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copied!<p>I'm not clear on the latter part of your question. Partly it's because your naming convention is confusing; I'd expect a collection named <code>BItems</code> to contain objects of type <code>B</code>, not <code>A</code>.</p> <p>So I'm going to change your nomenclature a bit so that I don't get confused. Instead of <code>A</code>, I'll call the first class <code>User</code>, and instead of <code>B</code>, I'll call the second class <code>Group</code>. A <code>Group</code> contains a collection of <code>User</code> objects, named <code>Users</code>. The global collections look like this:</p> <pre><code>List<User> Users; List<Group> Groups; </code></pre> <p>It's easy to determine if a given <code>User</code> <code>u</code> is in any group:</p> <pre><code>return Groups.Where(g => g.Users.Contains(u)).Any(); </code></pre> <p>Easy, but computationally expensive if you have many groups and they contain many users. We'll get back to that in a second.</p> <p>Right away, I see that one of your questions has got a problem:</p> <blockquote> <p>How can I make that checking a checkbox inserts its item to the BItems-Collection and unchecking removes it?</p> </blockquote> <p>What should happen if I check an unchecked user? Which group (or groups, since more than one group can contain a user) should it be added to?</p> <p>Since you say that you want checked items to be "assigned to the B-Object", I'm going to assume that the UI is only looking at one group at a time - we'll call it the <code>SelectedGroup</code>. This is good, because <code>g.Users.Contains(u)</code> is much less expensive than the query I showed above.</p> <p>If this is so, what you need to do is wrap your <code>User</code> in a class that exposes an <code>IsChecked</code> property. I'd call this class <code>UserViewModel</code>, since that's what it is. The class needs three properties (at a minimum):</p> <pre><code>public User User { get; set; } public Group SelectedGroup { get; set; } public bool IsChecked { get { return SelectedGroup.Users.Contains(this.User); } set { if (value != IsChecked) { if (IsChecked) { SelectedGroup.Users.Remove(this.User); } else { SelectedGroup.Users.Add(this.User); } } } } </code></pre> <p>Your <code>ListView</code> is bound to an <code>ObservableCollection<UserViewModel></code> named, say, <code>UserViewModels</code>. Whenever <code>SelectedGroup</code> is set, you need to rebuild this collection:</p> <pre><code>UserViewModels = new ObservableCollection<UserViewModel>( Users.Select(u => new UserViewModel { User=u, SelectedGroup=SelectedGroup })); </code></pre> <p>You could avoid rebuilding the collection by implementing <code>INotifyPropertyChanged</code> in the <code>UserViewModel</code> class, and having it raise <code>PropertyChanged</code> for the <code>IsChecked</code> property whenever <code>SelectedGroup</code> changes. </p> <p>Also, it would probably be responsible to include null-reference checking in the <code>IsChecked</code> property, so that the program doesn't throw an exception if <code>SelectedGroup</code> or <code>SelectedGroup.Users</code> is null.</p>

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