# Collections

We briefly introduced [MementoEntity and MementoEntityCollection](/release-1/memento/change-tracking-service/memento-entities.md) and seen how to [track changes in a simple graph](/release-1/memento/change-tracking-service/handling-change-tracking/simple-model.md).

We can go further and introduce collection change tracking:

```csharp
var list = new MementoEntityCollection<String>();
memento.Attach( list );

list.Add( "a" );
list.Add( "b" );
list.Add( "c" );

var count = list.Count; //3

memento.Undo();
var _count = list.Count; //2

memento.Redo();
var __count = list.Count; //3
```

The `MementoEntityCollection<T>` being a memento entity will keep track of changes applied to collection structure. Each change will be tracked, starting from `Add` and `Remove` to `Clear`, `Insert`, `InsertAt`, etc...

```csharp
var list = new MementoEntityCollection<Person>();
memento.Attach( list );

list.Add( new Person() );
```

Adding a `MementoEntity`, such as `Person`, will automatically trigger the memento that will start tracking the `Person` instance:

```csharp
var list = new MementoEntityCollection<Person>();
memento.Attach( list );

var person = new Person();
list.Add( person );

person.FirstName = "name";

memento.Undo(); //the person first name is reverted
memento.Undo(); //the person instance is removed from the collection
memento.Redo(); //the person instance is added once again to the collection
```

As expected the changes stack is handled in the correct order. The next step is to understand how to [handle change tracking in complex objects graph](/release-1/memento/change-tracking-service/handling-change-tracking/complex-graph.md).


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