Question

At the moment I have a Silverlight application in Windows Phone which uses the local database. Basically a listBox is generated to show the current list of stored ‘clients’, which works fine. Now I would like to be able to let the user Edit one of the client’s details. For this, I created a new page which is loaded whenever a user clicks on a button from the main page. The event is the following:

public ClientItem selectedClient;

public void Edit_Click(object sender, EventArgs e)
    {
        if (clientItemsListBox.SelectedItem != null)
        {
            selectedClient = clientItemsListBox.SelectedItem as ClientItem;

            NavigationService.Navigate(new Uri("/EditClient.xaml", UriKind.Relative));
        }
    }

The above simply checks which client is selected, stores it as selectedClient and navigates to the EditClient page.

In the EditClient class I have the following method:

 public void saveButton_Click(object sender, RoutedEventArgs e)
    {
        //Get the client that is selected

        ClientItem clientForDelete = mainPage.selectedClient;
        mainPage.ClientItems.Remove(clientForDelete);
        mainPage.clientDB.ClientItems.DeleteOnSubmit(clientForDelete);

        // Create a new client based on the text boxes
        ClientItem newClient = new ClientItem { ClientName = newClientNameTextBox.Text, ClientSurname = newClientSurnameTextBox.Text, ClientCompany = newClientCompanyTextBox.Text, ClientPhone = int.Parse(newClientPhoneTextBox.Text) };

        // Add new client to the database
        mainPage.clientDB.ClientItems.InsertOnSubmit(newClient);

        // Sava database changes
        mainPage.clientDB.SubmitChanges();

        // Go to main screen
        NavigationService.Navigate(new Uri("/MainPage.xaml", UriKind.Relative));
    }

When I run this code I get a nullReferenceException when it tries to execute:

mainPage.ClientItems.Remove(clientForDelete);

This is because the selectedClient is null. How can I get the object from the other class without it being null? Because I do not want to delete the item from the main class just in case the user decides to cancel the operation. Also I want to show the details of that client when the page is loaded, which I know how to do if I manage to get the object :). thanks

Answer 1

You can pass the client to the other page as query parameter:

   NavigationService.Navigate(
      new Uri("/MainPage.xaml?client="+clientId, UriKind.Relative));

Then in your MainPage OnNavigatedTo() retrieve the client:

   if (this.NavigationContext.QueryString.ContainsKey("client"))
          client = this.NavigationContext.QueryString["client"];