WPF Application Doesn't Load Resources in IDE

While working with WPF application I've experienced a problem: library assembly requests resource from System.Windows.Application which is defined in executable assembly in XAML file and this resource isn't loaded while I work in IDE! If I debug application all works fine, but XAML designer doesn't load resource, library method fails and window isn't rendered in designer. Of course I still can work with pure XAML and mostly I do so, but I prefer to have an ability to quickly watch results of my work in designer. So to workaround this problem I changed failed method so he will return not fully correct value which will not depend on application resources.