public abstract class CustomerBase
{
	public long CustomerNumber { get; set; }
	public string Name { get; set; }
}

public abstract class CustomerWithChildern<T> : CustomerBase
	where T: CustomerBase
{
	public IList<T> Childern { get; private set; }

	public CustomerWithChildern()
	{
		Childern = new List<T>();
	}
}

public class SalesOffice : CustomerWithChildern<NationalNegotiation>
{
}

public void WalkHierarchy(CustomerBase start)
	{
		Print(start.CustomerNumber);
		if (start is CustomerWithChildern<>)
		{
			foreach(ch in start.Childern)
			{
				WalkHierarchy(ch);
			}
		}
	}

public interface ICustomerWithChildern
{
	IEnumerable ChildernEnum { get; }
}

public abstract class CustomerWithChildern<T> : CustomerBase, ICustomerWithChildern
	where T: CustomerBase
{
	public IEnumerable ChildernEnum { get { return Childern; } }

	public IList<T> Childern { get; private set; }

	public CustomerWithChildern()
	{
		Childern = new List<T>();
	}
}

	public void WalkHierarchy(CustomerBase start)
	{
		var x = start.CustomerNumber;
		var c = start as ICustomerWithChildern;
		if (c != null)
		{
			foreach(var ch in c.ChildernEnum)
			{
				WalkHierarchy((CustomerBase)ch);
			}
		}
	}

if(start.GetType().GetGenericTypeDefinition() == typeof(CustomerWithChildern<>))

Type t = typeof(CustomerWithChildern<>)

typeof(CustomerWithChildern<>).IsAssignableFrom(CustomerWithChildern<Foo>)