# Property State

When dealing with change tracking systems it is important to know the state of the graph from 2 different point of views

* System point of view
* User point of view

The Radical [`ChangeTrackingService`](/memento/change-tracking-service.md) allows to gather both point of views with property level granularity.

Given a `Person` entity with a `Name` property we can basically be in 2 major scenarios:

```csharp
var person = new Person(name: "Mauro");

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

person.Name = "Giovanni"
```

Or something a bit more problematic:

```csharp
var person = new Person(name: "Mauro");

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

person.Name = "Giovanni"

//some other changes

person.Name = "Mauro"
```

In both cases from the `ChangeTrackingService` perspective the `Person` instance is changed, however from the user perspective maybe it's not since the `Name` property has the exact same value as when it was instantiated.

The `GetPropertyState(entity, property)` method of `ChangeTrackingService`, given a tracked entity and a property name, returns the current state of the property as seen by the tracking service.

The state can be:

* `None` : The property has no tracked states, thus is not changed either;
* `Changed`: The property has tracked states, thus has changes; This means that in some way the property changed over time, this state does not take into account the actual value of the property;
* `ValueChanged`: The property has tracked states and the actual value is different from the original one;

Using as a sample the above `Person` related changes:

```csharp
var state = memento.GetPropertyState(person, p => p.Name);
```

The value of `state` is `Changed` & `ValueChanged`. On the contrary the second one is `Changed` only, because the memento service has tracked some changes but the current value is the same as the original one.

### Extensions

In order to simply the query there is also an extension method for `IMemento` entities:

* `IsPropertyValueChanged(propertyName)`: Is an extension method to `IMemento` entities that acts like a shortcut to get if a property value changed over time or not;

Sample usage

```csharp
var person = new Person(name: "Mauro");

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

person.Name = "Giovanni"
  
var isNameValueChanged = person.IsPropertyValueChanged(p => p.Name); //will be true
```


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