help in combobox

Ok,
you have MyCombo and MyList on your form

1. handle the DoubleClick event of MyList.
2. Write your code :

private void MyList_DoubleClick(object sender, EventArgs e)
{
    MyCombo.Items.Add(MyList.SelectedItem);
    MyList.Items.Remove(MyList.SelectedItem);
}

Of course "MyList.Items.Remove(MyList.SelectedItem);" is optional.