AutoCompleteTextBox for comma-seperated terms in C# and WPF following the MVVM approach

In this article we will develop an auto-complete TextBox for comma-seperated terms. The suggestions are overlays that are intended to support rather than disturb you when writing. Suggestions that have already been entered are not displayed again and the capitalization is corrected automatically. With two bindings for the input and a list of possible suggestions we follow the MVVM approach. A suggestion is never sent to the ViewModel until it has been accepted.