[TestFixture]
public class When_Adding_an_item_to_a_basket
{
[Test]
public void Should_add_item_to_basket()
{
IBasket basket = new Basket();
IBasketItem basketItem = new BasketItem();

basket.AddItemToBasket(basketItem);

//EnumerableAssert.Contains(basket.Items, basketItem);
Specify.That(basket.Items).Contains(basketItem);

}
}

public class EnumerableAssert
{
private IEnumerable myList;

public EnumerableAssert(IEnumerable enumerable)
{
myList = enumerable;
}

public void Contains(T actual)
{
CollectionAssert.Contains(new List(myList), actual);
}
}

public class Specify
{
public static EnumerableAssert That(IEnumerable enumerable)
{
return new EnumerableAssert(enumerable);
}
}

I peeked in Reflector on MbUnit and basically the reason their CollectionAssert requires an ICollection is because they’re just newing up an ArrayList (which only takes an ICollection) and calling Contains on it.