Question

I am figuring out binding in WPF, and running into an issue with object binding.

I have a combobox with an itemsource set to a list of Users

ICollection<User> users = User.GetAll();
cmbContacts.ItemsSource = users;      

I also have an object in my UI which holds the selected user.

public partial class MainWindow : Window
{

    private User selectedUser = new User();

    public MainWindow()
    {
        InitializeComponent();
        ReloadContents();

        Binding b = new Binding();
        b.Source = selectedUser;
        b.Path = new PropertyPath("uFirstName");
        this.txtFirstName.SetBinding(TextBox.TextProperty, b);
   }

And in my combobox’s SelectChanged method…

selectedUser = (User)e.AddedItems[0];

However, the textbox is not updating! I can verify that my binding works by moving the binding code to the combobox SelectChanged methods

selectedUser = (User)e.AddedItems[0];    
Binding b = new Binding();
b.Source = selectedUser;
b.Path = new PropertyPath("uFirstName");
this.txtFirstName.SetBinding(TextBox.TextProperty, b);

Now the textbox updates fine. This seems like the incorrect way of doing things. Can anyone point me in the right direction?

Answer 1

In your code I see one bug – when you set field selectedUser you don’t notify that this data has been changed. Your sample should looks like this:

public partial class MainWindow : Window, INotifyPropertyChanged
{ 
    private User selectedUser;

    public User SelectedUser 
    {
       get 
       {
           return selectedUser;
       }
       set
       {
           selectedUser = value;
           NotifyPropertyChanged("SelectedUser");
       }
    }

    public MainWindow() 
    { 
        InitializeComponent(); 
        ReloadContents(); 

        // Now the source is the current object (Window), which implements
        // INotifyPropertyChanged and can tell to WPF infrastracture when 
        // SelectedUser property will change value
        Binding b = new Binding(); 
        b.Source = this; 
        b.Path = new PropertyPath("SelectedUser.uFirstName"); 
        this.txtFirstName.SetBinding(TextBox.TextProperty, b); 
   } 

   public event PropertyChangedEventHandler PropertyChanged;

   private void NotifyPropertyChanged(string info)
   {
        if (PropertyChanged != null)
        {
            PropertyChanged(this, new PropertyChangedEventArgs(info));
        }
   }
}

And don’t forget that now you will need to set new value to the property, don’t use field, so the SelectChanged should looks like:

SelectedUser = (User)e.AddedItems[0];