# MementoEntity and MementoEntityCollection

When we spoke about the [Change Tracking Service](https://docs.radicalframework.com/release-1/memento/change-tracking-service) we introduced a code snippet such as the following:

```csharp
var person = new Person();
person.FirstName = "first name value";
person.LastName = "last name value";
```

In order to leverage the full power of the memento services we can change the above snippet as follows:

```csharp
var memento = new ChangeTrackingService();

var person = new Person();
memento.Attach( person );

person.FirstName = "first name value";
person.LastName = "last name value";

var isChanged = memento.IsChanged; //true
var canUndo = memento.CanUndo; //true
```

Calling `memento.Undo()` will trigger the undo of the last operation, we can call undo until `CanUndo` is `true` rolling back change by change, that in the above sample will revert back the `LastName` property value to its default value.

The requirement to achieve the above is that the `Person` class is a Radical *memento* entity:

```csharp
class Person : MementoEntity
{   
    public String FirstName
    {
        get { return this.GetPropertyValue( () => this.FirstName ); }
        set { this.SetPropertyValue( () => this.FirstName, value ); }
    }

    public String LastName
    {
        get { return this.GetPropertyValue( () => this.LastName ); }
        set { this.SetPropertyValue( () => this.LastName, value ); }
    }
}
```

As we can see from the above snippet all we need to do is to create a class the inherits from the `MementoEntity` base class and declare all the properties we want to track as [Radical properties](https://github.com/RadicalFx/documentation/tree/3311fbb6d65f98ece74ce84f7fde9dec2a25a7ee/entities/property-system.md).

A similar approach can be used to keep track of items in a collection:

```csharp
var memento = new ChangeTrackingService();

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

list.Add( "a value" );
list.Add( "another value" );

var isChanged = memento.IsChanged; //true
var canUndo = memento.CanUndo; //true
```

Calling `memento.Undo()` will trigger the undo of the last operation that in the above sample will revert the collection status removing the last added value.

We can do more:

* [Handling change tracking in a simple model](https://docs.radicalframework.com/release-1/memento/change-tracking-service/handling-change-tracking/simple-model);
* [Handling change tracking in collections](https://docs.radicalframework.com/release-1/memento/change-tracking-service/handling-change-tracking/collections);
* [Handling change tracking in complex objects graph](https://docs.radicalframework.com/release-1/memento/change-tracking-service/handling-change-tracking/complex-graph);