But I can not understand why no object? It’s also a reference type?
Yes, both double[] and object are reference types, so null is implicitly convertible to both of them. However, member overloading generally favours more specific types, so the double[] constructor is used. See section 7.5.3 of the C# specification for more details (and boy are there a lot of details).
In particular, from section 7.5.3.5:
Given two different types T1 and T2, T1 is a better conversion target than T2 if at least one of the following holds:
- An implicit conversion from T1 to T2 exists, and no implicit conversion from T2 to T1 exists
That’s the case here, where T1 is double[] and T2 is object. There’s an implicit conversion from double[] to object, but no implicit conversion from object to double[], so double[] is a better conversion target than object.
If you want force the use of the object constructor, just cast:
D myD = new D((object) null);