how do i convert a IEnumberable<SubClass> to IEnumerable<ParentClass>

Question

How do i go from (cast?, convert?):

IEnumerable<Square>

to

IEnumerable<Shape>

Answer 1

var shapes = squares.Cast<Shape>();

Answer 2

If you're using anything lower than .NET 4.0, then you're going to have to use IEnumerable.Cast:

IEnumerable<Square> squares = new List<Square>();

// Fill the collection

IEnumerable<Shape> shapes = squares.Cast<Shape>();

.NET 4.0 introduces new Covariance and Contravariance features that allow you to make that cast directly:

IEnumerable<Square> squares = new List<Square>();
IEnumerable<Shape> shapes = (IEnumerable<Shape>)squares;

Answer 3

If you're using .NET 4.0, you don't have to.

From Covariance and Contravariance in Generics (emphasis added):

Polymorphism enables you to assign an instance of Derived to a variable of type Base. Similarly, because the type parameter of the IEnumerable(T) interface is covariant, you can assign an instance of IEnumerable<Derived> to a variable of type IEnumerable<Base>...

Since you're asking this question, I assume you're not using .NET 4.0 yet. But this might be a reason to upgrade, if you're able to.