How do I open a WPF window in AddNew mode before any data has been loaded?

I want to open a WPF4/EF4 form in AddNew mode so the user can start entering data in bound controls before any data has been selected from the database. I already have an "Add New Record" button but it only works with a populated DataContext (my CollectionViewSource). Here is the code so far:

private void btnAddNewRecord_Click(object sender, RoutedEventArgs e)
{
    LabSample newEntity = _labEntitiesContext.LabSamples.CreateObject<LabSample>();
    _labEntitiesContext.LabSamples.AddObject(newEntity);
    _labSamplesListCollectionView.AddNewItem(newEntity);
}

Background: This is a basic WPF app with bound controls. I started with an Entity Framework model that appears in my DataSources window. I dragged my LabSample entity from the DataSources window and let it create my CollectionViewSource (labSamplesViewSource) in the XAML's Windows.Resources section. The DataContext for all my controls is the labSamplesViewSource. I create a new LabEntities object called _labEntitiesContext as the window is instantiated. I use _labEntitiesContext to build my filtered ObjectQuery(of LabSample) and to SaveChanges, but I'm a little confused as to how this _labEntitiesContext is hooked up to my CollectionViewSource. If you could clarify this along with answering my question that would be helpful. Note: I'm not ready to use MVVM.

When the window loads I use this.FindResource to grab a reference to the CollectionViewSource in a class level variable named _labSamplesCollectionViewSource. I allow the user to enter search fields to populate the screen with data. My LoadData routine looks something like this:

System.Data.Objects.ObjectQuery<LabSample> labSamplesObjectQue开发者_开发知识库ry = this.GetLabSamplesFiltered_Query(_labEntitiesContext, sampleID_LIKE, xxx_LIKE, yyy_LIKE);
System.Data.Objects.ObjectResult<LabSample> labSamplesObjectResult = labSamplesObjectQuery.Execute(System.Data.Objects.MergeOption.AppendOnly);
_labSamplesCollectionViewSource.Source = new System.Collections.ObjectModel.ObservableCollection<LabSample>(labSamplesObjectResult);
_labSamplesListCollectionView = (ListCollectionView)_labSamplesCollectionViewSource.View;

The _labSamplesListCollectionView class level variable set above is used in my btnAddNewRecord_Click code. Before LoadData is called the _labSamplesListCollectionView is null causing my AddNew code to fail with "Object reference not set to an instance of an object".

How can I make this work? I'm wondering if I should be making use of _labSamplesListCollectionView.AddNew instead of my current technique but I couldn't get that work either. Your help will be greatly appreciated.

I am writing an app that does something similar. I am however using MVVM pattern, which allows me to do some neat things. In mine, I am working with Shipments. On the ShipmentsView, I can click an "Add New" button which fires off a bound command property which is housed in the associated ViewModel class. That command methods looks like the following: Note: Views in this context are not CollectionView but refer to MVVM View classes.

Dim NewShipment = New Shipment()
_Context.AddToShipments(NewShipment)
Dim ShipVM = New ShipmentViewModel(NewShipment)
ShipmentVMCollection.Add(ShipVM)
Dim NewShipmentView as ShipmentView(ShipVM)

My ShipmentView handles it's placement and visiblility, and my Shipment object has it's property values initialized so that it does not immediately present errors via it's validation handlers. This way the user can create a new shipment and if they get sidetracked they can save it and come back to it without having a bunch of mandatory fields.

When I use a CollectionViewSource, I populate it with an ObservableCollection of my entities, and then add the entities to that observable collection when I create them. ObservableCollection implements INotifyPropertyChanged and INotifyCollectionChanged events and notifies the UI when something happens, and it all works through the CollectionViewSource.

You might take a look at the MVVM pattern which is really good for moving data and keeping it in the proper scope, and there are some good MVVM frameworks out there that will help you make a nice application with MVVM.

MVVM may be a bit of overkill for your app if it is small. But if it gets over more than just a few Views it is going to get unwieldy and hard to move data back and forth and keep it current, and maintainable.

Wiki Article for MVVM - a pretty good place to start and get links

This is my Constructor for one of my ViewModels. I realize you don't want to implement in MVVM right now, but a code behind would be similar. In this instance, I am using a background worker to get my entity records, (the constructor call be for that and the View setting immediately afterward can be disregarded), then I link up my CVS, Populate it with my ObservableCollection, and set it's View to a field so I can filter on it later.

Public Sub New(ByRef MyView As NTMsView)
    Me.New(ViewManagerService.CreateInstance, ViewModelUIService.CreateInstance)
    NTMsBackgroundWorker.RunWorkerAsync()
    _View = MyView
    _NTMCollectionViewSource = _View.FindResource("NTMCollectionViewSource")
    _NTMCollectionViewSource.Source = NTMs
    _NTMCollectionView = _NTMCollectionViewSource.View
End Sub

This is an example of my AddRecord method. Then I instance a new object, add it to the appropriate collection in the Context, Save it, execute a stored procedure, then refresh the context since the stored procedure did a few things to the record. Then I add the object to my Observable.

Private Sub AddNTM()
    'Create an NTM Object.  
    Dim NewNTM As New NTMShipment()
    'Add it to the context
    _context.AddToNTMShipments(NewNTM)
    _context.SaveChanges()
    _context.MakeNewSecurityID(NewNTM.NTMShipID)
    _context.Refresh(RefreshMode.StoreWins, NewNTM)
    'Wrap it in a ViewModel and Add it to the NTMs collection
    NTMs.Add(New NTMViewModel(NewNTM))
End Sub

As for creating a new entity before your CollectionViewSource is created, a couple of questions. Is your edit forms datacontext related to the CVS? In my forms, the CVS is only used in conjunction with ItemsControls since it is displaying a collection of items. If your edit forms controls are dissociated from the CVS, you should not have much trouble populating them with a new entity and when it comes time to save, check to see if CVS is null and if so, create it then populate it.

If that is not a good answer, could you expand on how your application is structured?

Instead of opening the Window in AddNew mode I disable all data entry controls when the window loads or the when a search returns no records. When the "Add New Record" button is clicked I ALWAYS start over with a new data context that contains just one new entity. This means I have to prompt to save changes if any dirty (modified) records exist. The prompt allows the user to save changes, discard changes or continue editing (never entering AddNew mode). Here is the AddNew code:

MessageBoxResult response = PromptToSaveChanges(ReasonForPromptToSave.LoadingData);

if (response == MessageBoxResult.Cancel)  return;           

LabSample newEntity = _labEntitiesContext.LabSamples.CreateObject<LabSample>();
_labEntitiesContext.LabSamples.AddObject(newEntity);

_labSamplesCollectionViewSource.Source = new ObservableCollection<LabSample>();
_labSamplesListCollectionView = (ListCollectionView)_labSamplesCollectionViewSource.View;

_labSamplesListCollectionView.AddNewItem(newEntity);
_labSamplesListCollectionView.CommitNew();
_labSamplesListCollectionView.Refresh();

Here are my steps to put the window in AddNew mode:

1) Prompt to save changes.

2) Create a new entity and add it to my data context.

3) Create a new ObservableCollection of my entity type and assign it to the .Source of my CollectionViewSource. Note the _labSamplesCollectionViewSource is a reference to the XAML's CollectionViewSource that was auto-generated by dragging a table from the data sources window.

4) Assign the .View of the CollectionViewSource to a class level ListCollectionView variable.

5) Add the new entity to the ListCollectionView that was just created.

6) Call CommitNew and Refresh on the ListCollectionView

I have answered my own question here, but keep in mind that the answer is the result of trial and error and may not be ideal. Regarding my confusion as to how the _labEntitiesContext is hooked up to the CollectionViewSource I believe the answer is in the line that reads _labSamplesListCollectionView.AddNewItem(newEntity), but I'd like to see an explanation of how all of the objects reference each other.

My final comment/question is that I'm disappointed at how hard it is to find a standard reference application or document that teaches non-MVVM WPF/Entity Framework databinding in detail. Microsoft promotes drag-and-drop binding but leaves us without a reference on how to build a complete application. I'll move on to MVVM soon, meanwhile if anyone can direct me to a GREAT resource or feature complete application that is WPF, non-MVVM and Entity Framework I would greatly appreciate it.