Unexpected behavior with object initializer [duplicate]

Question

Not exactly sure how this is working. Not even sure how I can even use object initializer while building the Child class. And then how its overriding my default Test class.

internal class Program
{
    static void Main(string[] args)
    {
        Child child = new Child
        {
            Test = { Name = "hello" }
        };

        //child.Test.Name prints out "hello"
    }
}

public class Child : Parent
{
    public Child()
    {
    }
}

public abstract class Parent
{
    public Test Test { get; }

    protected Parent()
    {
        Test = new Test { Name = "Default" };
    }
}

public class Test
{
    public string Name { get; set; }
}

Expected an error can't I shouldn't be able to use object initializer on Test and the default can't be changed.

Answer 1

You are thinking of

Test = new Test { Name = "hello" }

which is indeed not allowed because Test does not have a setter.

On the other hand,

Test = { Name = "hello" }

is totally fine because it doesn't set Test. It just sets Test.Name, and Name indeed has a setter.

From the spec,

A member initializer that specifies an object initializer after the equals sign is a nested object initializer, i.e., an initialization of an embedded object. Instead of assigning a new value to the field or property, the assignments in the nested object initializer are treated as assignments to members of the field or property.

Answer 2

This is akin to:

Child child = new Child();
child.Test.Name = "hello";

Nothing illegal about that.

It is arguably a little unclear when it (the access on Test) is an assignment via set versus a mutate via the get, but one big clue here is the lack of a new; just using = {...} goes via the get.