Transfer NULL to the constructor

I can not understand why the constructor is executed with the parameter Double[]?

using System.Collections.Generic;
using System.Linq;
using System.Text;

namespace MyConsoleApp
{
    class Program
    {
        static void Main(string[] args)
        {
            D myD = new D(null);
            Console.ReadLine();           
        }

    }

    public class D
    {
        public D(object o)
        {
            Console.WriteLine("Object");
        }
        public D(double[] array)
        {
            Console.WriteLine("Array");
        }
        public D(int i)
        {
            Console.WriteLine("Int");
        }
    }
}

I think because the first constructor takes a parameter of reference type. The first constructor with a reference parameter because null is the default value for reference types.

But i don't understand why not object, it's also a reference type.

Jon Skeet
people
quotationmark

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);

people

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