Cannot assign null to anonymous property of type array

Question

I have any array of (Pilot) objects with a (Hanger) property, which may be null, which itself has a (List) property. For test

Answer 1

There are two things happening:

Firstly, when you construct an instance of an anonymous type using new { Name = Value}, in order to build the type the compiler needs to be able to work out the type of Value. Just null on its own doesn't have a type, so the compiler wouldn't know what type to give your Planes member.

Now, if you were using a named type for the value, you could just say (type)null and be done, BUT because you want an array of another anonymous type, there's no way to refer to is (it's anonymous!).

So how do you get null typed as array of an anonymous type? Well, the C# spec guarantees that anonymous types with members the same names and types (in the same order!) are unified; that is, if we say

var a = new { Foo = "Bar" };
var b = new { Foo = "Baz" };

then a and b have the same type. We can use this fact to get our suitably-typed null thus:

var array = (new[] { new { PlaneName = "" } });
array = null;

It's not pretty but it works - now array has the right type but a null value. So this compiles:

        var array = new[] { new { PlaneName = "" } };
        array = null;

        var expected = new[]
                           {
                               new
                                   {
                                       PilotName = "Higgins",
                                       Planes = array
                                   },
                               new
                                   {
                                       PilotName = "Higgins",
                                       Planes = new[]
                                                    {
                                                        new { PlaneName = "B-52" },
                                                        new { PlaneName = "F-14" }
                                                    }
                                   }
                           };