ObservableCollection.Contains() Doesn't Work Correctly

Question

Consider the following:

class Bind
{
    public string x { get; set; }
    public string y { get; set; }
}
public partial class MainWindow : Window
{
    public MainWindow()
    {
        InitializeComponent();
    }

    private void Window_Loaded(object sender, RoutedEventArgs e)
    {
        ObservableCollection<Bind> cX = new ObservableCollection<Bind>();
        ObservableCollection<Bind> cY = new ObservableCollection<Bind>();
        cX.Add(new Bind { x = "a", y = "1" });
        cX.Add(new Bind { x = "b", y = "2" });
        cY.Add(new Bind { x = "a", y = "1" });
        foreach (var i in cX)
        {
            if (!cY.Contains(i)) { lv.Items.Add(i); } //lv is a ListView control
        }
    }
}

Why does it add x = "a", y = "1" to the ListView?

If I change ObservableCollection to List or Collection, it does the same.

Answer 1

The 'Contains' method uses the Equals on object, and this simply checks that the memory addresses are different.

Consider changing your class to this...

 class Bind : IEquatable<Bind> {
     public string x { get; set; }
     public string y { get; set; }
     public bool Equals(Bind other)
     {
         return x == other.x && y == other.y; 
     } 
}

Your loop will then visit the strongly typed Equals method in your class, and this will result in the behaviour you are after.

NOTE: the string class ALSO inherits from IEquatable of T and that is what allows the equality operator to operate on the content of the string rather than the address of the string.