Deleting Cars from the Database

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

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

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