interface IBaseInterface
{
    string A { get; set; }
}
interface ISubInterface : IBaseInterface
{
    string B { get; set; }
}
class ImplementationClass : ISubInterface
{
    public string A { get; set; }
    public string B { get; set; }
}

public partial class MainPage : Page
{
    public ISubClass TheObject = new ImplementationClass { A = "1", B = "2" };
    //All the rest goes here
}

<TextBlock Text={x:Bind Path=TheObject.A}></TextBlock>

<TextBlock Text={x:Bind Path=TheObject.B}></TextBlock>