy with polymorphic methods

Classes and Structs as Operator The as operator tries to convert a variable to a specified type; if no such conversion is possible the result is null static void DoSomething(object o) { Car c = o as Car; if (c != null) c.Drive(); } More efficient than using is operator: test and convert in one operation Same design warning as with the is operator

Classes and Structs typeof Operator The typeof operator returns the System.Type object for a specified type Can then use reflection to dynamically obtain information about the type Console.WriteLine(typeof(int).FullName); Console.WriteLine(typeof(System.Int).Name); Console.WriteLine(typeof(float).Module); Console.WriteLine(typeof(double).IsPublic); Console.WriteLine(typeof(Car).MemberType);

Delegates Overview A delegate is a reference type that defines a method signature A delegate instance holds one or more methods Essentially an “object-oriented function pointer” Methods can be static or non-static Methods can return a value Provides polymorphism for individual functions Foundation for event handling

Delegates Overview delegate double Del(double x); // Declare static void DemoDelegates() { Del delInst = new Del(Math.Sin); // Instantiate double x = delInst(1.0); // Invoke }

Delegates Multicast Delegates A delegate can hold and invoke multiple methods Multicast delegates must contain only methods that return void , else there is a run-time exception Each delegate has an invocation list Methods are invoked sequentially, in the order added The += and -= operators are used to add and remove delegates, respectively += and -= operators are thread-safe

Delegates Multicast Delegates delegate void SomeEvent(int x, int y); static void Foo1(int x, int y) { Console.WriteLine("Foo1"); } static void Foo2(int x, int y) { Console.WriteLine("Foo2"); } public static void Main() { SomeEvent func = new SomeEvent(Foo1); func += new SomeEvent(Foo2); func(1,2); // Foo1 and Foo2 are called func -= new SomeEvent(Foo1); func(2,3); // Only Foo2 is called }

Delegates and Interfaces Could always use interfaces instead of delegates Interfaces are more powerful Multiple methods Inheritance Delegates are more elegant for event handlers Less code Can easily implement multiple event handlers on one class/struct

Events Overview Event handling is a style of programming where one object notifies another that something of interest has occurred A publish-subscribe programming model Events allow you to tie your own code into the functioning of an independently created component Events are a type of “callback” mechanism

Events Overview Events are well suited for user-interfaces The user does something (clicks a button, moves a mouse, changes a value, etc.) and the program reacts in response Many other uses, e.g. Time-based events Asynchronous operation completed Email message has arrived A web session has begun

Events Overview C# has native support for events Based upon delegates An event is essentially a field holding a delegate However, public users of the class can only register delegates They can only call += and -= They can’t invoke the event’s delegate Multicast delegates allow multiple objects to register with the same event

Events Example: Component-Side Define the event signature as a delegate Define the event and firing logic public delegate void EventHandler(object sender, EventArgs e); public class Button { public event EventHandler Click; protected void OnClick(EventArgs e) { // This is called when button is clicked if (Click != null) Click(this, e); } }

Events Example: User-Side Define and register an event handler public class MyForm: Form { Button okButton; static void OkClicked(object sender, EventArgs e) { ShowMessage("You pressed the OK button"); } public MyForm() { okButton = new Button(...); okButton.Caption = "OK"; okButton.Click += new EventHandler(OkClicked); } }

Attributes Overview It’s often necessary to associate information (metadata) with types and members, e.g. Documentation URL for a class Transaction context for a method XML persistence mapping COM ProgID for a class Attributes allow you to decorate a code element (assembly, module, type, member, return value and parameter) with additional information

Attributes Overview [HelpUrl(“http://SomeUrl/APIDocs/SomeClass”)] class SomeClass { [Obsolete(“Use SomeNewMethod instead”)] public void SomeOldMethod() { ... } public string Test([SomeAttr()] string param1) { ... } }

Attributes Overview Attributes are superior to the alternatives Modifying the source language Using external files, e.g., .IDL, .DEF Attributes are extensible Attributes allow to you add information not supported by C# itself Not limited to predefined information Built into the .NET Framework, so they work across all .NET languages Stored in assembly metadata

Attributes Overview Some predefined .NET Framework attributes COM Prog ID ProgId Transactional characteristics of a class Transaction Compiler will complain if target is used Obsolete Allows a class or struct to be serialized Serializable Should a property or event be displayed in the property window Browsable Description Attribute Name

Attributes Overview Attributes can be Attached to types and members Examined at run-time using reflection Completely extensible Simply a class that inherits from System.Attribute Type-safe Arguments checked at compile-time Extensive use in .NET Framework XML, Web Services, security, serialization, component model, COM and P/Invoke interop, code configuration…

Attributes Querying Attributes [HelpUrl("http://SomeUrl/MyClass")] class Class1 {} [HelpUrl("http://SomeUrl/MyClass"), HelpUrl("http://SomeUrl/MyClass”, Tag=“ctor”)] class Class2 {} Type type = typeof(MyClass); foreach (object attr in type.GetCustomAttributes() ) { if ( attr is HelpUrlAttribute ) { HelpUrlAttribute ha = (HelpUrlAttribute) attr; myBrowser.Navigate( ha.Url ); } }

C# provides preprocessor directives that serve a number of functions Unlike C++, there is not a separate preprocessor The “preprocessor” name is preserved only for consistency with C++ C++ preprocessor features removed include: #include : Not really needed with one-stop programming; removal results in faster compilation Macro version of #define : removed for clarity Preprocessor Directives Overview

Preprocessor Directives Overview Delimit outline regions #region , #end Specify line number #line Issue errors and warnings #error , #warning Conditionally skip sections of code #if, #elif , #else , #endif Define and undefine conditional symbols #define , #undef Description Directive

Preprocessor Directives Conditional Compilation #define Debug public class Debug { [Conditional("Debug")] public static void Assert(bool cond, String s) { if (!cond) { throw new AssertionException(s); } } void DoSomething() { ... // If Debug is not defined, the next line is // not even called Assert((x == y), “X should equal Y”); ... } }

Preprocessor Directives Assertions By the way, assertions are an incredible way to improve the quality of your code An assertion is essentially a unit test built right into your code You should have assertions to test preconditions, postconditions and invariants Assertions are only enabled in debug builds Your code is QA’d every time it runs Must read: “ Writing Solid Code ”, by Steve Maguire, Microsoft Press, ISBN 1-55615-551-4

XML Comments Overview Programmers don’t like to document code, so we need a way to make it easy for them to produce quality, up-to-date documentation C# lets you embed XML comments that document types, members, parameters, etc. Denoted with triple slash: /// XML document is generated when code is compiled with /doc argument Comes with predefined XML schema, but you can add your own tags too Some are verified, e.g. parameters, exceptions, types

XML Comments Overview Formatting hints <list>, <item>, ... Use of a parameter <paramref> Cross references <see>, <seealso> Sample code <example>, <c>, <code> Property <value> Exceptions thrown from method <exception> Permission requirements <permission> Method return value <returns> Method parameter <param> Type or member <summary>, <remarks> Description XML Tag

XML Comments Overview class XmlElement { /// <summary> /// Returns the attribute with the given name and /// namespace</summary> /// <param name="name"> /// The name of the attribute</param> /// <param name="ns"> /// The namespace of the attribute, or null if /// the attribute has no namespace</param> /// <return> /// The attribute value, or null if the attribute /// does not exist</return> /// <seealso cref="GetAttr(string)"/> /// public string GetAttr(string name, string ns) { ... } }

Unsafe Code Overview Developers sometime need total control Performance extremes Dealing with existing binary structures Existing code Advanced COM support, DLL import C# allows you to mark code as unsafe, allowing Pointer types, pointer arithmetic -> , * operators Unsafe casts No garbage collection

Unsafe Code Overview Lets you embed native C/C++ code Basically “inline C” Must ensure the GC doesn’t move your data Use fixed statement to pin data Use stackalloc operator so memory is allocated on stack, and need not be pinned unsafe void Foo() { char* buf = stackalloc char[256]; for (char* p = buf; p < buf + 256; p++) *p = 0; ... }

Unsafe Code Overview class FileStream: Stream { int handle; public unsafe int Read(byte[] buffer, int index, int count) { int n = 0; fixed (byte* p = buffer) { ReadFile(handle, p + index, count, &n, null); } return n; } [dllimport("kernel32", SetLastError=true)] static extern unsafe bool ReadFile(int hFile, void* lpBuffer, int nBytesToRead, int* nBytesRead, Overlapped* lpOverlapped); }

Unsafe Code C# and Pointers Power comes at a price! Unsafe means unverifiable code Stricter security requirements Before the code can run Downloading code

More Resources http://msdn.microsoft.com http://windows.oreilly.com/news/hejlsberg_0800.html http://www.csharphelp.com/ http://www.csharp-station.com/ http://www.csharpindex.com/ http://msdn.microsoft.com/msdnmag/issues/0900/csharp/csharp.asp http://www.hitmill.com/programming/dotNET/csharp.html http://www.c-sharpcorner.com/ http://msdn.microsoft.com/library/default.asp?URL=/library/dotnet/csspec/vclrfcsharpspec_Start.htm

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