Question

I’m working on a small WPF application in which I have a combobox that is bound to an ObservableCollection in the code behind:

public Molecule CurrentMolecule { get; set; }
public ObservableCollection<string> Formulas { get; set; }

public MainWindow()
{
     CurrentMolecule = new Molecule();
     Formulas = new ObservableCollection<string>(CurrentMolecule.FormulasList.ToList());
     DataContext = this;

     InitializeComponent();
}

<ComboBox x:Name="cmbFormula" ItemsSource="{Binding Path=Formulas}" SelectionChanged="cmbFormula_SelectionChanged"/>

This works fine to populate my combo box with the CurrentMolecule.FormulasList however if at some point I set CurrentMolecule to a new instance of Molecule the databinding no longer works. Do I need to implement some kind of OnPropertyChanged event so that no matter what the contents of the combo box will stay current with the CurrentMolecule.FormulasList?

Answer 1

You have to implement INotifyPropertyChanged, only then the changes will be updated in UI.

Here are the modifications that I’ve done to your code.

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Windows;
using System.Windows.Controls;
using System.Windows.Data;
using System.Windows.Documents;
using System.Windows.Input;
using System.Windows.Media;
using System.Windows.Media.Imaging;
using System.Windows.Navigation;
using System.Windows.Shapes;
using System.ComponentModel;
using System.Collections.ObjectModel;

namespace WpfApplication1
{
    /// <summary>
    /// Interaction logic for MainWindow.xaml
    /// </summary>
    public partial class MainWindow : Window, INotifyPropertyChanged
    {
        private Molecule _CurrentMolecule;

        public Molecule CurrentMolecule
        {
            get
            {
                return _CurrentMolecule;
            }
            set
            {
                _CurrentMolecule = value;
                OnPropertyChanged("CurrentMolecule");
                Formulas =  new ObservableCollection<string>(CurrentMolecule.FormulasList.ToList());
            }
        }

        private ObservableCollection<string> _Formulas;

        public ObservableCollection<string> Formulas
        {
            get { return _Formulas; }
            set
            {
                _Formulas = value;
                OnPropertyChanged("Formulas");
            }
        }

        public MainWindow()
        {
            InitializeComponent();

            CurrentMolecule = new Molecule();
            DataContext = this;
        }

        public event PropertyChangedEventHandler PropertyChanged;

        public void OnPropertyChanged(string propertyName)
        {
            if (PropertyChanged != null)
            {
                PropertyChanged(this, new PropertyChangedEventArgs(propertyName));
            }
        }
    }
}

Edit:
A better approach is to create a ViewModel and then bind it to the DataContext of the Window.

Define a new class called ViewModel as below. Note you might want to change the namespace

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.ComponentModel;
using System.Collections.ObjectModel;

namespace WpfApplication1
{
    public class ViewModel : INotifyPropertyChanged
    {
        #region Properties
        private Molecule _CurrentMolecule;
        public Molecule CurrentMolecule
        {
            get
            {
                return _CurrentMolecule;
            }
            set
            {
                _CurrentMolecule = value;
                OnPropertyChanged("CurrentMolecule");
                Formulas = new ObservableCollection<string>(CurrentMolecule.FormulasList.ToList());
            }
        }

        private ObservableCollection<string> _Formulas;
        public ObservableCollection<string> Formulas
        {
            get { return _Formulas; }
            set
            {
                _Formulas = value;
                OnPropertyChanged("Formulas");
            }
        }
        #endregion

        #region Constructor
        public ViewModel()
        {
            CurrentMolecule = new Molecule();
        }
        #endregion

        #region INotifyPropertyChanged implementation
        public event PropertyChangedEventHandler PropertyChanged;
        public void OnPropertyChanged(string propertyName)
        {
            if (PropertyChanged != null)
            {
                PropertyChanged(this, new PropertyChangedEventArgs(propertyName));
            }
        }
        #endregion
    }
}

Modify the MainWindow code behind file as below

public MainWindow()
{
    InitializeComponent();

    DataContext = new ViewModel();
}