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copied!<p>It's the business objects responsibility to determine what happens to the other items should one become primary, therefore you need some sort of object which manages the <code>IsPrimary</code> flag. </p> <p>However you do it (custom container type, mediator etc) you need something to mediate the changes</p> <p>e.g. </p> <pre><code>public class Mediator { IList<Contact> _contacts = null; public Mediator(IList<Contact> contacts) { _contacts = contacts; foreach(var c in contacts) { c.PropertyChanged += ContactPropertyChanged; } } private bool _isChanging = false; private void ContactPropertyChanged(object sender, PropertyChangedEventArgs e) { var current = sender as Contact; if(e.PropertyName == "IsPrimary" && !_isChanging && current.IsPrimary) { _isChanging = true; foreach(var c in _contacts.Where(x => x != current) { c.IsPrimary = false; } _isChanging = false; } } } </code></pre> <p>There are probably better ways, like having a container collection (which hooks the propertychanged etc on it's own ... also watch out for the event handlers!)</p> <p>You could write a more generic templated version which fires an overload (so you can easily just subclass to create different mediators etc)</p> <pre><code>public class Mediator<T> { IList<T> _items = null; public Mediator(IList<T> items, params string[] watchedProperties) { ... etc protected virtual OnWatchedPropertyChanged(T sender, string PropertyName) { } } public ContactMediator : Mediator<Contact> { public ContactMediator(IList<Contact> contacts, params string[] watchedProperties) { ... override OnWatchedPropertyChanged(Contact object, string propertyName) { ... etc } </code></pre>

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