C# – Setting a default selected item in ComboBox in WPF MVVM application

ccomboboxmvvmwpf

I've been stuck on this problem for hours… what I want to do is actually quite simple – set a default selected item in a ComboBox (I'm using the MVVM pattern).

I have the following XAML for the ComboBox in my View:

<ComboBox ItemsSource="{Binding Schools}" 
          DisplayMemberPath="Acronym" 
          SelectedValue="{Binding SelectedSchool}" 
          SelectedValuePath="Id" 
/>

In my ViewModel, I have an ObservableCollection, Schools:

 public ObservableCollection<School> Schools { get; private set; }

    public CourseFormViewModel()
    {
        Schools = new ObservableCollection<School>();

        try
        {
            // Gets schools from a web service and adds them to the Schools ObservableCollection
            PopulateSchools();
        }
        catch (Exception ex)
        {
            // ...
        }
    }

    public int SelectedSchool
    {
        get { return schoolId; }
        set
        {
            schoolId = value;
            OnPropertyChanged("SelectedSchool");
        }
    }

Finally, School is a simple business object:

[DataContract]
public class School
{
    [DataMember]
    public int Id { get; set; }
    [DataMember]
    public string Acronym { get; set; }
    [DataMember]
    public string Name { get; set; }
}

The problem is, that when the application is started, the combobox does not get a default value. I've tried setting the SelectedIndex to 0 in XAML, but to no avail. I've tried setting the SelectedIndex in a Window_Loaded event handler in the code-behind (which works), but since I'm using the MVVM pattern that feels kind of dirty. I'm still new to this whole WPF/MVVM stuff, so if someone could point me in the right direction I would be grateful.

Best Answer

You can set SelectedSchool like this:

public void CourseFormViewModel()
    {
        Schools = new ObservableCollection<School>();

        try
        {
            // Gets schools from a web service and adds them to the Schools ObservableCollection
            PopulateSchools();

            SelectedSchool = 3;
        }
        catch (Exception ex)
        {
            // ...
        }
    }

Test data:

 Schools.Add(new School { Id = 1, Name = "aaa", Acronym = "a" });
 Schools.Add(new School { Id = 2, Name = "bbb", Acronym = "b" });
 Schools.Add(new School { Id = 3, Name = "ccc", Acronym = "c" });

And you will get selected item "c".

If you want init ComboBox with the smallest Id you can use this code:

SelectedSchool = Schools.Min(x => x.Id);

Instead of assign constant value.

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

Related Topic