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State Design Pattern

Introduction:
Sometimes you want to build a class where its behavior changed according to its state. State Design pattern provides a good class structure to solve this issue. State Design pattern also known as Object for state pattern is used to represent the state of an object and the object behavior will changed according to state changes.
Example:

StateDesignPattern

 

static void Main(string[] args)
{
WaterCompound c = new WaterCompound(0);
c.IncreaseTempreture(10);
c.IncreaseTempreture(100);
c.DecreaseTempreture(99);
c.DecreaseTempreture(40);
c.DecreaseTempreture(50);
Console.ReadKey();
}
class WaterCompound : Compound
{
public WaterCompound(int temp)
{
_state = new LiquiedState(this,temp,0,100);
}
}
public class Compound
{
public State _state;
public void IncreaseTempreture(int temp)
{
_state.IncreaseTempreture(temp);
Console.WriteLine(” Balance = {0:D}”, _state.Tempreture);
Console.WriteLine(” Status = {0}\n”,
this._state.GetType().Name);
Console.WriteLine(“—————-“);
}
public void DecreaseTempreture(int temp)
{
_state.DecreasseTempreture(temp);
Console.WriteLine(” Balance = {0:D}”, _state.Tempreture);
Console.WriteLine(” Status = {0}\n”,
this._state.GetType().Name);
Console.WriteLine(“—————-“);
}
}
public abstract class State
{
protected Compound _compound;
public int LowerTempLimit { get; set; }
public int UpperTempLimit { get; set; }
public int Tempreture { get; set; }
public abstract void IncreaseTempreture(int temp);
public abstract void DecreasseTempreture(int temp);
}
class BoilingState:State
{
public BoilingState(Compound c,int temp,int lowerTemp,int Uppertemp)
{
Tempreture = temp;
base._compound = c;
LowerTempLimit = lowerTemp;
UpperTempLimit = Uppertemp;
}
public override void IncreaseTempreture(int temp)
{
Tempreture += temp;
}
public override void DecreasseTempreture(int temp)
{
Tempreture -= temp;
if (Tempreture < LowerTempLimit)
base._compound._state = new LiquiedState(_compound, Tempreture,0,100);
}
}
class FreezingState:State
{
public FreezingState(Compound c,int temp,int lowerTemp,int Uppertemp)
{
Tempreture = temp;
base._compound = c;
LowerTempLimit = lowerTemp;
UpperTempLimit = Uppertemp;
}
public override void IncreaseTempreture(int temp)
{
Tempreture += temp;
if (Tempreture > UpperTempLimit)
_compound._state = new LiquiedState(_compound, Tempreture,0,100);
}
public override void DecreasseTempreture(int temp)
{
Tempreture -= temp;
}
}
class LiquiedState:State
{
public LiquiedState(Compound c,int temp,int lowerTemp,int Uppertemp)
{
Tempreture = temp;
base._compound = c;
LowerTempLimit = lowerTemp;
UpperTempLimit = Uppertemp;
}
public override void IncreaseTempreture(int temp)
{
Tempreture += temp;
if (Tempreture > UpperTempLimit)
_compound._state = new BoilingState(_compound, Tempreture,101,100000);
}
public override void DecreasseTempreture(int temp)
{
Tempreture -= temp;
if (Tempreture < LowerTempLimit)
_compound._stae = new FreezingState(_compound, Tempreture,-150,-1);
}
}

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Command Design Pattern

Introduction:
Command Design Pattern enables you to encapsulate a request as an object, thereby letting you parameterize clients with different requests, queue or log requests, and support undoable operations.
Example:
The below code demonstrates the Command pattern to be used on a simple calculator. This caluclator doing unlimited number of undo’s & Redo’s .

One thing we might do to extend this Calculator project, is to create an IOperator Interface and for each operation have a concret Operator Class. for simplicity I am not going to do this. I will hard code couple of operator to make it very simple to be understood.

 

CommandDesignPattern

 

static void Main()
{
// Create user and let her compute
User user = new User();
// User presses calculator buttons
user.Calculate(‘+’, 100);
user.Calculate(‘-‘, 50);
user.Calculate(‘*’, 10);
user.Calculate(‘/’, 2);
// Undo 4 commands
user.UndoCalculation(4);
// Redo 3 commands
user.RedoCalculation(3);
// Wait for user
Console.ReadKey();
}
interface ICommand
{
void Execute();
void UnExecute();
}
class CalculationCommand : ICommand
{
private char _operator;
private int _operand;
private Calculator _calculator;
// Constructor
public CalculationCommand(Calculator calculator,
char @operator, int operand)
{
this._calculator = calculator;
this._operator = @operator;
this._operand = operand;
}
// Gets operator
public char Operator
{
set { _operator = value; }
}
// Get operand
public int Operand
{
set { _operand = value; }
}
// Execute new command
public  void Execute()
{
_calculator.Operation(_operator, _operand);
}
// Unexecute last command
public  void UnExecute()
{
_calculator.Operation(Undo(_operator), _operand);
}
// Returns opposite operator for given operator
private char Undo(char @operator)
{
switch (@operator)
{
case ‘+’: return ‘-‘;
case ‘-‘: return ‘+’;
case ‘*’: return ‘/’;
case ‘/’: return ‘*’;
default: throw new
ArgumentException(“@operator”);
}
}
}
class Calculator
{
private int _currentIndex = 0;
public void Operation(char @operator, int operand)
{
switch (@operator)
{
case ‘+’: _currentIndex += operand; break;
case ‘-‘: _currentIndex -= operand; break;
case ‘*’: _currentIndex *= operand; break;
case ‘/’: _currentIndex /= operand; break;
}
Console.WriteLine(
“Current value = {0,3} (following {1} {2})”,
_currentIndex, @operator, operand);
}
}
class User
{
// Initializers
private Calculator _calculator = new Calculator();
private List<ICommand> _commands = new List<ICommand>();
private int _currentIndex = 0;
public void RedoCalculation(int levels)
{
Console.WriteLine(“\n—- Redo {0} levels “, levels);
// Perform redo operations
for (int i = 0; i < levels; i++)
{
if (_currentIndex < _commands.Count – 1)
{
ICommand command = _commands[_currentIndex++];
command.Execute();
}
}
}
public void UndoCalculation(int levels)
{
Console.WriteLine(“\n—- Undo {0} levels “, levels);
// Perform undo operations
for (int i = 0; i < levels; i++)
{
if (_currentIndex > 0)
{
ICommand command = _commands[–_currentIndex] as ICommand;
command.UnExecute();
}
}
}
public void Calculate(char @operator, int operand)
{
// Create command operation and execute it
ICommand command = new CalculationCommand(
_calculator, @operator, operand);
command.Execute();
// Add command to undo list
_commands.Add(command);
_currentIndex++;
}
}

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Iterator Design Pattern

Introduction:
Sometimes you want to access a collection of objects in a sequential way without even expose how things is going inside your implementation . Iterator Design Pattern Provides you with a Skelton of how to design your classes to solve this issue.
Example:

IteratorDesignPattern

static void Main(string[] args)
{
ListAggregate a = new ListAggregate();
a[0] = “Item A”;
a[1] = “Item B”;
a[2] = “Item C”;
a[3] = “Item D”;
// Create Iterator and provide aggregate
ListIterator i = new ListIterator(a);
Console.WriteLine(“Iterating over collection:”);
string item = i.FirstItem();
while (item != null)
{
Console.WriteLine(item);
item = i.NextItem();
}
// Wait for user
Console.ReadKey();
}
class ListIterator : Iterator
{
private ListAggregate _aggregate;
private int _current = 0;
// Constructor
public ListIterator(ListAggregate aggregate)
{
this._aggregate = aggregate;
}
// Gets first iteration item
public override string FirstItem()
{
return (string)_aggregate[0];
}
// Gets next iteration item
public override string NextItem()
{
string ret = null;
if (_current < _aggregate.Count – 1)
{
ret = (string)_aggregate[++_current];
}
return ret;
}
// Gets current iteration item
public override string CurrentItem()
{
return (string)_aggregate[_current];
}
// Gets whether iterations are complete
public override bool HasMoreItems()
{
return _current >= _aggregate.Count;
}
}
class ListAggregate : Aggregate
{
private ArrayList _items = new ArrayList();
public override Iterator CreateIterator()
{
return new ListIterator(this);
}
// Gets item count
public int Count
{
get { return _items.Count; }
}
// Indexer
public string this[int index]
{
get { return (string)_items[index]; }
set { _items.Insert(index, value); }
}
}
abstract class Iterator
{
public abstract string FirstItem();
public abstract string NextItem();
public abstract bool HasMoreItems();
public abstract string CurrentItem();
}
abstract class Aggregate
{
public abstract Iterator CreateIterator();
}

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Mediator Design Pattern

Introduction:
Sometimes you want to design your classes set that interact with each other in a loosely coupled manner by keeping your classes away from referring each other directly… Mediator Design Pattern solve this issue by promoting the idea of loosely coupling classes

Example:

MediatorDesignPattern

 

static void Main(string[] args)
{
CountryList countrylist = new CountryList();

CityList city = new CityList();
PhoneNumberTextBox phone = new PhoneNumberTextBox();

countrylist.RegisterMonitorChangers(city);
countrylist.RegisterMonitorChangers(phone);

Console.WriteLine(“——————-“);
countrylist.SelectCountry(“USA”);
Console.WriteLine(“——————-“);
countrylist.SelectCountry(“Jordan”);

Console.ReadKey();

}

abstract class CountryListChangesMonitorBase
{
public CountryList TriggerList { get; set; }

public abstract void RecieveChange(string countryName);
}

class CountryList
{
List<string> countries = new List<string>();
string selectedCountry = “”;

List<CountryListChangesMonitorBase> participants = new List<CountryListChangesMonitorBase>();

public CountryList()
{
countries.Add(“USA”);
countries.Add(“UK”);
countries.Add(“Jordan”);
countries.Add(“Jaban”);
countries.Add(“Germani”);

}

public void SelectCountry(string countryName)
{
if (countries.Contains(countryName))
selectedCountry = countryName;

foreach (CountryListChangesMonitorBase p in participants)
p.RecieveChange(countryName);
}

public void RegisterMonitorChangers( CountryListChangesMonitorBase monitor)
{
if (!participants.Contains(monitor))
participants.Add(monitor);

monitor.TriggerList = this;
}

}

class CityList:CountryListChangesMonitorBase
{
public override void RecieveChange(string countryName)
{
Console.WriteLine(” Display available City List for : ” + countryName);
}
}

class PhoneNumberTextBox: CountryListChangesMonitorBase
{
public override void RecieveChange(string countryName)
{
Console.WriteLine(” Display phone number code for : ” + countryName);
}
}

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Interpreter Design Pattern

Introduction:
Sometimes you want to define a grammar for special language for certain reasons. After defining the syntax of this language, you need to build an interpreter to interpret the language and do actions accordingly. Interpreter Design Pattern help you to define a well structure object oriented classed to parse your expressions against the new language you defined
Example:

InterpreterDesignPattern

 

static void Main(string[] args)
{
string hxNumber = Console.ReadLine(); // Hexadecimal number
Context context = new Context(hxNumber);
HexadecimalNumber hex = new HexadecimalNumber();
hex.Interpret(context);
Console.WriteLine(“{0} HexaDecimal Numbering System = {1} Decimal Numbering System” ,hxNumber, context.Output);
// Wait for user
Console.ReadKey();
}
class HexadecimalNumber :Expression
{
public override string Symbol10()
{
return “A”;
}
public override string Symbol11()
{
return “B”;
}
public override string Symbol12()
{
return “C”;
}
public override string Symbol13()
{
return “D”;
}
public override string Symbol14()
{
return “E”;
}
public override string Symbol15()
{
return “F”;
}
public override bool IsSymbolNumberBetween0To9(string num)
{
int tmpNum;
return int.TryParse(num,out tmpNum);
}
public override int Multiplier()
{
return 16;
}
}
abstract class Expression
{
public void Interpret(Context context)
{
if (context.Input.Length == 0)
return;
int value = 0;
if (context.Input.StartsWith(Symbol10()))
{
value = 10;
}
else if (context.Input.StartsWith(Symbol11()))
{
value = 11;
}
else if (context.Input.StartsWith(Symbol12()))
{
value = 12;
}
else if (context.Input.StartsWith(Symbol13()))
{
value = 13;
}
else if (context.Input.StartsWith(Symbol14()))
{
value = 14;
}
else if (context.Input.StartsWith(Symbol15()))
{
value = 15;
}
else if (IsSymbolNumberBetween0To9(context.Input.Substring(0, 1)))
{
value = int.Parse(context.Input.Substring(0, 1));
}
else
{
Console.WriteLine(“Invalid Hexadecimal Number!”);
context.Output = 0;
return;
}
context.Input = context.Input.Substring(1);
context.Output += (int)(value * Math.Pow(Multiplier() , context.Input.Length ));
Interpret(context);
}
public abstract string Symbol10();
public abstract string Symbol11();
public abstract string Symbol12();
public abstract string Symbol13();
public abstract string Symbol14();
public abstract string Symbol15();
public abstract bool IsSymbolNumberBetween0To9(string num);
public abstract int Multiplier();
}
class Context
{
private string inputValue;
private int outputValue;
// Constructor
public Context(string input)
{
this.inputValue = input;
}
// Gets or sets input
public string Input
{
get { return inputValue; }
set { inputValue = value; }
}
// Gets or sets output
public int Output
{
get { return outputValue; }
set { outputValue = value; }
}
}

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Proxy Design Pattern

Introduction:
Sometimes you become into a case where you does not or can not reference an object directly, but you still want to interact with that object to do some job. The intent of the proxy design pattern is to control access to an object by providing placeholder for it. Below example will make the idea clear to you.
Example:

ProxyDesignPattern

 

static void Main()
{
// Create proxy Class
MathProxy proxy = new MathProxy();
// Call Methods
Console.WriteLine(“4 + 2 = ” + proxy.AddOperation(4, 2));
Console.WriteLine(“4 – 2 = ” + proxy.SubOperation(4, 2));
Console.WriteLine(“4 * 2 = ” + proxy.MultiplyOperation(4, 2));
Console.WriteLine(“4 / 2 = ” + proxy.DivsionOperation(4, 2));
// Exit
Console.ReadKey();
}
public interface IMath
{
double AddOperation(double a, double b);
double SubOperation(double a, double b);
double MultiplyOperation(double a, double b);
double DivsionOperation(double a, double b);
}
class Math : IMath
{
public double AddOperation(double a, double b)
{
return a + b;
}
public double SubOperation(double a, double b)
{
return a – b;
}
public double MultiplyOperation(double a, double b)
{
return a * b;
}
public double DivsionOperation(double a, double b)
{
if (b == 0)
return 0;
return a / b;
}
}
class MathProxy : IMath
{
private Math mathObject = new Math();
public double AddOperation(double a, double b)
{
return mathObject.AddOperation(a, b);
}
public double SubOperation(double a, double b)
{
return mathObject.SubOperation(a, b);
}
public double MultiplyOperation(double a, double b)
{
return mathObject.MultiplyOperation(a, b);
}
public double DivsionOperation(double a, double b)
{
return mathObject.DivsionOperation(a, b);
}
}

 

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Flyweight Design Pattern

Introduction:
Flyweight design pattern target to minimizes the memory usage by sharing as much data as possible with other similar objects. It is very useful when have large amount of objects in memory which have lot of similar values. by sharing the similar values between all the objects, memory usage will be much less.
Example:
Suppose you want to create an application to manage Employees and form UI perspective you want to display Employees list as Icons and let user control how that icon looks like. But for all employees icons, you will have only one icon style.
All employees sharing the same icon’s attributes, thus there is no need to include the icon’s attributes inside the Employee class. By doing so, you will decrease memory utilization and accordingly make your application more efficient.
Below is a diagram that showing how we will use Flyweight design pattern

FlyweightDesignPattern

 

enum Shape { Square, Circle, Regtangle };
class IconAttributes
{
public IconAttributes()
{
ObjectCreationCounter++;
}
public static int ObjectCreationCounter{ get; set; }
public Color IconColor { get; set; }
public Size  IconSize { get; set; }
public Shape IconShape { get; set; }
public Image IconImage { get; set; }
public int TransparancyPercentage { get; set; }
}
class Employee
{
public IconAttributes IconAttributes { get; set; }
public string EmployeeFirstName { get; set; }
public string EmployeeLastName { get; set; }
public DateTime EmployeeDob { get; set; }
public override string ToString()
{
return “FirstName:  ” + EmployeeFirstName +
”          LastName:  ” + EmployeeLastName +Environment.NewLine +
”          Dob:       ” + EmployeeDob.ToString () + Environment.NewLine +
“Icone Coloe” + IconAttributes.IconColor.ToString() +
”          Icon Shape:  ” + IconAttributes.IconShape.ToString()+ Environment.NewLine +
“=====================================” + Environment.NewLine;
}
}
static void Main(string[] args)
{
IconAttributes iconAttribue = new IconAttributes();
iconAttribue.IconColor = Color.Red;
iconAttribue.IconShape = Shape.Regtangle;
iconAttribue.IconSize = new Size(10, 10);
iconAttribue.TransparancyPercentage = 100;
Employee e;
List<Employee> employees = new List<Employee>();
for (int i = 0; i < 5; i++)
{
e = new Employee();
e.EmployeeFirstName = “FirstName ” + i.ToString();
e.EmployeeLastName = “LastName ” + i.ToString();
e.EmployeeDob = new DateTime(1982, 3, 20);
e.IconAttributes = iconAttribue;
employees.Add(e);
}
foreach (Employee emp in employees)
{
Console.WriteLine(emp.ToString());
}
Console.WriteLine(“++++++++++++++++++++++++++++++++++++++++”);
Console.WriteLine(“ObjectCreationCounter for IconAttributes   : ” + IconAttributes.ObjectCreationCounter.ToString());
Console.ReadKey();
}

 

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Façade (Facade) Design Pattern

Introduction:
Façade is one of the most used design patterns on software industry. Basically it is about providing a simple interface for large body of code, like class library and it will help you on :

  1. Make large body of code easier to be understand.
  2. Make client code more readable.
  3. Reduce dependency of client code on class library, thus you will be able to change the internal implementation of your component without affecting client code.
  4. Wrap a bad-designed APIs with a single well-designed ASPI

A Façade design pattern is used when you want to implement a new way to use component which is easier and more convenient. While Adapter design pattern is used when an interface must match a specific contract and it should support polymorphic behavior.
Example:

FacadeDesignPattern

 

class Program
{
static void Main(string[] args)
{
Facade fcadeInstance = new Facade();
Console.WriteLine(“Calling DoSometihng1() Method”);
fcadeInstance.DoSometihng1();
Console.WriteLine(“——————-“);
Console.WriteLine(“Calling DoSometihng2() Method”);
fcadeInstance.DoSometihng2();
Console.ReadKey();
}
class Facade
{
private System1 sys1 = new System1();
private System2 sys2 = new System2();
public void DoSometihng1()
{
Console.WriteLine(sys1.System1Mehtod1());
Console.WriteLine(sys2.System2Mehtod1());
}
public void DoSometihng2()
{
Console.WriteLine(sys1.System1Mehtod2());
Console.WriteLine(sys2.System2Mehtod2());
}
}
class System1
{
public string System1Mehtod1()
{
return “System   1  Mehtod   1 is called”;
}
public string System1Mehtod2()
{
return “System   1  Mehtod   2 is called”;
}
}
class System2
{
public string System2Mehtod1()
{
return “System   2  Mehtod   1 is called”;
}
public string System2Mehtod2()
{
return “System   2  Mehtod   2 is called”;
}
}

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Composite Design Pattern

Introduction:
Sometimes you want to treat a group of objects as a single instance. Composite design pattern is aiming to compose object into a structure of tree to represent part-whole hierarchy structures.
A simple example of this design pattern is directory structure.. Each directory might contains some entries and those entries might be a directory. Which leads us to a tree hierarchy structure.
Example:

CompositeDesignPattern

 

class Program
{
static void Main(string[] args)
{
Directory dir = new Directory(“c:”);
dir.Add(new File(“File 1”));
dir.Add(new File(“File 2”));
Directory dir2 = new Directory(“Program Files”);
dir2.Add(new File(“Program 1”));
dir2.Add(new File(“Program 2”));
dir.Add(dir2);
dir.Display(0,0);
Console.ReadKey();
}
}
abstract class DirectoryEntry
{
protected string name;
// Constructor
public DirectoryEntry(string name)
{
this.name = name;
}
public abstract void Add(DirectoryEntry entry);
public abstract void Remove(DirectoryEntry entry);
public abstract void Display(int depth,int leadingSpaces);
}
class File : DirectoryEntry
{
public File(string name ) :base(name)
{
}
public override void Add(DirectoryEntry entry)
{
Console.WriteLine(“Cannot add to a File”);
}
public override void Remove(DirectoryEntry entry)
{
Console.WriteLine(“Cannot remove from a File”);
}
public override void Display(int depth, int leadingSpaces)
{
Console.WriteLine(new string(‘ ‘, leadingSpaces) +  “|” + new String(‘-‘, depth) + name);
}
}
class Directory : DirectoryEntry
{
private List<DirectoryEntry> _children = new List<DirectoryEntry>();
public Directory(string name) : base(name)
{
}
public override void Add(DirectoryEntry entry)
{
_children.Add(entry);
}
public override void Remove(DirectoryEntry entry)
{
_children.Remove(entry);
}
public override void Display(int depth, int leadingSpaces)
{
Console.WriteLine( new string(‘ ‘ , leadingSpaces) + “|” + new string(‘-‘ ,  depth)  + name);
// Recursively display child nodes
foreach (DirectoryEntry entry in _children)
{
entry.Display(depth + 2, leadingSpaces+2);
}
}
}

 

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Bridge Design Pattern

Introduction:
Sometimes you want to decouple the abstraction from its implementation so both of them can be vary independently. Bridge design pattern helps you in implementing this decoupling easily. The bridge uses encapsulation, aggregation, and can use inheritance to separate responsibilities into different classes
Example:

BridgeDesignPattern

 

 

static void Main()
{
// Create Record Set Manager
RecordSet customers = new RecordSet(“Jordan”);
// Set ConcreteImplementor which is in this case is Customer Data.
customers.Data = new CustomersData();
// Exercise the bridge
customers.Show();
customers.Next();
customers.Show();
customers.Next();
customers.Show();
customers.Add(“New Customer”);
customers.ShowAll();
// Exit
Console.ReadKey();
}
class RecordSet : RecordSetBase
{
// Constructor
public RecordSet(string CustomerResidancyGroup)
: base(CustomerResidancyGroup)
{
}
public override void ShowAll()
{
Console.WriteLine();
Console.WriteLine(“************************”);
base.ShowAll();
Console.WriteLine(“************************”);
}
}
class RecordSetBase
{
private IDataObject _dataObject;
protected string _customerResidancyGroup;
public RecordSetBase(string customerResidancyGroup)
{
this._customerResidancyGroup = customerResidancyGroup;
}
// Property
public IDataObject Data
{
set { _dataObject = value; }
get { return _dataObject; }
}
public virtual void Next()
{
_dataObject.NextRecord();
}
public virtual void Prior()
{
_dataObject.PreviousRecord();
}
public virtual void Add(string customer)
{
_dataObject.AddRecord(customer);
}
public virtual void Delete(string customer)
{
_dataObject.DeleteRecord(customer);
}
public virtual void Show()
{
_dataObject.DisplayRecord();
}
public virtual void ShowAll()
{
Console.WriteLine(“Customer Group: ” + _customerResidancyGroup);
_dataObject.DisplayAllRecords();
}
}
class CustomersData : IDataObject
{
private List<string> _customers = new List<string>();
private int _current = 0;
public CustomersData()
{
// Loaded from a database
_customers.Add(“Customer 1”);
_customers.Add(“Customer 2”);
_customers.Add(“Customer 3”);
_customers.Add(“Customer 4”);
_customers.Add(“Customer 5″);
}
public void NextRecord()
{
if (_current <= _customers.Count – 1)
{
_current++;
}
}
public void PreviousRecord()
{
if (_current > 0)
{
_current–;
}
}
public void AddRecord(string customer)
{
_customers.Add(customer);
}
public void DeleteRecord(string customer)
{
_customers.Remove(customer);
}
public void DisplayRecord()
{
Console.WriteLine(_customers[_current]);
}
public void DisplayAllRecords()
{
foreach (string customer in _customers)
{
Console.WriteLine(” ” + customer);
}
}
}
interface IDataObject
{
void NextRecord();
void PreviousRecord();
void AddRecord(string name);
void DeleteRecord(string name);
void DisplayRecord();
void DisplayAllRecords();
}

 

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