NullReferenceException with object initializer suggested by resharper

Question

I have a strange issue with the object initializer syntax.

Here are my sample classes:

public class Foo
{
    public Bah BahProp { get; set; }
}

pub         


        

Answer 1

new Foo { BahProp = { Id = 1 } }

compiles to:

new Foo().BahProp.Id = 1;

or, little more verbose:

var foo3 = new Foo();
foo3.BahProp.Id = 1;

So BahProp is null. You're not constructing it.
(This is perhaps the most confusing syntax in all of C#)
Option 2 works because you're calling the constructor of Bah.

Option 3 would also work if you initialize BahProp inside the constructor of Foo. It will have been constructed by the time BahProp = { Id = 1 } is called.

The same is seen with collection initializers:

public class Foo {
    public List<int> Numbers { get; set; }
}

var foo = new Foo { Numbers = { 1, 2, 3 } };

This does not initialize the List. It only calls Add on it.

You really must see new MyObject() { X = 1, Y = 2 } as two distinct parts:
new MyObject() constructs a new object and
{ X = 1, Y = 2 } sets the values of its properties (and that's all it does).
Object and collection initializers can be nested. The top-level initializer must follow a constructor, but a nested initializer does not.