Question

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Recreating a Dictionary from an IEnumerable

When using the Where method on a dictionary of type Dictionary<TKey, TValue> you ends up with a IEnumerable<KeyValuePair<TKey, TSource>> and that is breaking the datatype that I have choose at the beginning. I would like to return a dictionary.

Maybe I am not using the correct filter function. So how do you usually filter element from a dictionary?

Thanks

Answer 1

IDictionary<TKey, TValue> actually extends IEnumerable<KeyValuePair<TKey, TValue>>. This is why you can use LINQ operators on an IDictionary<TKey, TValue> in the first place.

However, LINQ operators return IEnumerable<T> which are meant to provide deferred execution, meaning the results aren’t actually generated until you start iterating through the IEnumerable<T>.

The IEnumerable<T> implementation which is provided by IDictionary<TKey, TValue> comes by way of the ICollection<T> interface (where T is a KeyValuePair<TKey, TValue>), which means that if LINQ were to return IDictionary<TKey, TValue> instead of IEnumerable<KeyValuePair<TKey, TValue>> then it would have to materialize the list, violating it’s principals (hence the IEnumerable<KeyValuePair<TKey, TValue>> return value).

Of course, the way around it is to call the ToDictionary extension method on the Enumerable class (as others have mentioned), but a little back-story never hurts.