在数据网格之间切换时为空指针

Question

In a program I'm writing, there are two data grids on wpf form. One contains pet owners and the other contains the actual pets. When the user clicks on an owner, the pet data grid is populated with pets they own through a SQL database. The currently selected owner ID is saved to a variable. The same is done when the user clicks on a pet, the pet ID is saved to a variable.

Now, the problem I'm having is when an owners pet is selected. When clicking a different owner I get a null pointer exception and I'm not entirely sure why.'

Below is some of the code. It crashes on the PetDg_SelectionChanged method. Any help would be appreciated. Thanks!

public partial class UpdateWindowNew : Window
{
    int currentOwnerPk;
    int currentPetPk;

    public UpdateWindowNew()
    {
        InitializeComponent();
    }

    public void Window_Loaded(object sender, RoutedEventArgs e)
    {
        string conString = ConfigurationManager.ConnectionStrings["conString"].ConnectionString;
        string cmdString = "empty";

        using (SqlConnection con = new SqlConnection(conString))
        {
            cmdString = "Select * from Owner";
            SqlCommand query = new SqlCommand(cmdString, con);
            SqlDataAdapter gridAdapter = new SqlDataAdapter(query);
            DataTable dt = new DataTable("Owner");
            gridAdapter.Fill(dt);
            ownerDg.ItemsSource = dt.DefaultView;
        }
    }

    private void ownerDG_SelectionChanged(object sender, SelectionChangedEventArgs e)
    {
        currentOwnerPk = (int)(ownerDg.SelectedItem as DataRowView).Row.ItemArray[0];
        OwnerFname.Text = (string)(ownerDg.SelectedItem as DataRowView).Row.ItemArray[1];
        OwnerLname.Text = (string)(ownerDg.SelectedItem as DataRowView).Row.ItemArray[2];

        showOwnedPets(currentOwnerPk);
    }

    private void PetDg_SelectionChanged(object sender, SelectionChangedEventArgs e)
    {
        currentPetPk = (int)(PetDg.SelectedItem as DataRowView).Row.ItemArray[0];
    }

Answer 1

Its very simple, because when you select another owner, pet DataGrid get empty and fire this exception, you need to check for null value for selected item in PetDg_SelectionChanged event handler:

currentPetPk = PetDg.SelectedItem is DataRowView item ?
    (int)item.Row.ItemArray[0] : -1;

Answer 2

SelectedItem is probably null. You should check for it, with the Null-conditional Operator '?.'.

currentOwnerPk = (int)(ownerDg?.SelectedItem as DataRowView)?.Row.ItemArray[0] ?? -1;