WPF MVVM：ObservableCollectionの変更をUIに反映させる方法

私はWPFが比較的新しく、MVVMで作業しているアプリケーションを構築するために、MVVMパターンとObservableCollectionでのデータバインディングのしくみを理解しようとしています。メインウィンドウを持つアプリケーションのサンプルを作成しました。ここでは、ユーザーがどのボタンを押すかに応じて、異なるビュー（UserControl）が表示されます。一般的な考え方は、ユーザがデータベースからいくつかの要素のデータにアクセスし（例：顧客、製品など）、既存の要素を新規に追加したり編集したり削除したりできるようになるというものです。

So, there is a CustomerView, with its CustomerViewModel, and a ProductView, with its ProductViewModel respectively. Also, there are two classes (Customer.cs & Product.cs) that represent the Models. The structure of the project is displayed here.

MainWindow.xamlは次のとおりです。

そしてMainWindow.xaml.csの背後にあるコード：

public partial class MainWindow : Window
{
    public CustomerViewModel customerVM;
    public ProductViewModel productVM;

    public MainWindow()
    {
        InitializeComponent();
    }

    private void btnCustomers_Click(object sender, RoutedEventArgs e)
    {
        if (customerVM == null)
        {
            customerVM = new CustomerViewModel();
        }
        this.DataContext = customerVM;
    }

    private void btnProducts_Click(object sender, RoutedEventArgs e)
    {
        if (productVM == null)
        {
            productVM = new ProductViewModel();
        }
        this.DataContext = productVM;
    }
}

最後に、CustomerView.xamlは次のとおりです。

そしてCustomerViewModel.cs：

public class CustomerViewModel : ObservableCollection
{
    public CustomerViewModel()
    {
        LoadCustomers();
    }

    private void LoadCustomers()
    {
        for (int i = 1; i <= 5; i++)
        {
            var customer = new Customer()
            {
                Id = i,
                FirstName = "Customer_" + i.ToString(),
                LastName = "Surname_" + i.ToString()
            };
            this.Add(customer);
        }
    }

    public void AddNewCustomer(int id)
    {
        var customer = new Customer()
        {
            Id = id,
            FirstName = "Customer_" + id.ToString(),
            LastName = "Surname_" + id.ToString()
        };
        Add(customer);
    }
}

Please note that the ProductView.xaml & ProductViewModel.cs are similar. Currently, when the user presses the "Customers" or the "Products" button of the MainWindow, then the respective View is displayed and the collections are loaded according to the LoadCustomers (or LoadProducts) method, which is called by the ViewModel's constructor. Also, when the user selects a different object from the ComboBox, then its properties are displayed correctly (i.e. Id, Name, etc.). The problem is when the user adds a new (or deletes an existing) element.

Question 1: Which is the correct and best way to update a changed Observable Collection of an element and reflect its changes in the UI (Combobox, properties, etc.)?

Question 2: During testing this project I noticed that the constructor of the ViewModels (consequently the LoadCustomers & LoadProducts method) are called twice. However, it is only called when the user presses the Customers or the Products button respectively. Is it also called via the XAML data binding? Is this the optimum implementation?