Memento یک الگوی طراحی مفید و ساده است که برای ذخیره و بازیابی state یک object استفاده می‌شود. در بعضی از مقالات از آن به عنوان snapshot نیز یاد شده است! اگر با git  کار کرده باشید، این مفهوم را می‌توان در git بسیار یافت؛ هر commit به عنوان یک snapshot میباشد که میتوان به صورت مکرر آن را undo کرد و یا مثال خیلی ساده‌تر میتوان به ctrl+z در سیستم عامل اشاره کرد.

به مثال زیر توجه کنید:

Int temp;
Int a=1;
temp=a;
a=2;
.
.
a=temp;

شما قطعا در برنامه نویسی با کد بالا زیاد برخورد داشته‌اید و آن‌را به صورت مکرر انجام داده‌اید. کد بالا را در قالب یک object بیان میکنیم. به مثال زیر توجه کنید:

int main()
{
  MyClass One = new MyClass();
  MyClass Temp = new MyClass();
  // Set an initial value.
  One.Value = 10;
  One.Name = "Ten";
  // Save the state of the value.
  Temp.Value = One.Value;
  Temp.Name = One.Name;
  // Change the value.
  One.Value = 99;
  One.Name = "Ninety Nine";
  // Undo and restore the state.
  One.Value = Temp.Value;
  One.Name = Temp.Name;
}

در کد بالا با استفاده از یک temp، شیء مورد نظر را ذخیره کرده و در آخر مجدد داده‌ها را درون شیء، restore  میکنیم.

 از مشکلات کد بالا میتوان گفت :

۱- برای هر object باید یک شیء temp ایجاد کنیم.

۲- ممکن است بخواهیم که حالات یک object را بر روی هارد ذخیره کنیم. با روش فوق کدها خیلی پیچیده‌تر خواهند شد.

۳- نوشتن کد به این سبک برای پروژه‌های بزرگ، پیچیده و مدیریت آن سخت‌تر می‌شود.

پیاده سازی memento

ما این مثال را در قالب یک پروژه NET Core  onsole. ایجاد میکنیم. برای این کار یک پوشه‌ی جدید را ایجاد و درون ترمینال دستور زیر را وارد کنید:

dotnet new console

روش‌های زیادی برای پیاده سازی memento وجود دارند. برای پیاده سازی memento ابتدا یک abstract class را به شکل زیر ایجاد میکنیم: 

abstract class MementoBase
{
  protected Guid mementoKey = Guid.NewGuid();
  abstract public void SaveMemento(Memento memento);
  abstract public void RestoreMemento(Memento memento);
}

اگر به کلاس بالا دقت کنید، این کلاس قرار است parent کلاس‌های دیگری باشد که داری دو متد SaveMemento و RestoreMemento برای ذخیره و بازیابی و همچنین یک Guid برای نگهداری state‌های مختلف میباشد.

ورودی متدها از نوع memento میباشد. پس کلاس memento را به شکل زیر ایجاد می‌کنیم:

class Memento
{
    private Dictionary<Guid, object> stateList = new Dictionary<Guid, object>();
    public object GetState(Guid key)
    {
        return stateList[key];
    }
    public void SetState(Guid key, object newState)
    {
        stateList[key] = newState;
    }
    public Memento()
    {
    }
}

در کد بالا با یک Dictionary می‌توان هر object را با کلیدش ذخیره کنیم. توجه کنید که value دیکشنری از نوع object میباشد و چون object پدر تمام object‌های دیگر است پس می‌توانیم هر نوع داده‌ای را در آن ذخیره کنیم. تا اینجا، Memento پیاده سازی شده است. میتوان این کار را با جنریک‌ها نیز پیاده سازی کرد.

در ادامه می‌خواهیم یک کلاس بسازیم و حالت‌های مختلف را در آن بررسی کنیم. کلاس زیر را ایجاد کنید:

class ConcreteOriginator : MementoBase
{
  private int value = 0;
  public ConcreteOriginator(int newValue)
  {
    SetData(newValue);
  }
  public void SetData(int newValue)
  {
    value = newValue;
  }
  public void Speak()
  {
    Console.WriteLine("My value is " + value.ToString());
  }
  public override void SaveMemento(Memento memento)
  {
    memento.SetState(mementoKey, value);
  }
  public override void RestoreMemento(Memento memento)
  {
    int restoredValue = (int)memento.GetState(mementoKey);
    SetData(restoredValue);
  }
}

کلاس ConcreteOriginator از کلاس MementoBase ارث بری کرده و دو متد RestoreMemento و SaveMemento را پیاده سازی میکند و همچنین دارای یک مشخصه value می‌باشد. برای خروجی گرفتن، متد main را به صورت زیر پیاده سازی می‌کنیم:

static void Main(string[] args)
{
  Memento memento = new Memento();
  // Create an originator, which will hold our state data.
  ConcreteOriginator myOriginator = new ConcreteOriginator("Hello World!", StateType.ONE);
  ConcreteOriginator anotherOriginator = new ConcreteOriginator("Hola!", StateType.ONE);
  ConcreteOriginator2 thirdOriginator = new ConcreteOriginator2(7);
  // Set some state data.
  myOriginator.Speak();
  anotherOriginator.Speak();
  thirdOriginator.Speak();
  // Save the states into our memento.
  myOriginator.SaveMemento(memento);
  anotherOriginator.SaveMemento(memento);
  thirdOriginator.SaveMemento(memento);
  // Now change our originators' states.
  myOriginator.SetData("Goodbye!", StateType.TWO);
  anotherOriginator.SetData("Adios!", StateType.TWO);
  thirdOriginator.SetData(99);
  myOriginator.Speak();
  anotherOriginator.Speak();
  thirdOriginator.Speak();
  // Restore our originator's state.
  myOriginator.RestoreMemento(memento);
  anotherOriginator.RestoreMemento(memento);
  thirdOriginator.RestoreMemento(memento);
  myOriginator.Speak();
  anotherOriginator.Speak();
  thirdOriginator.Speak();
  Console.ReadKey();
}

Hello World! I'm in state ONE
Hola! I'm in state ONE
My value is 7
Goodbye! I'm in state TWO
Adios! I'm in state TWO
My value is 99
Hello World! I'm in state ONE
Hola! I'm in state ONE
My value is 7

 <div>
                              <div>
                                  <div>
                                      <p><span>First Name </span>: Jonathan</p>
                                  </div>
   </div>
                          </div>

@model System.Web.UI.WebControls.ListItem


<div>
    <p><span>@Model.Text </span>: @Model.Value</p>
</div>

@using System.Web.UI.WebControls
@using ZekrWepApp.Filters
@model ZekrModel.Admin

    <div>
        <h1>Bio Graph</h1>
        <div>
            
            @{
                ListItemCollection collection = GetCustomProperties.Get(Model,exclude:new string[]{"Poems","Id"});
                foreach (var item in collection)
                {
                    Html.RenderPartial(MVC.Shared.Views._BioRow, item);
                }
            }

                    </div>
    </div>

  public class GetCustomProperties
    {
        private static PropertyInfo[] getObjectsInfos(object obj,string[] inclue,string[] exclude )
        {
            var list = obj.GetType().GetProperties();

            PropertyInfo[] outputPropertyInfos = null;

            if (inclue != null)
            {           
                return list.Where(propertyInfo => inclue.Contains(propertyInfo.Name)).ToArray();
            }
            if (exclude != null)
            {
                return list.Where(propertyInfo => !exclude.Contains(propertyInfo.Name)).ToArray();
            }
            return list;
        }
    }

 public static ListItemCollection Get(object obj,string[] inclue=null,string[] exclude=null)
        {
            var propertyInfos = getObjectsInfos(obj, inclue, exclude);
            if (propertyInfos == null) throw new ArgumentNullException("propertyInfos is null");

            var collection = new ListItemCollection();

            foreach (PropertyInfo propertyInfo in propertyInfos)
            {


                string name = propertyInfo.Name;

                foreach (Attribute attribute in propertyInfo.GetCustomAttributes(true))
                {
                    DisplayAttribute displayAttribute = attribute as DisplayAttribute;

                    if (displayAttribute != null)
                    {
                        name = displayAttribute.Name;
                        break;
                    }                  
                }

                string value = "";
                object objvalue = propertyInfo.GetValue(obj);
                if (objvalue != null) value = objvalue.ToString();

                collection.Add(new ListItem(name,value));
            }
            return collection;
        }

using System;
using System.Collections.Generic;
using System.ComponentModel.DataAnnotations;
using System.Linq;
using System.Linq.Expressions;
using System.Reflection;
using System.Web;
using System.Web.Mvc.Html;
using System.Web.UI.WebControls;
using Links;

namespace ZekrWepApp.Filters
{
    
    public class GetCustomProperties
    {
        public static ListItemCollection Get(object obj,string[] inclue=null,string[] exclude=null)
        {
            var propertyInfos = getObjectsInfos(obj, inclue, exclude);
            if (propertyInfos == null) throw new ArgumentNullException("propertyInfos is null");

            var collection = new ListItemCollection();

            foreach (PropertyInfo propertyInfo in propertyInfos)
            {
                string name = propertyInfo.Name;
                foreach (Attribute attribute in propertyInfo.GetCustomAttributes(true))
                {
                    DisplayAttribute displayAttribute = attribute as DisplayAttribute;

                    if (displayAttribute != null)
                    {
                        name = displayAttribute.Name;
                        break;
                    }                  
                }

                string value = "";
                object objvalue = propertyInfo.GetValue(obj);
                if (objvalue != null) value = objvalue.ToString();

                collection.Add(new ListItem(name,value));
            }
            return collection;
        }
        private static PropertyInfo[] getObjectsInfos(object obj,string[] include,string[] exclude )
        {
            var list = obj.GetType().GetProperties();

            PropertyInfo[] outputPropertyInfos = null;

            if (include != null)
            {           
                return list.Where(propertyInfo => include.Contains(propertyInfo.Name)).ToArray();
            }
            if (exclude != null)
            {
                return list.Where(propertyInfo => !exclude.Contains(propertyInfo.Name)).ToArray();
            }
            return list;
        }  
    }   
}

amount=1000&orderId=452&Pid=xxx&....

using System;
using System.Collections.Generic;
using System.Linq;

namespace Utils
{
    public class QueryStringParametersList
    {
        private string Symbol = "&";
        private List<KeyValuePair<string, string>> list { get; set; }

        public QueryStringParametersList()
        {
            list = new List<KeyValuePair<string, string>>();
        }
        public QueryStringParametersList(string symbol)
        {
            Symbol = symbol;
           list = new List<KeyValuePair<string, string>>(); 
        }

        public int Size
        {
            get { return list.Count; }
        }
        public void Add(string key, string value)
        {
            list.Add(new KeyValuePair<string, string>(key, value));
        }

        public string GetQueryStringPostfix()
        {
            return string.Join(Symbol, list.Select(p => Uri.EscapeDataString(p.Key) + "=" + Uri.EscapeDataString(p.Value)));
        }
    }
}

QueryStringParametersList queryparamsList = new QueryStringParametersList();

            ueryparamsList.Add("consumer_key", requestPayment.Consumer_Key);
            queryparamsList.Add("amount", requestPayment.Amount.ToString());
            queryparamsList.Add("callback", requestPayment.Callback);
            queryparamsList.Add("description", requestPayment.Description);
            queryparamsList.Add("email", requestPayment.Email);
            queryparamsList.Add("mobile", requestPayment.Mobile);
            queryparamsList.Add("name", requestPayment.Name);
            queryparamsList.Add("irderid", requestPayment.OrderId.ToString());

   private QueryStringParametersList ReadParams(object obj)
        {
            PropertyInfo[] propertyInfos = obj.GetType().GetProperties();

            QueryStringParametersList queryparamsList = new QueryStringParametersList();
            for (int i = 0; i < propertyInfos.Count(); i++)
            {
                queryparamsList.Add(propertyInfos[i].Name.ToLower(),propertyInfos[i].GetValue(obj).ToString() );              
            }
            return queryparamsList;
        }

<ResourceDictionary xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
                    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
                    xmlns:system="clr-namespace:System;assembly=mscorlib">
    <system:String x:Key="button1">Hello!</system:String>
    <system:String x:Key="button2">How Are You?</system:String>
    <system:String x:Key="button3">Are You OK?</system:String>
 
</ResourceDictionary>

<Application x:Class="BeRMOoDA.WPF.LocalizationSample.App"
             xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
             StartupUri="MainWindow.xaml">
    <Application.Resources>
 
        <ResourceDictionary>
            <ResourceDictionary.MergedDictionaries>
                <ResourceDictionary Source="Culture_en-US.xaml"/>
            </ResourceDictionary.MergedDictionaries>
        </ResourceDictionary>
 
    </Application.Resources>
</Application>

<Button Content="{DynamicResource ResourceKey=button1}" />
<Button Content="{DynamicResource ResourceKey=button2}" />
<Button Content="{DynamicResource ResourceKey=button3}" />

<ResourceDictionary xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
                    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
                    xmlns:system="clr-namespace:System;assembly=mscorlib">
    <ResourceDictionary.MergedDictionaries>
        <ResourceDictionary Source="Culture_fa-IR_Farsi.xaml"/>
    </ResourceDictionary.MergedDictionaries>
    <system:String x:Key="button1">سلام!</system:String>
    <system:String x:Key="button2">حالت چطوره؟</system:String>
    <system:String x:Key="button3">خوبی؟</system:String>
</ResourceDictionary>

public class CultureAssemblyModel
    {
        //the text will be displayed to user as language name (like Farsi)
        public string DisplayText { get; set; }
        //name of .dll file (like Culture_fa-IR_Farsi.dll)
        public string Name { get; set; }
        //standar notation of this culture (like fa-IR)
        public string Culture { get; set; }
        //name of resource dictionary file inside the loaded .dll (like Culture_fa-IR.xaml)
        public string XamlFileName { get; set; }
    }

//will keep information about loaded assemblies
public List<CultureAssemblyModel> CultureAssemblies { get; set; }
 
//loads assmeblies in languages folder and adds their info to list
 void LoadCultureAssemblies()
 {
      //we should be sure that list is empty before adding info (do u want to add some cultures more than one? of course u dont!)
      CultureAssemblies.Clear();
      //creating a directory represents applications directory\languages
      DirectoryInfo dir = new DirectoryInfo(System.IO.Path.GetDirectoryName(Assembly.GetExecutingAssembly().Location) + "\\languages");
      //getting all .dll files in the language folder and its sub dirs. (who knows? maybe someone keeps each culture file in a seperate folder!)
      var assemblies = dir.GetFiles("*.dll", SearchOption.AllDirectories);
      //for each found .dll we will create a model and set its properties and then add to list  for (int i = 0; i < assemblies.Count(); i++)
      {

                string name = assemblies[i].Name;

                CultureAssemblyModel model = new CultureAssemblyModel() { DisplayText = name.Split('.', '_')[2], Culture = name.Split('.', '_')[1], Name = name  , XamlFileName =name.Substring(0, name.LastIndexOf(".")) + ".xaml" };
                CultureAssemblies.Add(model);
      }
 }

comboboxLanguages.ItemsSource = CultureAssemblies;

<ComboBox HorizontalAlignment="Left" Margin="10" VerticalAlignment="Top" MinWidth="100" Name="comboboxLanguages">
            <ComboBox.ItemTemplate>
                <DataTemplate>
                    <Label Content="{Binding DisplayText}"/>
                </DataTemplate>
            </ComboBox.ItemTemplate>
</ComboBox>

//loads selected culture
 public void LoadCulture(CultureAssemblyModel culture)
 {
     //creating a FileInfo object represents .dll file of selected cultur
     FileInfo assemblyFile = new FileInfo("languages\\" + culture.Name);
     //loading .dll into memory as a .net assembly
     var assembly = Assembly.LoadFile(assemblyFile.FullName);
     //getting .dll file name
     var assemblyName = assemblyFile.Name.Substring(0, assemblyFile.Name.LastIndexOf("."));
     //creating string represents structure of a pack uri (something like this: /{myassemblyname;component/myresourcefile.xaml}
     string packUri = string.Format(@"/{0};component/{1}", assemblyName, culture.XamlFileName);
     //creating a pack uri
     Uri uri = new Uri(packUri, UriKind.Relative);
     //now we have created a pack uri that represents a resource object in loaded assembly
     //and its time to load that as a resource dictionary (do u remember that we had resource dictionary in culture assemblies? don't u?)
     var dic = Application.LoadComponent(uri) as ResourceDictionary;
     dic.Source = uri;
     //here we will remove current merged dictionaries in our resource dictionary and add recently-loaded resource dictionary as e merged dictionary
     var mergedDics = this.Resources.MergedDictionaries;
     if (mergedDics.Count > 0)
          mergedDics.Clear();
     mergedDics.Add(dic);
 }

void comboboxLanguages_SelectionChanged(object sender, SelectionChangedEventArgs e)
 {
     var selectedCulture = (CultureAssemblyModel)comboboxLanguages.SelectedItem;
     App app = Application.Current as App;
     app.LoadCulture(selectedCulture);
 }

اما در مدل EAV، خواص داینامیک را به درون جدول دومی منتقل خواهیم کرد:

create table EmployeeEav
(
   Id int auto_increment
   primary key
);

create table EmployeeAttributes
(
  Id int auto_increment
  primary key,
  EmployeeId int not null,
  AttributeName text null,
  AttributeValue text null,
  constraint FK_EmployeeAttributes_EmployeeEav_EmployeeId
  foreign key (EmployeeId) references EmployeeEav (Id)
  on delete cascade
);

create index IX_EmployeeAttributes_EmployeeId
on EmployeeAttributes (EmployeeId);

تعریف جداول فوق نیز در Entity Framework به اینصورت خواهند بود:

public class EmployeeEav
{
    public int Id { get; set; }
    public virtual ICollection<EmployeeAttribute> Attributes { get; set; }
}

public class EmployeeAttribute
{
    public int Id { get; set; }
    public virtual EmployeeEav Employee { get; set; }
    public int EmployeeId { get; set; }
    public string AttributeName { get; set; }
    public string AttributeValue { get; set; }
}

public class MyDbContext : DbContext
{

    public DbSet<EmployeeEav> EmployeeEav { get; set; }
    public DbSet<EmployeeAttribute> EmployeeAttributes { get; set; }

    protected override void OnConfiguring(DbContextOptionsBuilder options)
    {
        options.UseMySQL(_configuration.GetConnectionString("DataConnection"));
    }
}

SELECT `e`.`Id`, `e`.`DateOfBirth`, `e`.`FirstName`, `e`.`LastName`
FROM `Employees` AS `e`
WHERE `e`.`DateOfBirth` > @__endDate_0

USE [WideWorldImporters];
GO

SET STATISTICS IO ON;
GO


/*
What's are the inner and outer
data sets?
*/
SELECT
    [ol].[OrderLineID],
    [o].[CustomerID]
FROM [Sales].[OrderLines] [ol]
    INNER JOIN [Sales].[Orders] [o]
    ON [ol].[OrderID] = [o].[OrderID]
WHERE [o].[CustomerID] = 185;
GO

SELECT
    [ol].[OrderLineID],
    [o].[CustomerID]
FROM [Sales].[OrderLines] [ol]
    INNER JOIN [Sales].[Orders] [o]
    ON [ol].[OrderID] = [o].[OrderID]
WHERE [o].[CustomerID] = 185
OPTION (FORCE ORDER);

SELECT
    [ol].[OrderLineID],
    [o].[CustomerID]
FROM [Sales].[OrderLines] [ol]
    INNER LOOP JOIN [Sales].[Orders] [o]
    ON [ol].[OrderID] = [o].[OrderID]
WHERE [o].[CustomerID] = 185
OPTION (FORCE ORDER);
GO

SELECT
    [ContactPersonID],
    [OrderDate],
    [CustomerPurchaseOrderNumber]
FROM [Sales].[Orders]
WHERE [ContactPersonID] = 3144;

ALTER TABLE [Sales].[Orders] ADD  CONSTRAINT [PK_Sales_Orders] PRIMARY KEY CLUSTERED 
(
[OrderID] ASC
)

GO

CREATE NONCLUSTERED INDEX [FK_Sales_Orders_ContactPersonID] ON [Sales].[Orders]
(
[ContactPersonID] ASC
)

CREATE NONCLUSTERED INDEX [FK_Sales_Orders_ContactPersonID]
ON [Sales].[Orders] (
[ContactPersonID] ASC
)
INCLUDE (
[OrderDate], [CustomerPurchaseOrderNumber]
)
WITH (DROP_EXISTING = ON)
ON [USERDATA];
GO

SELECT
    [ContactPersonID],
    [OrderDate],
    [CustomerPurchaseOrderNumber]
FROM [Sales].[Orders]
WHERE [ContactPersonID] = 3144;

USE [WideWorldImporters]
GO

DROP TABLE [Sales].[Copy_Orders]
GO

SELECT *
INTO [Sales].[Copy_Orders]
FROM [Sales].[Orders];
GO

CREATE NONCLUSTERED INDEX [NCI_Copy_Orders_ContactPersonID]
ON [Sales].[Copy_Orders] (
[ContactPersonID]
);
GO

SELECT
    [ContactPersonID],
    [OrderDate],
    [CustomerPurchaseOrderNumber]
FROM [Sales].[Copy_Orders]
WHERE [ContactPersonID] = 3144;

CREATE NONCLUSTERED INDEX [NCI_Copy_Orders_ContactPersonID]
ON [Sales].[Copy_Orders] (
[ContactPersonID] ASC
)
INCLUDE (
[OrderDate], [CustomerPurchaseOrderNumber]
)
WITH (DROP_EXISTING = ON)
ON [USERDATA];
GO

using System.Collections.Generic;
using System.Runtime.CompilerServices;

namespace ConditionalWeakTableSamples
{
    public static class AttachedProperties
    {
        public static ConditionalWeakTable<object,
            Dictionary<string, object>> ObjectCache = new ConditionalWeakTable<object,
                Dictionary<string, object>>();

        public static void SetValue<T>(this T obj, string name, object value) where T : class
        {
            var properties = ObjectCache.GetOrCreateValue(obj);

            if (properties.ContainsKey(name))
                properties[name] = value;
            else
                properties.Add(name, value);
        }

        public static T GetValue<T>(this object obj, string name)
        {
            Dictionary<string, object> properties;
            if (ObjectCache.TryGetValue(obj, out properties) && properties.ContainsKey(name))
                return (T)properties[name];
            return default(T);
        }

        public static object GetValue(this object obj, string name)
        {
            return obj.GetValue<object>(name);
        }
    }
}

    public static class ISecurityPrincipalExtension
    {
        public static bool Disclaimer(this IPrincipal principal)
        {
            return principal.GetValue<bool>("Disclaimer");
        }

        public static void SetDisclaimer(this IPrincipal principal, bool value)
        {
            principal.SetValue("Disclaimer", value);
        }
    }

using System.Runtime.CompilerServices;

namespace ConditionalWeakTableSamples
{
    public static class PropertyExtensions
    {
        private class ExtraPropertyHolder
        {
            public bool IsDirty { get; set; }
        }

        private static readonly ConditionalWeakTable<object, ExtraPropertyHolder> _isDirtyTable
                = new ConditionalWeakTable<object, ExtraPropertyHolder>();

        public static bool IsDirty(this object @this)
        {
            return _isDirtyTable.GetOrCreateValue(@this).IsDirty;
        }

        public static void SetIsDirty(this object @this, bool isDirty)
        {
            _isDirtyTable.GetOrCreateValue(@this).IsDirty = isDirty;
        }
    }
}

public void Closed(DbConnection connection, DbConnectionInterceptionContext interceptionContext)
{
}

using System;
using System.Runtime.CompilerServices;

namespace ConditionalWeakTableSamples
{
    public static class UniqueIdExtensions
    {
        static readonly ConditionalWeakTable<object, string> _idTable = 
                                    new ConditionalWeakTable<object, string>();

        public static string GetUniqueId(this object obj)
        {
            return _idTable.GetValue(obj, o => Guid.NewGuid().ToString());
        }

        public static string GetUniqueId(this object obj, string key)
        {
            return _idTable.GetValue(obj, o => key);
        }
    }
}