如何让WPF / XAML设计器使用具有依赖关系的自定义类?

I'm using a custom class to perform validation in a WPF app and have defined a class which inherits from ValidationRule. The class has a dependancy which I would like AutoFac to inject at runtime, but the Visual Studio 2010 XAML designer needs a parameterless constructor in order to work. I'm using a custom class to perform validation