Why can't Dictionary> be cast to Dictionary>?

Question

I\'m wondering why I can\'t just cast (I have a vague idea this might have something to do with that co/contravariance stuff?), and am I forced to copy the elements of the f

Answer 1

You cannot do this because they aren't the same type. Consider:

        var x = new Dictionary>();

        // won't compile, but assume it could...
        Dictionary> xPrime = x;

        // uh oh, would allow us to legally add array of int!
        xPrime["Hi"] = new int[13];

Does this make sense? Because Dictionary> says the TValue is List which means you can Add() a List as a value. If you could cast this to a Dictionary> it would mean the value type is IEnumerable which would mean you could Add() any IEnumerable (int[], HashSet, etc) which would violate the original type.

So, a List can be converted to a IEnumerable reference because List implements IEnumerable, but that does not mean that Dictionary> implements/extends Dictionary>.

Put more simply:

Dictionary 

Can't convert to

Dictionary

Because the latter would allow you to add Cat, Squirrel, etc.