C# – ComboBox SelectedIndex MVVM WPF

ccomboboxmvvmwpfxaml

I have a ComboBox bound to an ObservableCollection of tbPublications which populates as it should. I then select a row from a DataGrid which fires another Create form in which I insert a new record into tbPublications, all good.

When I close said create form and return to my ComboBox form I am clearing and re-reading in the one new item to my ObservableCollection, returning the user to the item they've just created. The ComboBox then displays the one item from my newly populated collection, all good.

My problem is that on returning to my ComboBox form, the new publication is not set as selected item in the ComboBox display, the user has to click the ComboBox then select the item.

I can't use SelectedIndex = "0" in my view XAML as I want to show the whole ObservableCollection in my ComboBox on page load.

Is there any way to use a method in the ViewModel maybe to solve this issue, something maybe such as..

      private void SetSelectedIndex()
      {
        if (MyObservableCollection.Count == 1)
        {
            //Set selected indexer to "0";
        }
      }

Found a solution to this, not sure if it's the cleanest 'MVVM' solution…

After reading in my ObservableCollection I invoke this method:

 if (_ModelPublicationsObservableList.Count == 1)
                {
                    SelectedPublication = _ModelPublication;
                    SetSelectedIndex();
                }

Here's the method which gets the current main window and sets the SelectedIndex:

 private void SetSelectedIndex()
    {
        ArticlesDataGridWindow singleOrDefault = (ComboBoxWindow)Application.Current.Windows.OfType<ComboBoxWindow>().SingleOrDefault(x => x.IsLoaded);
        singleOrDefault.comboBox1.SelectedIndex = 0;        
    }

Best Answer

Did you consider using the SelectedItem property of combobox? You can bind the selected item property of combobox to get or set the selected item.

XAML

<ComboBox ItemsSource="{Binding Path=Publications}" SelectedItem="{Binding Path=SelectedPublication, Mode=TwoWay}" />

ViewModel

public class ItemListViewModel
{
    public ObservableCollection<Publication> Publications {get; set;}

    private Publication _selectedPublication;
    public Publication SelectedPublication 
    {
        get { return _selectedPublication; }
        set
        {
            if (_selectedPublication== value) return;
            _selectedPublication= value;
            RaisePropertyChanged("SelectedPublication");
        }
    }
}

If you want to set the selected item from View model,You can set the SelectedPublication property as-

SelectedPublication = Publications[0];

Or you can locate the required item in the Publications collection and assign it to SelectedPublication property.

Related Solutions

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.

WPF MVVM ComboBox SelectedItem or SelectedValue not working

Setting IsSynchronizedWithCurrentItem="True" worked for me!

Best Answer

Related Solutions

C# – Binding a WPF ComboBox to a custom list

WPF MVVM ComboBox SelectedItem or SelectedValue not working

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