Description

Singleton pattern restricts the instantiation of a class to one object. It is used when you want to have one instance of a class that is shared across the application.

Example in .NET :

Singleton

 
using System.Data.OleDb;

namespace Singleton;
/// <summary>
/// 
///sealed class Singleton
///{
///    private Singleton() { }
///    public static readonly Singleton Instance = new Singleton();
///}
///
/// </summary>
internal class SingletonDemo
{
    public static void GetFactory()
    {
        //cannot do new
        //OleDbFactory factory=new OleDbFactory();
        //get singleton instance
        OleDbFactory factory = OleDbFactory.Instance;
    }
}

Learn More

Wikipedia

Homework

Implement a singleton that will allow you to create a single instance of a logger that logs to a file and to a console.

Description

Strategy pattern allows a client to choose from a family of algorithms at runtime. It is used when the client expects to have multiple algorithms and wants to choose one of them at runtime.

Example in .NET :

Strategy

using System;
using System.Collections.Generic;

namespace Strategy;
internal class StrategyDemo
{
    public static void SortWithDifferentStrategies()
    {
        List<int> al = new ();
        al.Add(102);
        al.Add(201);
        //sort ascending
        al.Sort((x, y) =&gt; x.CompareTo(y));

        for (int i = 0; i &lt; al.Count; i++)
        {
            Console.WriteLine(al[i]);
        }

        Console.WriteLine("---------------");

        //sort descending
        al.Sort((y, x) =&gt; x.CompareTo(y));
        for (int i = 0; i &lt; al.Count; i++)
        {
            Console.WriteLine(al[i]);
        }
        Console.WriteLine("---------------");
        //sort custom
        al.Sort((x, y) =&gt; LastDigit(x).CompareTo(LastDigit(y)));
        for (int i = 0; i &lt; al.Count; i++)
        {
            Console.WriteLine(al[i]);
        }

        var array = al.FindAll(it =&gt; it &gt; 10);


    }

    static int LastDigit(int x)
    {
        return x % 10;
    }

}

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Wikipedia

Homework

Image you want to serialize classes to XML,JSON and CSV . Implement a strategy that will allow you to choose between XML , JSON and CSV serialization at runtime.

Description

Visitor pattern is a way of separating an algorithm from an object structure on which it operates. A practical result of this separation is the ability to add new operations to existing object structures without modifying the structures.

Example in .NET :

Visitor

using Microsoft.CodeAnalysis;
using Microsoft.CodeAnalysis.CSharp;
using Microsoft.CodeAnalysis.CSharp.Syntax;

namespace Visitor;
internal class VisitorDemo
{
    public static void VisitMethods()
    {
        var Code = @"""
using System;
namespace Test1
{
    class Program
    {
        static void Main(string[] args)
        {
              var dt=DateTime.Now;
			  Console.WriteLine(dt);
        }
     }
}

""";
        var tree = CSharpSyntaxTree.ParseText(Code);

        var node = tree.GetRoot();

        MethodVisiting LG = new MethodVisiting();
        //start visiting
        var sn = LG.Visit(node);

    }
}
public class MethodVisiting : CSharpSyntaxRewriter
{
    public override SyntaxNode? VisitMethodDeclaration(MethodDeclarationSyntax node)
    {
        if (node.Body == null || node.Body.Statements.Count == 0)
            return base.VisitMethodDeclaration(node);

        var parent = node.Parent as ClassDeclarationSyntax;
        
        if (parent == null)
            return base.VisitMethodDeclaration(node);

        var nameMethod = node.Identifier.Text;
        var nameClass = parent.Identifier.Text;
        Console.WriteLine($"visiting {nameMethod} from {nameClass}");

        return base.VisitMethodDeclaration(node);

    }
}

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Wikipedia

Homework

Implement a visitor that will allow you to calculate the total price of a shopping cart. The shopping cart should contain items with a price and a quantity.Visit every item and make the sum

Description

Adapter design pattern allows the interface of an existing class to be used as another interface.It is often used to make existing classes work with others without modifying their source code.

Examples in .NET :

SQLiteDataAdapter

namespace Adaptor;
internal class SQLiteDataAdapterDemo
{
    /// <summary>
    ///Adaptee  - Command 
    ///Target  - DataTable
    ///Adapter  - SqlDataAdapter
    ///Target Method - Fill(Dataset instance)
    /// </summary>
    /// <returns></returns>
    public static async Task MainSqliteAdapterAsync()
    {
        var dataFormats = new DataTable();
        Console.WriteLine(dataFormats.Rows.Count);
        Console.WriteLine(dataFormats.Columns.Count);
        using (var con = new SQLiteConnection())
        {
            con.ConnectionString = "Data Source=CatalogRo.sqlite3";
            await con.OpenAsync();
            using (var cmd = new SQLiteCommand())
            {
                cmd.CommandText = "select * from Format";
                cmd.Connection = con;
                using (var adapt = new SQLiteDataAdapter(cmd))
                {
                    adapt.Fill(dataFormats);
                }
            }

        }
        Console.WriteLine(dataFormats.Rows.Count);
        Console.WriteLine(dataFormats.Columns.Count);
    }
}

EncodingAdapter

namespace Adaptor;
internal class EncodingAdapterDemo
{
    /// <summary>
    ///Adaptee  - string 
    ///Target  - bytes
    ///Adapter  - encoding
    ///Target Method - GetBytes
    /// </summary>
    public static void AdapterStringByte()
    {
        var url = "http://msprogrammer.serviciipeweb.ro";
        Encoding e = new ASCIIEncoding();
        var b = e.GetBytes(url);

        Console.WriteLine($"from {e.EncodingName} number bytes {b.Length}");

        e = new UTF32Encoding();
        b = e.GetBytes(url);
        Console.WriteLine($"from {e.EncodingName} number bytes {b.Length}");

    }


}

Learn More

Wikipedia

Homework

iPhone 7 does not have a headphone jack. Implement an adapter that will allow you to use your old headphones , that have jack, with the iPhone 7.

Description

The intent of the Builder design pattern is to separate the construction of a complex object from its representation

Examples in .NET :

UriBuilder

namespace Builder;
static class UriBuilderDemo
{
    public static void UriMod()
    {
        //start example 1
        var uri = new Uri("https://msprogrammer.serviciipeweb.ro/category/friday-links/");
        var b = new UriBuilder(uri);
        //changing part
        b.Scheme = "http";
        //now we have http://msprogrammer.serviciipeweb.ro/category/friday-links/
        Console.WriteLine(b.Uri);
        //changing part
        b.Path = "2018/03/05/design-patterns-class/";
        //now we have http://msprogrammer.serviciipeweb.ro/2018/03/05/design-patterns-class/
        Console.WriteLine(b.Uri);
        //end example 1
    }
}

SqlConnectionStringBuilder

namespace Builder;
internal class ConnectionStringDemo
{
    public static void ConnectionString()
    {
        //start example 2
        var build = new SqlConnectionStringBuilder();
        build.DataSource = ".";
        build.InitialCatalog = "MyDatabase";
        build.ConnectTimeout = 30;
        //here is the connection string built
        Console.WriteLine(build.ConnectionString);
        //end example 2
    }
}

Learn More

Wikipedia

Homework

Imagine that you have a logger that logs to a file and to a console. Implement a builder that will allow you to create a logger with different configurations. For example, you can set the log level, the log format, and the log destination.

Description

Iterator design pattern allows to traverse a container and access the container’s elements.

Example in .NET :

DirectoryEnumerable

namespace Iterator;
internal class DirectoryEnumerableDemo
{
    public static void DirectoryEnumerableFiles(int nrMaxFiles)
    {
        var count = 0;
        //what if we called Directory.GetFiles        
        foreach (var file in Directory.EnumerateFiles(@"c:\","*.*",SearchOption.AllDirectories))
        {
            count++;
            if (count > nrMaxFiles)
                break;
            Console.WriteLine(file);
        }
    }
}

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Wikipedia

Homework

With the Yield keyword implement a function that return an IEnumerable of generic int that will return the first 10 numbers of the Fibonacci sequence

Description

Instead of returning null , use an object which implements the expected interface, but whose method body is empty.

Examples in .NET :

EmptyFolder

using System;
using System.IO;

namespace NullObject;
internal class EmptyFolderDemo
{
    public static void DemoWithCreateNewFolder()
    {
        //start example 1
        var env = Environment.CurrentDirectory;
        var guid = Guid.NewGuid().ToString("X");
        var fldEmpty = Path.Combine(env, guid);
        //create empty folder
        Directory.CreateDirectory(fldEmpty);
        var files= Directory.GetFiles(fldEmpty);
        Console.WriteLine($"files.Length:{files.Length}");
        //end example 1
        
    }
}

NullLogger

namespace NullObject;
internal class LogWithData
{
    ILogger _logger;
    public LogWithData(ILogger? logger=null)
    {
        _logger = logger ?? NullLogger.Instance;   
    }
    public void DoWork(string message)
    {
        _logger.LogInformation($"start work with {message}");
    }
}

Learn More

Wikipedia

Homework

When retrieving data( e.g. a Person with ID =-1 ) from a database , return a NullObject instead of null. How you will verify that the object is a NullObject?