Question

I’m trying to pass a Dictionary<int, List<T>> into a constructor that requires an IDictionary<int, IEnumerable<T>>.

Unfortunately, the generic IDictionary is not defined as IDictionary<TKey, out TValue>. Perhaps this wouldn’t make sense, but is there a cast that would allow me to pass my dictionary into the constructor?

The obvious cast ((IDictionary<int, IEnumerable<T>>)dictionary) fails at runtime.

Answer 1

No, because that would not be safe:

Dictionary<int, List<int>> mydictionary = whatever;
Dictionary<int, IEnumerable<int>> converted = (Dictionary<int, IEnumerable<int>>) mydictionary;
converted.Add(10, new int[] { 1, 2, 3 } );

And you just added an array to a dictionary that can only hold lists. This isn’t allowed because there is no way to make it safe.

You note that the dictionary interface is not covariant; it is precisely this reason why it cannot be covariant. The “out” in the covariant annotation is a mnemonic that is telling you “the value parameter is only used for output”, but clearly the value parameter is used for input into a dictionary.