ComboBox will not update its display list unless you change selections first

Question

Update: I checked the answer before I fully tested it still does not work. I updated the code below so you should just be able to paste in to a empty WinForms project and it

Answer 1

Use a BindingList instead of a List. It was designed to address such issues. Dinesh Chandnani, a member of the .NET Client Team, states the following in a blog post:

BindingList<T> is the new generic implementation of IBindingList which fires ListChanged event when items are added/removed/inserted/etc. from the list. bindingSource hooks on to these events and is thus “aware” of these changes and can notify controls bound thos this BindingSource.

I was able to reproduce the issue you described in your updated entry, but didn't quite reproduce the original issue without tweaking the code slightly.

By using a BindingList<Holder> I was able to get an immediate response when focus left the textbox. It's possible to get instant updates by using an overloaded method when adding a new data binding. I also set the BindingSource's DataSource directly since using a null dataMember in the overloaded constructor was not yielding the expected behavior.

Here's the code I ended up with based off your sample code:

public partial class Form1 : Form
{
    private BindingSource bs;
    private BindingList<Holder> bList;

    public Form1()
    {
        InitializeComponent();

        bList = new BindingList<Holder>();
        bList.Add(new Holder("test1"));
        bList.Add(new Holder("test2"));

        bs = new BindingSource();
        bs.DataSource = bList;

        cmb.DataSource = bs;
        cmb.DisplayMember = "Name";
        cmb.ValueMember = "Name";

        // updates when focus leaves the textbox
        txt.DataBindings.Add("Text", bs, "Name");

        // updates when the property changes
        //txt.DataBindings.Add("Text", bs, "Name", false, DataSourceUpdateMode.OnPropertyChanged);
    }
}

Comment out the first txt binding and uncomment the one below it to see the DataSourceUpdateMode.OnPropertyChanged in action.

Here are some BindingList resources:

• BindingSource and BindingList Of T - DataBinding made simple!
• BindingSource - A Closer Look...
• Databinding - Bindinglist, BindingSource and BusinessObjects - Part 1
• Databinding - BindingList, BindingSource, and BusinessObjects: Part 2
• Behind the Scenes: Improvements to Windows Forms Data Binding in the .NET Framework 2.0, Part 2

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  EDIT: in reply to the issues you're facing with your updated code, please make these changes:

1) Replace bsBroken = new BindingSource(lstBroken, null); with:

bsBroken = new BindingSource();
bsBroken.DataSource = lstBroken;

Or in one line: bsBroken = new BindingSource() { DataSource = lstBroken };

This yields the expected behavior with an immediate response to changes (I also mentioned this before above). Do not use the overload that accepts a dataMember and set it to null. Doing so gives the buggy behavior you're experiencing.

2) After doing the above, I see no need for the txtBroken_TextChanged event. Comment out the event handler assignment to test, but you should be able to remove it completely.