Why can’t I assign a List to a List?

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The simplest way to understand why this is not allowed is the following example:

abstract class Fruit
{
}

class Apple : Fruit
{
}

class Banana : Fruit
{
}

// This should intuitively compile right? Cause an Apple is Fruit.
List<Fruit> fruits = new List<Apple>();

// But what if I do this? Adding a Banana to a list of Apples
fruits.Add(new Banana());

The last statement would ruin the type safety of .NET.

Arrays however, do allow this:

Fruit[] fruits = new Apple[10]; // This is perfectly fine

However, putting a Banana into fruits would still break type safety, so therefor .NET has to do a type check on every array insertion and throw an exception if it’s not actually an Apple. This is potentially a (small) performance hit, but this can be circumvented by creating a struct wrapper around either type as this check does not happen for value types (because they can’t inherit from anything). At first, I didn’t understand why this decision was made, but you’ll encounter quite often why this can be useful. Most common is String.Format, which takes params object[] and any array can be passed into this.

In .NET 4 though, there’s type safe covariance/contravariance, which allows you to make some assignments like these, but only if they’re provably safe. What’s provably safe?

IEnumerable<Fruit> fruits = new List<Apple>();

The above works in .NET 4, because IEnumerable<T> became IEnumerable<out T>. The out means that T can only ever come out of fruits and that there’s no method at all on IEnumerable<out T> that ever takes T as a parameter, so you can never incorrectly pass a Banana into IEnumerable<Fruit>.

Contravariance is much the same but I always forget the exact details on it. Unsurprisingly, for that there’s now the in keyword on type parameters.

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