Entity Framework saves only one child of my POCO entity (the HashSet concept)

I was trying to create an entity with several children entities and persist it to the database using Entity Framework. I was generating the entity, set its entry state to Added, saved the changes in the DbContext, everything right. However, in the database I had one parent entity and one child entity. I suspected it had something to do with the way I created the tables, the foreign keys, something related to the way I was declaring the connection between the entities in the model. It was none of that.

If you look at the way EF generates entities from database tables, it creates a two directional relationship from any foreign key: the parent entity gets an ICollection<Child> property to hold the children and the child entity a virtual property of type Parent to hold the parent. Moreover, the parent entity instantiates the collection in the constructor in the form of a HashSet<Child>. It doesn't have to be a HashSet, though. It works just as well if you overwrite it when you create the entity with something like a List. However, the HashSet approach tells something important of the way EF behaves when considering collections of child objects.

In my case, I was doing something like

var parent = new Parent { 
  Children = Enumerable
                .Repeat(new Child { SomeProperty = SomeValue }, 3)
                .ToList() 
};

Can you spot the problem? When I changed the code to

var parent = new Parent();
Enumerable
   .Repeat(new Child { SomeProperty = SomeValue }, 3)
   .ToList()
   .ForEach(child => parent.Children.Add(child));

I was shocked to see that my Parent had only a list of Children with count 1. Why? Because Enumerable.Repeat takes the instance of the object you give it and repeats it:

Enumerable.Repeat(something,3).Distinct().Count() == 1 !

There was no problem that I was setting the Children collection to a List instead of a HashSet, but when saving the children, Entity Framework was considering the distinct instances of Child.

The solution here was to generate different instances of the same object, something like

var parent = new Parent {
   Children = Enumerable.Range(0,3)
                .Select(i => new Child { SomeProperty = SomeValue }, 3)
                .ToList()
};

or, to make it more clear for this case:

var parent = new Parent {
    Children = Enumerable
                .Repeat(() => new Child { SomeProperty = SomeValue }, 3)
                .Select(generator => generator())
                .ToList()
};