Auto property initializers vs Expression-bodied members. Explained here: http://thebillwagner.com/Blog/Item/2015-07-16-AC6gotchaInitializationvsExpressionBodiedMembers

FailingTest.cs

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

    public abstract class Widget
    {
        public abstract Name Name { get; }
    }

    public class Cog : Widget
    {
        public override Name Name => new Name { Text = "Cog" };
    }

    [Fact]
    public void what_the_heck()
    {
        Widget widget = new Cog();

        widget.Name.Text = "New Cog";

        Assert.Equal(widget.Name.Text, "New Cog");
    }
}

WorkingTest.cs

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

    public abstract class Widget
    {
        public abstract Name Name { get; }
    }

    public class Cog : Widget
    {
        public override Name Name { get; } = new Name { Text = "Cog" };
    }

    [Fact]
    public void what_the_heck()
    {
        Widget widget = new Cog();

        widget.Name.Text = "New Cog";

        Assert.Equal(widget.Name.Text, "New Cog");
    }
}

The first is supposed to fail ... Everytime you access the Name variable in the Cog class, it returns a new instance of Name.

Even if Text is set, it would revert to default when Name is accessed again, making the Assert.Equal fail