I have a simple list box that contains some string values. This list box is initially populated by binding to a entity collection (via DataSource property of listbox). However later user will be able to add some values into it also. When I just do a listbox.Items.Add(mystring), this new values never show up in the listbox itself. Any ideas what I am doing wrong? Or may be there are standard patterns for dealing with this type of situation?

Is your app a Winform or WebForm? Could you please show the declarative aspx?

Its a winform

I think you need to choose binding to a collection or using the items collection. In my experience you can't do both.

if you bind to a set using databinding, you can't add a list of values which aren't in the set to choose from. Typical example: the '<none>' value in a combo box in this case: you can't add that to the combobox, as all values in the combobox are obtained by winforms from the datasource.

You may try to add new entities to the databound entityCollection.