Retrieving Cars from the Database and Disposing the Context

In this step you will add functionality to the MainViewModel which allows the retrieval of cars from the database and disposes of the context when it is no longer needed.

1. In the constructor of the model add a call to a method which will be defined shortly - RetrieveCarsToDisplay().

   this.RetrieveCarsToDisplay();
   

   Me.RetrieveCarsToDisplay()
   

2. Define the RetrieveCarsToDisplay() method. It saves the id of the currently selected car, retrieves the cars from the database according to the currently active filter and then restores the selected car using its id. At the end it updates the status according to the number of retrieved cars. As you can see, the retrieved cars are exposed through the CarsToDisplay property to which a RadGridView control on MainWindow is bound.

   private void RetrieveCarsToDisplay()
   {
       this.ClearCache();
       int? selectedCarId = null;
       //save the Id of the currently selected car
       if (this.SelectedCar != null)
       {
           selectedCarId = this.SelectedCar.CarID;
       }
       if (string.IsNullOrEmpty(this.carMakerFilter))
       {
           //retrieve all cars
           this.CarsToDisplay = this.context.Cars.ToList();
       }
       else
       {
           //retrieve cars which comply to the filter
           this.CarsToDisplay = this.context.Cars.Where(car => car.Make == this.carMakerFilter).ToList();
       }
       //restore the selected car using its saved Id
       if (selectedCarId.HasValue)
       {
           this.SelectedCar = this.CarsToDisplay.FirstOrDefault(car => car.CarID == selectedCarId);
       }
       this.Status = string.Format(STATUS_MESSAGE, this.CarsToDisplay.Count); 
   }
   

   Private Sub RetrieveCarsToDisplay()
       Me.ClearCache()
       Dim selectedCarId As Integer? = Nothing
       'save the Id of the currently selected car
       If IsNothing(Me.SelectedCar) = False Then
           selectedCarId = Me.SelectedCar.CarID
       End If
       If String.IsNullOrEmpty(Me.CarMakerFilter) Then
           'retrieve all cars
           Me.CarsToDisplay = Me._context.Cars.ToList()
       Else
           'retrieve cars which comply to the filter
           Me.CarsToDisplay = Me._context.Cars.Where(Function(car) car.Make = Me.CarMakerFilter).
                               ToList()
       End If
       'restore the selected car using its saved Id
       If selectedCarId.HasValue Then
           Me.SelectedCar = Me.CarsToDisplay.FirstOrDefault(Function(car) car.CarID = selectedCarId)
       End If
       Me.Status = String.Format(STATUS_MESSAGE, Me.CarsToDisplay.Count)
   End Sub
   

3. Define the ClearCache() method. It is called from RetrieveCarsToDisplay and clears the context cache, so that changes to single properties can be detected whenever retrieving cars.

   private void ClearCache()
   {
       if (this.CarsToDisplay != null)
       {
           this.context.ClearChanges();
       }
   }
   

   Private Sub ClearCache()
       If IsNothing(Me._context) = False Then
           Me._context.ClearChanges()
       End If
   End Sub
   

Disposing the context

Whenever an OpenAccessContext object is initialized, a connection to the database is opened. This connection stays opened as long as the Context object lives. It is a good practice to dispose of the Context object when it is no longer needed. In the current view model the context is no longer needed once the view closes and the view model is disposed.

1. In MainViewModel implement the IDisposable interface to make it disposable. In the Dispose method, dispose the OpenAccessContext object. This way when the view model is no longer needed and thus disposed, the context will be disposed as well.

   public void Dispose()
   {
       //The context is local to the ViewModel and must be disposed together with it
       if (this.context != null)
       {
           this.context.Dispose();
       }
   }
   

   Public Sub Dispose() Implements IDisposable.Dispose
       'The context is local to the ViewModel and must be disposed together with it
       If IsNothing(Me._context) = False Then
           _context.Dispose()
       End If
   End Sub
   

Next step: Initializing MainViewModel