C#: How can Dictionary<K,V> implement ICollection<KeyValuePair<K,V>> without having Add(KeyValuePair<K,V>)?

Question

Looking at System.Collections.Generic.Dictionary<TKey, TValue>, it clearly implements ICollection<KeyValuePair<TKey, TValue>>, but doesn't have the required "void Add(KeyValuePair<TKey, TValue> item)" function.

This can also be seen when trying to initialize a Dictionary like this:

private const Dictionary<string, int> PropertyIDs = new Dictionary<string, int>()
{
    new KeyValuePair<string,int>("muh", 2)
};

which fails with

No overload for method 'Add' takes '1' arguments

Why is that so?

Answer 1

The expected API is to add via the two argument Add(key,value) method (or the this[key] indexer); as such, it uses explicit interface implementation to provide the Add(KeyValuePair<,>) method.

If you use the IDictionary<string, int> interface instead, you will have access to the missing method (since you can't hide anything on an interface).

Also, with the collection initializer, note that you can use the alternative syntax:

Dictionary<string, int> PropertyIDs = new Dictionary<string, int> {
  {"abc",1}, {"def",2}, {"ghi",3}
}

which uses the Add(key,value) method.

Answer 2

Some interface methods are implemented explicitly. If you use reflector you can see the explicitly implemented methods, which are:

void ICollection<KeyValuePair<TKey, TValue>>.Add(KeyValuePair<TKey, TValue> keyValuePair);
bool ICollection<KeyValuePair<TKey, TValue>>.Contains(KeyValuePair<TKey, TValue> keyValuePair);
void ICollection<KeyValuePair<TKey, TValue>>.CopyTo(KeyValuePair<TKey, TValue>[] array, int index);
bool ICollection<KeyValuePair<TKey, TValue>>.Remove(KeyValuePair<TKey, TValue> keyValuePair);
IEnumerator<KeyValuePair<TKey, TValue>> IEnumerable<KeyValuePair<TKey, TValue>>.GetEnumerator();
void ICollection.CopyTo(Array array, int index);
void IDictionary.Add(object key, object value);
bool IDictionary.Contains(object key);
IDictionaryEnumerator IDictionary.GetEnumerator();
void IDictionary.Remove(object key);
IEnumerator IEnumerable.GetEnumerator();

Answer 3

It doesn't implement ICollection> direclty. It implements IDictionary.

IDictionary derives from ICollection>