Косвенное приведение long [] к ulong [] через Array дает разные типы (GetType vs. is ulong [])

Question

Я не понимаю, почему d is long[] и d is ulong[] дают true. Любые намеки на то, почему это работает? И второй вопрос, почему не работает приведение к double[]?

using System;
                    
public class Program
{
    public static void Main()
    {
        long[] a = new Int64[10];
        ulong[] b = (ulong[]) (Array) a;
        
        Array d;
        
        // not allowed
        // double[] c = (double[]) (Array) a;
        
        a[0] = -5;
        
        // check types of a
        d = a;
        Console.WriteLine("d.GetType() {0}", d.GetType());
        Console.WriteLine("a.GetType() {0}", a.GetType());
        if(d is ulong[])
            Console.WriteLine("{0}","d is ulong[]");
        if(d is long[])
            Console.WriteLine("{0}","d is long[]");
        if(a is ulong[])
            Console.WriteLine("{0}","a is ulong[]");
        if(a is long[])
            Console.WriteLine("{0}","a is long[]");
        
        // check types of b
        d = b;
        Console.WriteLine("d.GetType() {0}", d.GetType());
        Console.WriteLine("b.GetType() {0}", b.GetType());
        if(d is long[])
            Console.WriteLine("{0}", "d is long[]");
        if(d is ulong[])
            Console.WriteLine("{0}", "d is ulong[]");
        if(b is long[])
            Console.WriteLine("{0}", "b is long[]");
        if(b is ulong[])
            Console.WriteLine("{0}", "b is ulong[]");
        
        Console.WriteLine("{0} {1}", a[0], b[0]);
    }
}

Вывод:

d.GetType() System.Int64[]
a.GetType() System.Int64[]
d is ulong[]
d is long[]
a is long[]
d.GetType() System.Int64[]
b.GetType() System.Int64[]
d is long[]
d is ulong[]
b is ulong[]
-5 18446744073709551611