Which CheckedListBox event triggers after a item is checked?

There are lots of related StackOverflow posts on this... As well as Branimir's solution, here are two more simple ones:

Delayed execution on ItemCheck (also here):

    void checkedListBox1_ItemCheck(object sender, ItemCheckEventArgs e)
    {
        this.BeginInvoke((MethodInvoker) (
            () => Console.WriteLine(checkedListBox1.SelectedItems.Count)));
    }

Using the MouseUp event:

    void checkedListBox1_MouseUp(object sender, MouseEventArgs e)
    {
        Console.WriteLine(checkedListBox1.SelectedItems.Count);
    }

I prefer the first option, as the second would result in false positives (i.e. firing too often).

You can use the ItemCheck event, if you also check the new state of the item which is being clicked. This is available in the event args, as e.NewValue. If NewValue is checked, include the current item along with the collection proper in your logic:

    private void checkedListBox1_ItemCheck(object sender, ItemCheckEventArgs e)
    {                     
        List<string> checkedItems = new List<string>();
        foreach (var item in checkedListBox1.CheckedItems)
            checkedItems.Add(item.ToString());

        if (e.NewValue == CheckState.Checked)
            checkedItems.Add(checkedListBox1.Items[e.Index].ToString());
        else
            checkedItems.Remove(checkedListBox1.Items[e.Index].ToString());

        foreach (string item in checkedItems)
        {
            ...
        }
    }

As another example, to determine if the collection will be empty after this item is (un-)checked:

private void ListProjects_ItemCheck(object sender, ItemCheckEventArgs args)
{
    if (ListProjects.CheckedItems.Count == 1 && args.NewValue == CheckState.Unchecked)
        // The collection is about to be emptied: there's just one item checked, and it's being unchecked at this moment
        ...
    else
        // The collection will not be empty once this click is handled
        ...
}

I tried this and it worked:

private void clbOrg_ItemCheck(object sender, ItemCheckEventArgs e)
{
    CheckedListBox clb = (CheckedListBox)sender;
    // Switch off event handler
    clb.ItemCheck -= clbOrg_ItemCheck;
    clb.SetItemCheckState(e.Index, e.NewValue);
    // Switch on event handler
    clb.ItemCheck += clbOrg_ItemCheck;

    // Now you can go further
    CallExternalRoutine();        
}