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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

  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)
        bool saveChangesIsSuccessful = false;
            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)
    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
        Dim saveChangesIsSuccessful As Boolean = False
            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
        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
        this.command = new RelayCommand(this.DeleteCar, this.IsCarSelected);
        return this.command;
        this.command = value;
Public Property DeleteCarCommand As RelayCommand
        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


The sample application can now delete cars from the database.

Next step: Preparing AddEditViewModel