using System;
using MyNamespace.One;

public class Test
{
    public static void Main()
    {
        var item = new Product();
        Console.WriteLine(item is ILastModified); //RETURNS FALSE??!
    }
}
 
interface ILastModified
{
    DateTime LastModified { get; set; }
}

namespace MyNamespace.One
{
    public partial class Product
    {
        public int ID { get; set; }
    }
}

namespace MyNamespace.Two
{
    public partial class Product : ILastModified
    {
        public DateTime LastModified { get; set; }
    }
}