Question

In the following example, why can’t I cast collectionA to collectionB given that the compiler knows that a TItem is a A<T>?

public class A<T>
{
}

public void Foo<TItem, T> () where TItem : A<T>
{
    var collectionA = new List<TItem>();
    var collectionB = (List<A<T>>)collectionA; // "Cannot cast" error here
}

Answer 1

The problem is that it would allow you to place inappropriate items into collectionA.

Here’s a simplified reworking of it, which hopefully makes it easier to see the problem:

Suppose you have (pseudocode):

class Animal {...}

class Dog: Animal { Bark(){} }

class Cat: Animal { Meow(){} }

Now imagine you could do this:

var dogs = new List<Dog>();

dogs.Add(new Dog());

dogs[0].Bark();

var animals = (List<Animal>) dogs;

Then you would be able to do this:

animals.Add(new Animal()); // Adds an Animal to the list 'dogs', which 'animals' references.

dogs[1].Bark(); // dogs will now have two elements, but the second isn't a dog -
                // so calling Bark() will explode.