Question

I’ve set up a simple form. A ListBox takes values from a list in the ‘business object’ displaying the Name property and providing the Value property.

In additon the ListBox’s SelectedItem property is bound to a property in the same business object.

Using the UI to select a value from the list correctly changes the objects property (checked when the button is clicked) and the correct value is available. So far so good.

However, if the ListBox’s SelectedIndex property is changed in the code, then the UI correctly changes as expected but the business property does not change – it would appear to have missed the change event. This is true for both setting in the constructor and in the button event handler (see the code).
What have I missed or what am I doing incorrectly.
(I’ve only included the code I’ve written – not VS wizard generated stuff)

class Frequency
{
    public String Name { get; set; }
    public Int16 Value { get; set; }

    public Frequency(String name, Int16 value)
    {
        Name = name;
        Value = value;
    }
}

class FrequencyList : System.ComponentModel.BindingList<Frequency>
{
}

class Model
{
    public static FrequencyList FrequencyValues = new FrequencyList() 
    {
        new Frequency("Slowest", 100),
        new Frequency("Slow", 150),
        new Frequency("Medium", 1000),
        new Frequency("Fast", 5500),
        new Frequency("Fastest", 10000)
    };

    public Frequency StartFrequency { get; set; }

    public void DoStuff()
    {
        if (StartFrequency == null)
            return;

        Int16 freq = StartFrequency.Value;
    }
}

    public partial class Form1 : Form
{
    private Model myModel = new Model();

    public Form1()
    {
        InitializeComponent();

        // Bind the list to a copy of the static model data
        this.listBox1.DataSource = Model.FrequencyValues;
        // Bind the control to the model value
        this.listBox1.DataBindings.Add("SelectedItem", myModel, "StartFrequency");
        // Select the start value
        this.listBox1.SelectedIndex = 3;

    }

    private void button1_Click(object sender, EventArgs e)
    {
        Int16 f = (Int16)listBox1.SelectedValue;

        this.myModel.DoStuff();
        int new_index = listBox1.SelectedIndex + 1;
        if (new_index >= listBox1.Items.Count)
            new_index = 0;
        listBox1.SelectedIndex = new_index;
    }
}

Answer 1

You don’t want the Click event, you want the SelectedIndexChanged event. This will trigger regardless of whether the user or the program instigates the change.