Question about tekpub Mastering C#4.0 series 2

I have tried to make the following text as accurate as I could hear from the video training

http://shop.tekpub.com/products/csharp4

interface IFoo<in T>
{
    void TakeAnInstanceOfT(T instance);
}

class Program
{
    static void Main(string[] args)
    {
        IFoo<Fruit> fruit = null;
        IFoo<Apple> apple = fruit; // <<< The following description was used
                                       // <<< to explain this line.

        // Apple is fruit.
        // That's ok. Because anytime when fruit is expected to be 
            // passed in, is perfectly happy
        // if you actually pass-in apple
    }
}

Question> Based on my understand, the reason why the following statement

IFoo<Apple> apple = fruit;

is correct is because that the interface IFoo accepts contra-variance parameters. In other words, it allows you to pass in instances of parent class o开发者_C百科f T. While, the video interpretation tells me the exact opposite thing.

I expect the author to say the following (I should be wrong this case:)

IFoo<Apple>, anytime when Apple is expected, you are allowed to pass in fruits.

Not sure about the video, but here's an explanation. Say you have these implementations:

public class FruitImpl : IFoo<Fruit>
{
    public void TakeAnInstanceOfT(Fruit instance)
    {
    }
}

public class AppleImpl : IFoo<Apple>
{
    public void TakeAnInstanceOfT(Apple instance)
    {
    }
}

Now let's say you do this:

IFoo<Fruit> fruit = new FruitImpl();
IFoo<Apple> apple = fruit;

if we then call apple.TakeAnInstanceOfT(new Apple()) (you have to pass in an instance of Apple, since it's now strongly typed to IFoo<Apple>) then the method in FruitImpl will get called (remember, apple points to an instance of FruitImpl) and you'll be passing in an instance of an Apple to a method that accepts a Fruit which is totally legal. Makes sense?