如何仅从字典中获取特定类型的键值对

Question

I have an IDictionary (not generic, so I am dealing with object 's here), and I would like to get from it only the elements in that IDictionary which correspond to a specific key type K and a specific value type V. There are a billion ways to do this, but the most elegant way I have found seems to be this:

Dictionary<K, V> dict = myIDictionary.OfType<KeyValuePair<K, V>>();

This compiles fine so I am assuming this method CAN work for a dictionary (it's declared by IEnumerable) but the resulting dictionary is empty, although there are definitely KVPs in there that meet those conditions. Obviously I was assuming that KeyValuePair<K, V> is the format it is expecting, since that's what's used in enumeration over a dictionary, but is it something else? Is there something I'm missing?

Answer 1

A non-generic IDictionary doesn't use a KeyValue<T,U> type, so you'll never have any matches.

You would need to parse the DictionaryEntry items, and convert them:

Dictionary<K,V> dict = myIDictionary
                          .Cast<DictionaryEntry>()
                          .Where(de => de.Key is K && de.Value is V)
                          .ToDictionary(de => (K)de.Key, de => (V)de.Value);

Answer 2

The OfType function does filter the list based on the specified type... but you don't have anything of the type KeyValuePair<K,V> in your base IDictionary . You've only got whatever type the non-generic IDictionary object contains (I'm not positive at the moment - maybe DictionaryEntry ?). Even if it "matches" by the generic types of the pair, it's not really the same outer type.

So that function won't do what you're looking for. You need to do something with a Where statement. Something like:

var dict = myIDictionary.Where(e => e.Key is typeof(K) && e.value is typeof(V))

though that is straight pseudocode, not positive what the actual syntax is.

Edit: Looks like Reed has the full solution spelled out quite nicely.