Deleting Cars from the Database

This article is relevant to entity models that utilize the deprecated Visual Studio integration of Telerik Data Access. The current documentation of the Data Access framework is available here.

In this step you will add functionality to MainViewModel which will allow you to delete cars from the database. You will need to implement the methods required for that and define properties which expose commands based on them.

• Defining the required methods
• Exposing commands to the view

Defining the required methods

1. In MainViewModel add a new method called DeleteCar. This method displays a confirmation message box and if the delete operation is confirmed, the currently selected car is deleted from the database. It will be used to form the delete command.

   private void DeleteCar()
   {
       Car carToDelete = this.SelectedCar;
       MessageBoxResult deleteConfirmation = MessageBox.
           Show(string.Format(CONFIRM_DELETE_MESSAGE, carToDelete.Make, carToDelete.Model),
                              CONFIRM_DELETE_TITLE, MessageBoxButton.YesNo, MessageBoxImage.Question);
       if (deleteConfirmation != MessageBoxResult.Yes)
       {
           return;
       }
       this.context.Delete(carToDelete);
       bool saveChangesIsSuccessful = false;
       try
       {
           this.context.SaveChanges();
           saveChangesIsSuccessful = true;
       }
       catch (DataStoreException ex)
       {
           MessageBox.Show(DELETE_ERROR_RELATED_ENTRY_MESSAGE + ex.BackendError.Description,
                           DELETE_ERROR_TITLE, MessageBoxButton.OK, MessageBoxImage.Error);
       }
       catch (Exception ex)
       {
           MessageBox.Show(DELETE_ERROR_MESSAGE + ex.Message, DELETE_ERROR_TITLE,
                           MessageBoxButton.OK, MessageBoxImage.Error);
       }
       if (saveChangesIsSuccessful)
       {
           this.RetrieveCarsToDisplay();
       }
   }
   

   Private Sub DeleteCar()
       Dim carToDelete As Car = Me.SelectedCar
       Dim deleteConfirmation As MessageBoxResult = MessageBox.
           Show(String.Format(CONFIRM_DELETE_MESSAGE, carToDelete.Make, carToDelete.Model),
                CONFIRM_DELETE_TITLE, MessageBoxButton.YesNo, MessageBoxImage.Question)
       If deleteConfirmation <> MessageBoxResult.Yes Then
           Exit Sub
       End If
       Me._context.Delete(carToDelete)
       Dim saveChangesIsSuccessful As Boolean = False
       Try
           Me._context.SaveChanges()
           saveChangesIsSuccessful = True
       Catch ex As DataStoreException
           MessageBox.Show(DELETE_ERROR_RELATED_ENTRY_MESSAGE + ex.BackendError.Description,
                           DELETE_ERROR_TITLE, MessageBoxButton.OK, MessageBoxImage.Error)
       Catch ex As Exception
           MessageBox.Show(DELETE_ERROR_MESSAGE + ex.Message,
                           DELETE_ERROR_TITLE, MessageBoxButton.OK, MessageBoxImage.Error)
       End Try
       If saveChangesIsSuccessful Then
           Me.RetrieveCarsToDisplay()
       End If
   End Sub
   

2. In MainViewModel add a new method called IsCarSelected. This method will be used to determine if the delete command can be executed. The delete command can be executed only when there is a selected car available.

   private bool IsCarSelected()
   {
       return this.SelectedCar != null;
   }
   

   Private Function IsCarSelected() As Boolean
       Return IsNothing(Me.SelectedCar) = False
   End Function
   

Exposing commands to the view

In MainViewModel define a property DeleteCommand. In its getter initialize a RelayCommand object based on the DeleteCar and IsCarSelected methods and return it.

public RelayCommand DeleteCarCommand
{
    get
    {
        this.command = new RelayCommand(this.DeleteCar, this.IsCarSelected);
        return this.command;
    }
    set
    {
        this.command = value;
    }
}

Public Property DeleteCarCommand As RelayCommand
    Get
        Me._command = New RelayCommand(AddressOf Me.DeleteCar, AddressOf Me.IsCarSelected)
        Return Me._command
    End Get
    Set(value As RelayCommand)
        Me._command = value
    End Set
End Property

Checkpoint

The sample application can now delete cars from the database.

Next step: Preparing AddEditViewModel