C# databinding on combobox

Question

Hello,

I can't understand the following 2 issues given this code. I mapped a combobox to a custom object and I want each time that the selected value change on the combobox, the custom object changes too.

public partial class MainForm : Form
{
    private Person _person;
    public MainForm()
    {
        InitializeComponent();
        _person = new Person();

        //Populating the combox, we have this.comboBoxCities.DataSource = this.cityBindingSource;
        cityBindingSource.Add(new City("London"));
        cityBindingSource.Add(new City("Paris"));
        _person.BirthCity = new City("Roma");
        cityBindingSource.Add(_person.BirthCity);
        cityBindingSource.Add(new City("Madrid"));

        //Doing the binding
        comboBoxCities.DataBindings.Add("SelectedItem", _person, "BirthCity");
    }

    private void buttonDisplay_Click(object sender, EventArgs e)
    {
        MessageBox.Show("BirthCity=" + _person.BirthCity.Name);
    }

    private int i = 0;
    private void buttonAddCity_Click(object sender, EventArgs e)
    {
        City city = new City("City n°" + i++);
        cityBindingSource.Add(city);
        comboBoxCities.SelectedItem = city;
    }

}

public class Person
{
    private City _birthCity;
    public City BirthCity
    {
        get { return _birthCity; }
        set
        {
            Console.WriteLine("Setting birthcity : " + value.Name);
            _birthCity = value;
        }
    }
}

public class City
{
    public string Name { get; set; }
    public City(string name) { Name = name; }
    public override string ToString() { return Name; }
}

1 - why when I manually select twice in a row (or more) different value on the combobox, I got only one call to BirthCity.Set witht he last selected value (and the call seems firing only when the combobox lost the focus) ?

2 - why when I click buttonAddCity and then buttonDisplay, the diplayed city is not the one selected (not the one displayed in the comobox )

Answer 1

You should override GetHashCode() and Equals() methods.

Answer 2

why when I manually select twice in a row (or more) different value on the combobox, I got only one call to BirthCity.Set witht he last selected value (and the call seems firing only when the combobox lost the focus) ?

That's how Data Binding works, data is moved from the control to the property when validation occurs, and validation occurs when the control loses focus.

why when I click buttonAddCity and then buttonDisplay, the diplayed city is not the one selected (not the one displayed in the comobox )

I don't know. I created a simple form (Visual C# Express 2008 using .Net 3.5 SP1) and pasted your code pretty much verbatim, and it works as expected: it shows the new city in the combo box.

If you add comboBoxCities.Focus (); to the end of buttonAddCity_Click (), you'll make sure the new city gets pushed into _person.BirthCity earlier rather than on ValidateChildren ().