C# – Winforms Binding a ComboBox SelectedItem to an Object Property

ccomboboxdata-bindingwinforms

I have two simple classes:

public class Customer
{
    public String CustomerID { get; set; }
    public String Forename { get; set; }
    public String Surname { get; set; }
}

and

public class Order
{
    public String OrderID { get; set; }
    public Decimal Value { get; set; }
    public Customer OrderedBy { get; set; }
}

I then create a list of Customer objects:

List<Customer> customers = new List<Customer>();
customers.Add(new Customer() { CustomerID = "1", Forename = "John", Surname = "Smith"});
customers.Add(new Customer() { CustomerID = "2", Forename = "Jeremy", Surname = "Smith" });

And I have a combo box, against which I set the data source to be my list of Customers, and the DisplayMember to be the Forename property of the Customer object:

comboBox1.DisplayMember = "Forename";
comboBox1.DataSource = customers;

And the result is a combo box with two items, "John" and "Jeremy". Up to now I'm not too confused.

What I would like to be able to do though, is set the "OrderedBy" property of an instance of Order, based on the selection from the Combobox – Can complex types be bound to ComboBoxes like this?

I've tried this, but it doesnt seem to be updating the OrderedBy property of the Order instance:

Order myOrder = new Order();
comboBox1.DataBindings.Add("SelectedItem", myOrder, "OrderedBy");

I dont know if what I'm trying to do is possible, or if it is beyond the capabilities of Data Binding in WinForms.

I'd like to avoid having to update my Order object as part of an event handler on the ComboBox, and solely use Data Binding if possible.

Best Answer

Your code will update a property of the bounded object, but only after ComboBox loses focus.

If you want to update the property straight after the SelectedItem changed, the fastest approach will be to send a message about changes manually.
In a ComboBox, when the SelectedItemchanges, it will fire the SelectionChangesCommitted event.

You can create an event handler to take care of the change and manually call the binding:

private void combobox1_SelectionChangesCommitted(Object sender, EventArgs e)
{
    ((ComboBox)sender).DataBindings["SelectedItem"].WriteValue();
}

You could also use the ComboBox.ValueMember property and bind your object property to the SelectedValue.

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Since C# 6.0, you can specify initial value in-line. The syntax is:

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C# – Binding a WPF ComboBox to a custom list

You set the DisplayMemberPath and the SelectedValuePath to "Name", so I assume that you have a class PhoneBookEntry with a public property Name.

Have you set the DataContext to your ConnectionViewModel object?

I copied you code and made some minor modifications, and it seems to work fine. I can set the viewmodels PhoneBookEnty property and the selected item in the combobox changes, and I can change the selected item in the combobox and the view models PhoneBookEntry property is set correctly.

Here is my XAML content:

<Window x:Class="WpfApplication6.Window1"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        Title="Window1" Height="300" Width="300">
<Grid>
    <StackPanel>
        <Button Click="Button_Click">asdf</Button>
        <ComboBox ItemsSource="{Binding Path=PhonebookEntries}"
                  DisplayMemberPath="Name"
                  SelectedValuePath="Name"
                  SelectedValue="{Binding Path=PhonebookEntry}" />
    </StackPanel>
</Grid>
</Window>

And here is my code-behind:

namespace WpfApplication6
{

    /// <summary>
    /// Interaction logic for Window1.xaml
    /// </summary>
    public partial class Window1 : Window
    {
        public Window1()
        {
            InitializeComponent();
            ConnectionViewModel vm = new ConnectionViewModel();
            DataContext = vm;
        }

        private void Button_Click(object sender, RoutedEventArgs e)
        {
            ((ConnectionViewModel)DataContext).PhonebookEntry = "test";
        }
    }

    public class PhoneBookEntry
    {
        public string Name { get; set; }

        public PhoneBookEntry(string name)
        {
            Name = name;
        }

        public override string ToString()
        {
            return Name;
        }
    }

    public class ConnectionViewModel : INotifyPropertyChanged
    {
        public ConnectionViewModel()
        {
            IList<PhoneBookEntry> list = new List<PhoneBookEntry>();
            list.Add(new PhoneBookEntry("test"));
            list.Add(new PhoneBookEntry("test2"));
            _phonebookEntries = new CollectionView(list);
        }

        private readonly CollectionView _phonebookEntries;
        private string _phonebookEntry;

        public CollectionView PhonebookEntries
        {
            get { return _phonebookEntries; }
        }

        public string PhonebookEntry
        {
            get { return _phonebookEntry; }
            set
            {
                if (_phonebookEntry == value) return;
                _phonebookEntry = value;
                OnPropertyChanged("PhonebookEntry");
            }
        }

        private void OnPropertyChanged(string propertyName)
        {
            if (PropertyChanged != null)
                PropertyChanged(this, new PropertyChangedEventArgs(propertyName));
        }
        public event PropertyChangedEventHandler PropertyChanged;
    }
}

Edit: Geoffs second example does not seem to work, which seems a bit odd to me. If I change the PhonebookEntries property on the ConnectionViewModel to be of type ReadOnlyCollection, the TwoWay binding of the SelectedValue property on the combobox works fine.

Maybe there is an issue with the CollectionView? I noticed a warning in the output console:

System.Windows.Data Warning: 50 : Using CollectionView directly is not fully supported. The basic features work, although with some inefficiencies, but advanced features may encounter known bugs. Consider using a derived class to avoid these problems.

Edit2 (.NET 4.5): The content of the DropDownList can be based on ToString() and not of DisplayMemberPath, while DisplayMemberPath specifies the member for the selected and displayed item only.

Best Answer

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