L>>Object-oriented programming (OOP) is a programming paradigm using "objects" – data structures consisting of data fields and methods together with their interactions – to design applications and computer programs.

L>>The most important distinction is whereas procedural programming uses procedures to operate on data structures, object-oriented programming bundles the two together so an "object", which is an instance of a class, operates on its "own" data structure.

I>var rep = new AccountRepository(Application.Db);
I>var account1 = rep.LookUpFor(id_1);
I>var account2 = rep.LookUpFor(id_2);
I>var svc = new TransferService(account1, account2, Application.CurrencyRates);

I>svc.Transfer(100,Currency.USD);
I>

I>var rep = new AccountRepository(Application.Db);
I>var account1 = rep.LookUpFor(id_1);
I>var account2 = rep.LookUpFor(id_2);

I>account1.TransferTo(account2, 100, Currency.USD); // опаньки, нарушение инкапсуляции
I>

A consequence of this situation is if two objects are identical, a change to one will always affect the other.

S>class Monostate 
S>{
S>    static int a;
S>    public int A { get { return a; } set { a = value; } }
S>}
S>var a = new Monostate();
S>var b = new Monostate();
S>a.A = 42;
S>Assert(42 == b.A);
S>

L>Object-oriented programming (OOP) is a programming paradigm using "objects" – data structures consisting of data fields and methods together with their interactions – to design applications and computer programs.

L>The most important distinction is whereas procedural programming uses procedures to operate on data structures, object-oriented programming bundles the two together so an "object", which is an instance of a class, operates on its "own" data structure.

I>account1.TransferTo(accout2, 100, Currency.USD);
I>

G>3 . 3 . 1 . 1 IUnknown:

G>HRESULT IUnknown::QueryInterface(iid, ppv)

G>Return a pointer within this object instance that implements the indicated interface. Answer NULL if the receiver does not contain an implementation of the interface.

G>It is required that any query for the specific interface IUnknown always returns the same
G>actual pointer value, no matter through which interface derived from IUnknown it is called.

G>This enables the following identity test algorithm to determine whether two pointers in
G>fact point to the same object: call QueryInterface(IID_IUnknown, ...) on both and compare the results.
G>
G>...
G>This requirement isthe basis for what is called COM identity.

S>Identity allows comparison of references. Two references can be compared whether they are equal or not. Due to the identity property, this comparison has special properties. If the comparison of references indicates that the references are equal, then it's clear that the two objects pointed by the references are the same object. If the references do not compare equal, then it's not necessarily guaranteed that the identity of the objects behind those references is different. The object identity of two objects of the same type is the same, if every change to either object is also a change to the other object.
S>(http://en.wikipedia.org/wiki/Identity_%28object-oriented_programming%29)

S>var a = new object();
S>var b = new object();
S>var c = a;
S>Debug.Assert(a == c);
S>Debug.Assert(a != b);
S>

G>var a = "string";
G>var b = "string"

G>Debug.Assert((object)a != (object)b); //(1)
G>Debug.Assert(Object.Equals(a,b)); //(2)

G>

A consequence of this situation is if two objects are identical, a change to one will always affect the other.

G>8.2.5.1 Identity

G>...
G>• Otherwise, if their exact type is a reference type, then they are identical if and only if the
G>locations of the values are the same. (1)

G>Identity is implemented on System.Object via the ReferenceEquals method. (2)

S>>>ссылка обеспечивает все что нужно, что бы обратится к объекту с identity. Но из этого не следует что из идентичности следует совпадение ссылок. По крайней мере для логической идентичности это не следует (http://c2.com/cgi/wiki?ObjectIdentity).
G>>Для конкретного языка — следует, можешь обратный пример привести? Я таких не знаю.
G>Для C# две строки с одинаковым содержимым.

A>>Console.WriteLine(ex.Equals(o));//true
A>>Console.WriteLine(object.ReferenceEquals(a, ex));//false!!!
A>>

G>8.2.5.1 Identity

G>The identity operator is defined by the CTS as follows.
G>• If the values have different exact types, then they are not identical.
G>• Otherwise, if their exact type is a value type, then they are identical if and only if the bit
G>sequences of the values are the same, bit by bit.
G>• Otherwise, if their exact type is a reference type, then they are identical if and only if the
G>locations of the values are the same.

G>Identity is implemented on System.Object via the ReferenceEquals method.

A>>class Something
A>>{
A>>  private readonly ID id;
  
A>>  public Something()
A>>  {
A>>    id = ID.GetUniqueId();
A>>    //INSERT INTO table (ID, State) VALUES ($id, 0)
A>>  }

A>>  private Something(ID id)
A>>  {
A>>    this.id = id;
A>>  }
  
A>>  public static IEnumerable<Something> GetAll()
A>>  { 
A>>     foreach(id in //SELECT ID FROM table)
A>>       yield return new Something(id)
A>>  }

A>>  public int State
A>>  {
A>>    get { //SELECT State FROM table WHERE ID = $id }
A>>    set { //UPDATE table SET State=$value WHERE ID = $id }
A>>  }
A>>}
A>>

A>bool IsIdentical(object o1, object o2)
A>{
A>  return false;
A>}
A>

S>>bool MyIdentityEquals(object a, object b)
S>>{
S>>    if (ReferenceEquals(a,b))
S>>       return true; // ослабить а не изменить в корне
    
S>>    if (a == null || b == null)
S>>       return false; // не знаю что определено в штатном порядке, пусть будет так.

S>>    var type = a.GetType();

S>>    if (type != b.GetType())
S>>       return false;

S>>    if (type == typeof(string) && StringComparer.Ordinal.Equals((string)a, (string)b))
S>>        return true;

S>>    // тут обработка хороших типов.
S>>    if (IsImmutable(type) && IsStructuralEquals(a, b))
S>>        return true;

S>>    return false;
S>>}
S>>

G>[Immutable]
G>class A
G>{
G>    public B B { get; }
G>}

G>class B //не-immutable
G>{
G>    public int Prop {get; set;}
G>}

G>var a = new A { B = new B { Prop = 1 } };
G>var b = new A { B = new B { Prop = 1 } };

G>Debug.Assert(MyIdentityEquals(a,b)); //Упадет?

G>b.B.Prop = 1;
G>Debug.Assert(MyIdentityEquals(a,b)); //А тут?
G>

S>>>var sqrt2 = new Sqrt(2);
S>>>Assert(sqrt2 * sqrt2 == 2);
S>>>