WPF - How to clear items before repopulating combobox

Question

I am trying to make cascading combo boxes. When I select values in combo boxes for the first time everything is OK, but when I return to first combo box and change to some other value, new items in the other combos are appended to the old items. This is my code bellow:

public Apartment()
    {
        InitializeComponent();
    }
    SqlConnection con = new SqlConnection(Connection.cnnDatabase1);
    Database1DataSet ds = new Database1DataSet();

 private void FillLocation()
    {
        //locationCombo.Items.Clear() 
        SqlDataAdapter daLocation= new SqlDataAdapter("select * from Location", con);

        daLocation.Fill(ds, "Location");
        con.Open();
        locationCombo.ItemsSource = ds.Tables["Location"].DefaultView;
        locationCombo.DisplayMemberPath = "Location";
        locationCombo.SelectedValuePath = "IdLocation";
        con.Close();
    }
    private void FillCity(String IdLocation)
    {
        /*cityCombo.Items.Clear() -- I have tried inserting this, 
        but I am getting an error "Operation is not valid while ItemsSource is in use. 
        Access and modify elements with ItemsControl.ItemsSource instead." on that part 
        when I reselect the combo.*/
        SqlDataAdapter daCity= new SqlDataAdapter("select * from City where IdLocation= " + IdLocation, con);

        daCity.Fill(ds, "City");
        con.Open();
        cityCombo.ItemsSource = ds.Tables["City"].DefaultView;
        cityCombo.DisplayMemberPath = "City";
        cityCombo.SelectedValuePath = "IdCity";
        con.Close();
    }
    private void FillStreet(String IdCity)
    {
        //cityCombo.Items.Clear()
        SqlDataAdapter daStreet= new SqlDataAdapter("select * from Street where IdCity= " + IdCity, con);

        daStreet.Fill(ds, "Street");
        con.Open();
        cityCombo.ItemsSource = ds.Tables["Street"].DefaultView;
        cityCombo.DisplayMemberPath = "Street";
        cityCombo.SelectedValuePath = "IdStreet";
        con.Close();
    }

private void Window_Loaded_1(object sender, RoutedEventArgs e)
    {
        FillLocation();
    }

    private void locationCombo_SelectionChanged(object sender, SelectionChangedEventArgs e)
    {
        String idLocation= locationCombo.SelectedValue.ToString();
        FillCity(idLocation);

    }
    private void cityCombo_SelectionChanged(object sender, SelectionChangedEventArgs e)
    {
        String idCity = cityCombo.SelectedValue.ToString();
        FillStreet(idCity);

    }

Answer 1

Since cityCombo.Items.Clear() is not an option in this case as mentioned in the code comments, you can clear the datatable instead:

ds.Tables["City"].Clear();

And in order to avoid the NullReferenceException in cityCombo_SelectionChanged you can check the cityCombo.SelectedValue for null.

Answer 2

Try this:

public Apartment()
{
    InitializeComponent();
}
SqlConnection con = new SqlConnection(Connection.cnnDatabase1);
Database1DataSet ds = new Database1DataSet();

private void FillLocation()
{
    //locationCombo.Items.Clear() 
    SqlDataAdapter daLocation= new SqlDataAdapter("select * from Location", con);

    ds.Tables["Location"].Clear();

    daLocation.Fill(ds, "Location");
    con.Open();
    locationCombo.ItemsSource = ds.Tables["Location"].DefaultView;
    locationCombo.DisplayMemberPath = "Location";
    locationCombo.SelectedValuePath = "IdLocation";
    con.Close();
}
private void FillCity(String IdLocation)
{
    /*cityCombo.Items.Clear() -- I have tried inserting this, 
    but I am getting an error "Operation is not valid while ItemsSource is in use. 
    Access and modify elements with ItemsControl.ItemsSource instead." on that part 
    when I reselect the combo.*/
    if(!String.IsNullOrWhiteSpace(IdLocation))
    {
        ds.Tables["City"].Clear();
        SqlDataAdapter daCity= new SqlDataAdapter("select * from City where IdLocation= " + IdLocation, con);

        daCity.Fill(ds, "City");
        con.Open();
        cityCombo.ItemsSource = ds.Tables["City"].DefaultView;
        cityCombo.DisplayMemberPath = "City";
        cityCombo.SelectedValuePath = "IdCity";
        con.Close();
    }
}
private void FillStreet(String IdCity)
{
    if(!String.IsNullOrWhiteSpace(IdCity))
    {
        s.Tables["Street"].Clear();
        SqlDataAdapter daStreet= new SqlDataAdapter("select * from Street where IdCity= " + IdCity, con);

        daStreet.Fill(ds, "Street");
        con.Open();
        cityCombo.ItemsSource = ds.Tables["Street"].DefaultView;
        cityCombo.DisplayMemberPath = "Street";
        cityCombo.SelectedValuePath = "IdStreet";
        con.Close();
    }
}

private void Window_Loaded_1(object sender, RoutedEventArgs e)
{
    FillLocation();
}

private void locationCombo_SelectionChanged(object sender, SelectionChangedEventArgs e)
{
    if(locationCombo.SelectedValue != null)
    {
        String idLocation= locationCombo.SelectedValue.ToString();
        FillCity(idLocation);
    }

}
private void cityCombo_SelectionChanged(object sender, SelectionChangedEventArgs e)
{
    if(cityCombo.SelectedValue != null)
    {
        String idCity = cityCombo.SelectedValue.ToString();
        FillStreet(idCity);
    }
}

Answer 3

You are reusing the DataSet everywhere. So it will keep appending. As I said in the comments, either clear the DataSet or make a local one to each function.