How to set combobox by default to no selected item

Question

Hi, I bind a list(ObservableCollection) to a ComboBox. There is always one item selected. I want that by default no item is selected. How can I do this?

 <ComboBox Width="200" Height="25" ItemsSource="{Binding CountryView}" DisplayMemberPath="Country" SelectedItem="{Binding SelectedCountry}"/>

Answer 1

Case 1 : If your CountryView is an ObservableCollection, the Combobox below has no items selected by default.

MainWindow.xaml:

  <Grid>  
        <ComboBox Width="200" Height="25" ItemsSource="{Binding CountryView}" DisplayMemberPath="Country"  
                  SelectedItem="{Binding SelectedCountry}"/>  
    </Grid>  

MainWindow.xaml.cs:

using System;  
using System.Collections.ObjectModel;  
using System.Windows;  
using System.Windows.Data;  
  
namespace ComboboxBindCollectionView  
{  
  public partial class MainWindow : Window  
  {  
    public MainWindow()  
    {  
      InitializeComponent();  
      DataContext = new ViewModel();  
    }  
  }  
  public class ViewModel  
  {  
    public ObservableCollection<MyCountry> CountryView { get; set; }  
     
    public MyCountry SelectedCountry { get;set;}  
    public ViewModel()  
    {  
      CountryView = new ObservableCollection<MyCountry>();  
      CountryView.Add(new MyCountry() { Country = "Beijing"});  
      CountryView.Add(new MyCountry() { Country = "NewYork" });  
      CountryView.Add(new MyCountry() { Country = "Shanghai"});  
      CountryView.Add(new MyCountry() { Country = "Paris" });  
     
    }  
  }  
  public class MyCountry  
  {  
    public string Country { get; set; }  
  }  
}  

Case 2: If your CountryView is a CollectionViewSource, the Combobox below has no items selected by default.
MainWindow.xaml:

 <Window.Resources>  
        <CollectionViewSource x:Key="CountryView"/>  
    </Window.Resources>  
    <Grid>  
        <ComboBox Width="200" Height="25" ItemsSource="{Binding Source={StaticResource CountryView}}" DisplayMemberPath="Country"  
                  SelectedItem="{Binding SelectedCountry}"/>  
    </Grid>  

MainWindow.xaml.cs:

using System.Collections.ObjectModel;  
using System.Windows;  
using System.Windows.Data;  
  
namespace ComboboxBindCollectionView  
{  
  public partial class MainWindow : Window  
  {  
    public ObservableCollection<MyCountry> CountryView { get; set; }  
    public MyCountry SelectedCountry { get; set; }  
    public MainWindow()  
    {  
      InitializeComponent();  
  
      CountryView = new ObservableCollection<MyCountry>();  
      CountryView.Add(new MyCountry() { Country = "Beijing" });  
      CountryView.Add(new MyCountry() { Country = "NewYork" });  
      CountryView.Add(new MyCountry() { Country = "Shanghai" });  
      CountryView.Add(new MyCountry() { Country = "Paris" });  
      CollectionViewSource cvs = (FindResource("CountryView") as CollectionViewSource);  
      cvs.Source = CountryView;  
      DataContext = this;  
    }  
  }  
   
  public class MyCountry  
  {  
    public string Country { get; set; }  
  }  
}  

Case 3: If your CountryView is a ICollectionView, the Combobox below has no items selected by default.

<Grid>  
        <ComboBox Width="200" Height="25" ItemsSource="{Binding ConutryView}" DisplayMemberPath="Country"  
                   SelectedItem="{Binding SelectedCountry}"  IsSynchronizedWithCurrentItem = "True"/>  
    </Grid>  

MainWindow.xaml.cs:

using System.Collections.ObjectModel;  
using System.ComponentModel;  
using System.Windows;  
using System.Windows.Data;  
  
namespace ComboboxNoSelectedItemDefault  
{  
  public partial class MainWindow : Window  
  {  
    public MainWindow()  
    {  
      InitializeComponent();  
      DataContext = new ViewModel();  
    }  
  }  
  public class ViewModel  
  {  
    public ObservableCollection<MyCountry> Countries { get; set; }  
    public MyCountry SelectedCountry { get; set; }  
    private ICollectionView conutryView;  
  
    public ICollectionView ConutryView  
    {  
      get { return conutryView; }  
    }  
  
    public ViewModel()  
    {  
      ObservableCollection<MyCountry> countries =new ObservableCollection<MyCountry>();  
      Countries = new ObservableCollection<MyCountry>();  
      Countries.Add(new MyCountry() { Country = "Beijing" });  
      Countries.Add(new MyCountry() { Country = "NewYork" });  
      Countries.Add(new MyCountry() { Country = "Shanghai" });  
      Countries.Add(new MyCountry() { Country = "Paris" });  
      conutryView = CollectionViewSource.GetDefaultView(Countries);  
    }  
  }  
  public class MyCountry  
  {  
    public string Country { get; set; }  
  }  
}  

The result:

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Answer 2

Try setting the initial value of SelectedCountry to null, or use this binding:

 SelectedItem="{Binding SelectedCountry, Mode=OneWayToSource}"