Nested Interfaces: Cast IDictionary> to IDictionary>?

Question

I would think it\'s fairly straightforward to cast an IDictionary> object to an IDictionary

Answer 1

I would think it's fairly straightforward to cast an IDictionary> object to an IDictionary>

Absolutely not. It wouldn't be type-safe. Here's an example of why not:

// This is fine...
IDictionary> dictionary = new Dictionary>();

// Suppose this were valid...
IDictionary> badDictionary = dictionary;

// LinkedList doesn't implement IList
badDictionary["foo"] = new LinkedList();

// What should happen now?
IList bang = dictionary["foo"];

As you can see, that's going to cause problems - we'd be trying to get a LinkedList out when we expect all the values to implement IList. The point of generics is to be type-safe - so which line would you expect to fail? The first, third and fourth lines look pretty clearly valid to me - so the second one is the only one which can fail to compile, and it does...

Now in some cases, it can be done safely. For example, you can convert (in C# 4) from IEnumerable to IEnumerable