Get Bound Selected Item from Combobox

Question

My combo cbStudents is bound to StudentsList :

    <ComboBox Name="cbStudents"
        ItemsSource="{Binding Path=Students}"
        SelectedItem="{Binding Path=SelectedStudent, Mode=TwoWay}"
        DisplayMemberPath="Name"/>


    public class Student
    {
        private string _name;
        public string Name
        {
            get { return _name; }
            set { _name = value; }
        }

        private int _age;
        public int Age
        {
            get { return _age; }
            set { _age= value; }
        }
    }

I want to get the SelectedItem :

I tried-

    private ObservableCollection<Language> _students;

    public ObservableCollection<Language> Students
    {
        get { return _students; }
        set { _students= value; }
    }

    private string _selectedStudent;

    public string SelectedStudent
    {
        get { return _selectedStudent; }
        set { _selectedStudent= value; }
    }

    private void btnGetOutput(object sender, RoutedEventArgs e)
    {
        MessageBox.Show("Your selected item is: " + SelectedStudent);
    }

Result: I get Null output. No error code, nothing. Any help is appreciated.

Answer 1

It seems you want to select a student by its name, ie a string .

Instead of SelectedItem , you should then use SelectedValue and SelectValuePath :

<ComboBox
    ItemsSource="{Binding Students}"
    DisplayMemberPath="Name"
    SelectedValuePath="Name"
    SelectedValue="{Binding SelectedStudent}" />

The view model would of course have to implement INotifyPropertyChanged and should declare the Students property as a readonly:

public class ViewModel : INotifyPropertyChanged
{
    public event PropertyChangedEventHandler PropertyChanged;

    public ObservableCollection<Student> Students { get; }
        = new ObservableCollection<Student>();

    private string selectedStudent;

    public string SelectedStudent
    {
        get { return selectedStudent; }
        set
        {
            selectedStudent = value;
            PropertyChanged?.Invoke(this,
                new PropertyChangedEventArgs(nameof(SelectedStudent)));
        }
    }
}

Answer 2

First of all, you have defined ObservableCollection<Language> Students , I think it should be ObservableCollection<Student> Students instead.

Then, you have to bind to that collection, not to StudentList , which doesn't exist in your sample (maybe you didn't present full code?).

Then, you have to add items to that list somewhere, Students.Add(...) .

Then, if items of your combo box are of type Student , then property bound to SelectedItem also must have type of Student , not string .

Last but not least: you have to bind class with all this fieelds defined to your view, so you must write: view.DataContext = objectWithData; , where view is your view and objectWithData is object with those fields defined.

Answer 3

You can implement INotifyPropertyChanged interface. As in the example below:

public class Data : INotifyPropertyChanged
{
    // boiler-plate
    public event PropertyChangedEventHandler PropertyChanged;
    protected virtual void OnPropertyChanged(string propertyName)
    {
        PropertyChangedEventHandler handler = PropertyChanged;
        if (handler != null) handler(this, new PropertyChangedEventArgs(propertyName));
    }

    protected bool SetValue<T>(ref T field, T value, string propertyName)
    {
        if (EqualityComparer<T>.Default.Equals(field, value)) return false;
        field = value;
        OnPropertyChanged(propertyName);
        return true;
    }
    
    private ObservableCollection<Language> _students;

    public ObservableCollection<Language> Students
    {
       get { return _students; }
       set { _students= value; }
    }

    private Language _selectedStudent;

    public Language SelectedStudent
    {
       get { return _selectedStudent; }
       set { SetValue(_selectedStudent, value, "SelectedStudent"); }
    }
}