WPF ComboBox SelectedItem – change to previous value

comboboxselecteditemwpf

I have a ComboBox that has the SelectedItem bound to the ViewModel.

<ComboBox SelectedItem="{Binding SelItem, Mode=TwoWay}" ItemsSource="{Binding MyItems}">

When the user selects a new Item in the View ComboBox, I want to display a prompt and verify that they want to make the change.

In the SetItem Property setter in the View Model, I display a Dialog to confirm the selection. When they say yes, it works fine.

My problem is, when the user clicks on "No" I am not sure who to get the ComboBox
to revert back to the previous value. The Property in the ViewModel has the correct
older value, however in the View the ComboBox displays the newly Selected Value.

I want the user to select an item, confirm they want to go ahead with it, and if they
decide not to, I want the ComboBox to revert back to the previous item.

How can I accomplish this?
Thanks!

Best Answer

When the user says "no", WPF is unaware that the value has changed. As far as WPF is concerned, the value is whatever the user selected.

You might try raising a property changed notification:

public object SelItem
{
    get { ... }
    set
    {
        if (!CancelChange())
        {
            this.selItem = value;
        }

        OnPropertyChanged("SelItem");
    }
}

The problem is, the change notification happens within the same context of the selection event. Thus, WPF ignores it because it already knows the property has changed - to the item the user selected!

What you need to do is raise the notification event in a separate message:

public object SelItem
{
    get { ... }
    set
    {
        if (CancelChange())
        {
            Dispatcher.BeginInvoke((ThreadStart)delegate
            {
                OnPropertyChanged("SelItem");
            });
            return;
        }

        this.selItem = value;
        OnPropertyChanged("SelItem");
    }
}

WPF will then process this message after it's done processing the selection changed event and will therefore revert the value in the view back to what it should be.

Your VM will obviously need access to the current Dispatcher. See my blog post on a base VM class if you need some pointers on how to do this.

Related Solutions

.net – Databinding an enum property to a ComboBox in WPF

You can create a custom markup extension.

Example of usage:

enum Status
{
    [Description("Available.")]
    Available,
    [Description("Not here right now.")]
    Away,
    [Description("I don't have time right now.")]
    Busy
}

At the top of your XAML:

    xmlns:my="clr-namespace:namespace_to_enumeration_extension_class

and then...

<ComboBox 
    ItemsSource="{Binding Source={my:Enumeration {x:Type my:Status}}}" 
    DisplayMemberPath="Description" 
    SelectedValue="{Binding CurrentStatus}"  
    SelectedValuePath="Value"  />

And the implementation...

public class EnumerationExtension : MarkupExtension
  {
    private Type _enumType;


    public EnumerationExtension(Type enumType)
    {
      if (enumType == null)
        throw new ArgumentNullException("enumType");

      EnumType = enumType;
    }

    public Type EnumType
    {
      get { return _enumType; }
      private set
      {
        if (_enumType == value)
          return;

        var enumType = Nullable.GetUnderlyingType(value) ?? value;

        if (enumType.IsEnum == false)
          throw new ArgumentException("Type must be an Enum.");

        _enumType = value;
      }
    }

    public override object ProvideValue(IServiceProvider serviceProvider)
    {
      var enumValues = Enum.GetValues(EnumType);

      return (
        from object enumValue in enumValues
        select new EnumerationMember{
          Value = enumValue,
          Description = GetDescription(enumValue)
        }).ToArray();
    }

    private string GetDescription(object enumValue)
    {
      var descriptionAttribute = EnumType
        .GetField(enumValue.ToString())
        .GetCustomAttributes(typeof (DescriptionAttribute), false)
        .FirstOrDefault() as DescriptionAttribute;


      return descriptionAttribute != null
        ? descriptionAttribute.Description
        : enumValue.ToString();
    }

    public class EnumerationMember
    {
      public string Description { get; set; }
      public object Value { get; set; }
    }
  }

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

Related Solutions

.net – Databinding an enum property to a ComboBox in WPF

C# – Binding a WPF ComboBox to a custom list

Related Topic