WinForms data binding

You should probably set the ValueMember on the combo and the list box. You might need to also handle bindingSource.CurrentChanged and rebind the listbox to the currently selected list.

Use BindingSource for all your complex data binding needs:

// bind to List<Foo>
BindingSource comboBoxBindingSource = new BindingSource();
comboBoxBindingSource.DataSource = foos;
// use this binding source in comboBox1
// for display use FooName
comboBox1.DataSource = comboBoxBindingSource;
comboBox1.DisplayMember = "FooName";

// bind to comboBox1's SelectedValue
// it will point to the Foo selected in combobox
BindingSource listBoxBindingSource = new BindingSource();
listBoxBindingSource.DataSource = comboBox1;
listBoxBindingSource.DataMember = "SelectedValue";
// use this binding source in listBox1
// for display use BarName
listBox1.DataSource = listBoxBindingSource;
listBox1.DisplayMember = "BarName";

// bind to comboBox'1s SelectedValue (reusing listBoxBindingSource)
// and set Text to value of BarDesc
textBox1.DataBindings.Add("Text", listBoxBindingSource, "BarDesc");

To make all this work, I had to add the Items property to the Foo class. This is the "link/relationship" between the two binding sources.

public partial class Form1 : Form {
    public class Foo : List<Bar> {
        public string FooName { get; set; }
        public Foo(string name) { this.FooName = name; }
        public List<Bar> Items { get { return this; } }
    }
    public class Bar {
        public string BarName { get; set; }
        public string BarDesc { get; set; }
        public Bar(string name, string desc) {
            this.BarName = name;
            this.BarDesc = desc;
        }
    }
    public Form1() {

        InitializeComponent();

        List<Foo> foos = new List<Foo>();

        Foo a = new Foo("letters");
        a.Add(new Bar("a", "aaa"));
        a.Add(new Bar("b", "bbb"));
        foos.Add(a);

        Foo b = new Foo("digits");
        b.Add(new Bar("1", "111"));
        b.Add(new Bar("2", "222"));
        b.Add(new Bar("3", "333"));
        foos.Add(b);

        //Simple Related Object List Binding
        //http://blogs.msdn.com/bethmassi/archive/2007/04/21/simple-related-object-list-binding.aspx

        BindingSource comboBoxBindingSource = new BindingSource();
        BindingSource listBoxBindingSource = new BindingSource();

        comboBoxBindingSource.DataSource = foos;
        listBoxBindingSource.DataSource = comboBoxBindingSource;
        listBoxBindingSource.DataMember = "Items";

        comboBox1.DataSource = comboBoxBindingSource;
        comboBox1.DisplayMember = "FooName";

        listBox1.DataSource = listBoxBindingSource;
        listBox1.DisplayMember = "BarName";

        textBox1.DataBindings.Add("Text", listBoxBindingSource, "BarDesc");

    }
}