xmlns:local="clr-namespace:BestChat.IRC.Global.View" and xmlns:IrcCtrls="clr-namespace:BestChat.IRC.Global.View.Ctrls;assembly=BestChat.IRC.Global.View.Ctrls" declared on the root element. The binding that complains about not being able to find the dialog class is {Binding Msg, Mode=OneWay, NotifyOnSourceUpdated=True, RelativeSource={RelativeSource FindAncestor, AncestorType={x:Type local:NetworkEditorDlg}}}. The usage of the control is <IrcCtrls:NetworkLogInModesComboBox ... and that control is declared as NetworkLogInModesComboBox in namespace BestChat.IRC.Global.View.Ctrls in the assembly BestChat.IRC.Global.View.Ctrls.dll.