We Keep Coding

Why isn't this ListView binding to this list? [duplicate]

I used the List<Person> collection as the ItemsSource for the DataGrid control.
But it did not update the View if i remove the item from the List collection. I was struggling for a long time for the solution.
Then Instead of the List<Person> collection in my ViewModel. I changed this into ObservableCollection<Person> collection. Now it updates the view when ever there is a change in the collection.
I am not sure why it updates only for ObservableCollection<Person> ? Anyone ?

Well its in the name. A simple List doesn't tell the ui to update, in other words "the view can't observe the list". There is no weird magic behind Databinding. WPF and DataBinding needs the Data model to tell them "this is new" or "this is changed", you propably already saw INotifyPropertyChanged, INotifyCollectionChanged is the same but for collections, and the List<> doesn't implement it, ObservableCollection does.

Because List does not implement INotifyCollectionChanged

Because the update of a databinding is not a kind of magic, there are several requirements to make databinding working correctly. If you have a single property to bind on this property must be either a dependency property or its parent class must implement the INotifyPropertyChanged interface to notify the wpf binding system about changes of the property value.
For a collection there is a simelar mechanism: it must implement INotifyPropertyChanged to inform the wpf binding system about removed/moved/added items.
See here for more details: http://msdn.microsoft.com/en-us/library/system.collections.specialized.inotifycollectionchanged.aspx

ObservableCollection<T> fires change events every time you change the items in the collection. List<T> doesn't. That's the reason.
DataBinding is lazy. If you don't tell your view that something has changed, it won't bother updating. Under the hoods, WPF DataBinding registers for change notifications, so that your ViewModel can tell the view when it has changed. It does this with interfaces like INotifyPropertyChanged and INotifyCollectionChanged.
ObservableCollection<T> implements the interface INotifyCollectionChanged. This interface defines the event CollectionChanged, which your View basically attaches its own event handler to. That handler will update the view when the event is raised by the collection.

INotifyPropertyChanged vs Two way binding

I am newbie in c# and i want to know why we have to implement INotifyPropertyChanged interface when we use the TwoWay binding ?? and for the OneWay also ??
Thank you

In brief to support OneWay/TwoWay bindings, the underlying data must implement INotifyPropertyChanged.
Then the OneWay/TwoWay binding is just to choose the bind direction, you can find more here :
Various wpf binding modes

INotifyPropertyChanged,like the name, is to notify your client that your property has changed, see MSDN
You will need it for updating your UI when properties change.
OneWay(Source to Target): Propertychange will cause UI update and UI operation will NOT cause propertychange. *
TwoWay(Both way): Proerty and UI are fully binded, any of them change will affect the other one.
Binding works as long as you implement INotifyPropertyChanged for your property in this case.
If you don't, nothing will change even if you use Twoway.

Implementing INotifyPropertyChanged just offers classes (others than the one implementing it) the possibility to react on property changes.
This is not possible if that interface is not implemented because if a class instance, say A sets a property on B, a third class instance C cannot track that information. Setting that property is now only a concern of A and B. If Chowever knows that B does implement INotifyPropertyChanged, it can simply add an event handler to the event PropertyChanged (which is part of the interface) and react on it - still hoping that B will throw the event as expected.
Bindings need that information to update the model (data) or the view depending where the change happened. So basically, they are C listening for changes of other objects (A & B).

Controls are updated by INotifyPropertyChanged but not the list in the DataGrid

I've implemented INotifyPropertyChanged and the update in my GUI works as supposed to. If the view model changes a value, the control in the window updates.
However, I also have a DataGrid control and the update in view model doesn't propagate to the GUI. If I change something in the the table and invoke store, the changed values are available for VM, including newly added rows.
I've looked at other classes, like INotifyCollectionChanged but got no wiser. Should I move over and use ObservableCollection or is there a more recommended approach?

IObservable is the way to go to notify the View of changes. As you have mentioned you models are already implenting INotifyPropertyChanged, so your in a good spot to take advantage of it.

Property change notification when property members change

Say I have a property named Location of type Point in my ViewModel, to which my View binds. I'm using MVVM Light's ObservableObject and RaisePropertyChanged setup to raise property change notifications. The problem is that when I execute the following in the ViewModel:
this.Location = new Point(100,100);
my View updates correctly. But if I do:
this.Location.Offset(10,10);
my View doesn't update, means the Location property does not notify the View about the change. Why is this so? How do sub-properties of built-in types such as Point or Size deal with change notification?

The fundamental problem is that Point is mutable.
You could write your own ImmutablePoint that allows conversion to and from the existing Point class for API compatibility, but doesn't allow methods like Offset.
It's not an ideal solution, but it gets around the problem of changing the fields on a property rather than the property itself.

In MVVM land, your ViewModel is responsible for notifying the View when properties change. The majority of the .NET classes do not implement INotifyPropertyChanged themselves, so you won't get automatic wiring when using them. You'll have to raise PropertyChanged yourself on the Location property each time you modify any properties of Location
Alternatively, you could implement your own Point object that implements INotifyPropertyChanged.

You can add INotifyPropertyChanged to your class members automatically using either Castle Dynamic Proxy (which wraps your models in a proxy at runtime) or PropertyChanged.Fody (which modifies the model's IL at compile time). Google is your friend.

How should the Model let the ViewModel know about changes in its state?

I am building and MVVM application where some of my Models contain the state of realworld-object that their are modelling. The state changes from outside events, e.g. my Model gets information over TCP/IP about a stock that changes, and updates its state to reflect that change.
Now I would like the change to propagate to my View, and the way to do that, is to let my ViewModel know about the change.
I can think of two ways of doing this.
One, I let the Model implement INotifyPropertyChanged, and fire the event when the property changes. However, this seems to be frowned upon for some reason.
Two, I implement an event for each property that can change in the Model, events that the ViewModel explicitly can bind to.
What is the preferred way? Are there other ways (better ways) of doing it?
Edit:
I have now read, both in the comment from slugster and here, that having the Model have a state is not the purpose of the model.
However, in John Gossmans original MVVM post, we find: "The Model is defined as in MVC; it is the data or business logic, completely UI independent, that stores the state and does the processing of the problem domain."

Option 2 is viable, albeit dirty. You can mitigate the dirtiness somewhat by exposing the public events and functions via an interface, and the viewmodel uses the model only via that interface.
Having said that, the model should be treated as a conduit of information, rather than a container for information. This means that ideally your model should not contain any state information. If you have a persistently open channel in your model that needs to notify other components when something is received then I suggest you use the EventAggregator in Prism - this is a weak event pub/sub system, your viewmodel can subscribe to an event of your choosing, and your model can publish that event when it needs to (along with an appropriate payload).

The model classes typically provide property and collection change
notification events through the INotifyPropertyChanged and
INotifyCollectionChanged interfaces. This allows them to be easily
data bound in the view. Model classes that represent collections of
objects typically derive from the ObservableCollection class.
MSDN
I wouldn't say that a Model typically implements INotifyPropertyChanged, but I wouldn't categorically refuse to do so. In simpler - non-PRISM - environments it's totally fine in my opinion.
And then?
To clarify: INotifyPropertyChanged is an interface for the notification in case of a property change, as the name says and most people know. However, it is not more than this, especially it is not WPF specific! Furthermore, as according to the the MVVM principles, the Model should be agnostic of the ViewModel. Therefore, it is clear, that you should use a Publisher(Model)-Subscriber(ViewModel)-pattern (events). That's all. So, as an answer to your question: How to set up the communication between Model/ViewModel depends on your style and the specific task:
If you just want to react on changed properties, just use INotifyPropertyChanged, as that's what it was made for. In the ViewModel, just subscribe to the PropertyChanged-event of the Model and process the changes, which is basically just 'map them forward', that is raise PropertChanged for the ViewModel-property, which is effected by the change of the model property.
Alternatively, if you need to wrap the model changes in a more specific event, lets say 'MyDataUpdated', then I don't see any problem about that as well. Just listen to this event in the ViewModel, and handle it as you wish. That's perfectly fine to.
I hope this helps.
EDIT: As a side-note: I recommend using the PropertyWeaver extension, so that you donÄt mess up your model with all the PropertyChanged-stuff.

I will go with option1. Keep a reference to the model in your viewmodel, however do not use any properties directly. Instead, populate the viewmodel's own properties from the model.Your view model can listen to the property changing notifications for the concerned properties from the Model, and repopulate its own properties from the model. The model is still UI independent, it is merely notifying property changed.
In my opinion, doing your own publisher-subscriber logic other than inotifypropertychanged could be just an overkill as there might not be any benefits doing that. However, this depends on whether you can afford to have a model reference in the view model and my answer is based on the assumption that you can have a reference of model in view model.

For for data binding you can use **Binding**
Data Binding Overview