Combobox 项目来源绑定不起作用

Question

I'm trying to fill my Combobox ItemsSource using Binding in Xaml with a collection of data ObservableCollection but I always get the Combobox ItemsSource null.

WPF UserControl Cs Code: ( Update the code )

    public ObservableCollection<User> users { get; set; }


      public partial class MainWindow : Window

         
         {
            InitializeComponent();
            User user1 = new User("Mariah", 15);
            User user2 = new User("John", 19 );
            users = new ObservableCollections<User>();
            users.Add(user1);
            users.Add(user2);
            this.DataContext = this;          
            Console.WriteLine(ComboBoxUsers.Items.Count); // always 0
            Console.WriteLine(ComboBoxUsers.ItemsSource); // always null
         }

WPF UserControl Xaml Code: ( Updated my code )

          <ComboBox SelectedIndex = "0" x:Name="ComboBoxUsers" ItemsSource="{Binding users, UpdateSourceTrigger=PropertyChanged}" FontFamily="Arial" FontSize="11" Grid.Row="3" Height="30"  Margin="10,5,5,5">
<ComboBox.ItemTemplate>
    <DataTemplate>
        <StackPanel Orientation="Horizontal" >
             <Image  IsEnabled="False" Source="{Binding Image}"/>
             <TextBlock x:Name="textblock" IsEnabled="False"   Text="{Binding Name} />

        </StackPanel>
        <DataTemplate.Resources>
            <Style TargetType="ComboBoxItem">
                <Setter Property="IsEnable" Value="{Binding IsEnable}"/>
            </Style>
        </DataTemplate.Resources>
    </DataTemplate>
</ComboBox.ItemTemplate>
                    </StackPanel>

                </DataTemplate>

            </ComboBox.ItemTemplate>
            <ComboBox.ItemContainerStyle>
                <Style TargetType="ComboBoxItem" BasedOn="{StaticResource {x:Type ComboBoxItem}}">
                    <Setter
                Property="Visibility"
                Value="{Binding IsHidden}" />
                </Style>
            </ComboBox.ItemContainerStyle>

        </ComboBox>

Class User

 public class User 
    {
       public string Name  { get; set; }
       public int Age { get; set; }
       public User(string name, int age)
           {
              this.Name = name;
              this.Age = age;
           }
    }

What is the source of this problem?

Answer 1

Discarding unnecessary (which is not relevant to the question or has syntax errors), your example with a little formatting works fine.

XAML:

<ComboBox x:Name="ComboBoxUsers"
          ItemsSource="{Binding Users}" 
          FontFamily="Arial"
          FontSize="11"
          Grid.Row="3" 
          Height="30" 
          Margin="10,5,5,5">
    <ComboBox.ItemTemplate>
        <DataTemplate>
            <StackPanel Orientation="Horizontal" >
                <TextBlock IsEnabled="False" 
                           Text="{Binding Name}" />
            </StackPanel>
        </DataTemplate>
    </ComboBox.ItemTemplate>
</ComboBox>

Code-behind:

public class User
{
    public string Name { get; }
    public int Age { get; }

    public User(string name, int age)
    {
        Name = name;
        Age = age;
    }
}

public partial class MainWindow : Window
{
    public ObservableCollection<User> Users { get; }

    public MainWindow()
    {
        InitializeComponent();

        // Initialize collection with some items
        Users = new ObservableCollection<User>
        {
            new User("Mariah", 15),
            new User("John", 19)
        };

        DataContext = this;
    }
}

Result:

Remarks:

Your

Console.WriteLine(ComboBoxUsers.Items.Count); // always 0 Console.WriteLine(ComboBoxUsers.ItemsSource); // always null

because you use Binding . You needn't access ItemsSource or Items.Count through ComboBox - you should use binded collection Users (eg Users.Count ) to manipulate or get ComboBox content.

EDIT.

About SelectedItem . You should define for yourself, you want use Bindings or work with ComboBox directly .

Binding push you to NOT use ComboBox.SelectedItem/Index/Value whatever. Even not access ComboBoxUsers to get some data. Binding is closely related to, for example, the MVVM Pattern . If you decided to use Bindings - forget about accessing directly to ComboBox or it data properties SelectedItem/Index/Value or similar.

If you use Bindings - you should create a property (eg SelectedUser ) for SelectedItem (same as you create property Users for your ItemsSource ) and bind to it:

XAML (introducing binding for SelectedItem property and SelectionChanged handler) :

<ComboBox x:Name="ComboBoxUsers"
          ItemsSource="{Binding Users}" 
          SelectedItem="{Binding SelectedUser}"
          SelectionChanged="OnUserSelectionChanged"
          FontFamily="Arial"
          FontSize="11"
          Grid.Row="3" 
          Height="30" 
          Margin="10,5,5,5">
</ComboBox>

Code-behind (introducing property SelectedUser and OnUserSelectionChanged handler) :

public partial class MainWindow : Window
{
    public ObservableCollection<User> Users { get; }
    // Here would be stored your Binded selected item
    public User SelectedUser { get; set; }

    // And here is your handler when selection changed
    private void OnUserSelectionChanged(object sender, SelectionChangedEventArgs e)
    {
        // SelectedUser property stores selected in ComboBox item,
        // so you can use it directly
        _ = MessageBox.Show(SelectedUser.Name);

        // Even if you wish to get directly - it is possible 
        // (thanks to @Clemens):
        var selectedUser = (sender as ComboBox).SelectedItem as User;
        var selectediIndex = (sender as ComboBox).SelectedIndex;
    }

    public MainWindow()
    {
        InitializeComponent();

        Users = new ObservableCollection<User>
        {
            new User("Mariah", 15),
            new User("John", 19)           
        };

        DataContext = this;
    }
}

Repeat same algorithm for each property you wish to Bind (eg SelectedIndex ):

SelectedIndex="{Binding SelectedUserIndex}"
public int SelectedUserIndex { get; set; }

Result:

Decide for yourself, what you need. Nice modern Bindings or old boring (sender as ComboBox).SelectedItem .