When would you use a List<KeyValuePair<T1, T2>> instead of a Dictionary<T1, T2>?

What is the difference between a List of KeyValuePair and a Dictionary for the same types? Is there an appropriate time to use one or the other?

When you don't need fast lookups on key - maintaining the hashtable used by Dictionary has a certain overhead.

In short, the list does not enforce uniqueness of the key, so if you need that semantic then that's what you should use.

Dictionary is generic type that contains a collection of key-value pairs. Dictionary is fast for lookup operations, because is using hash function internally. That means, all the keys must be unique in dictionary.

Consider this examples:

List<KeyValuePair<int, string>> pairs = new List<KeyValuePair<int, string>>();
pairs.Add(new KeyValuePair<int, string>(1, "Miroslav"));
pairs.Add(new KeyValuePair<int, string>(2, "Naomi"));
pairs.Add(new KeyValuePair<int, string>(2, "Ingrid"));

Dictionary<int, string> dict = new Dictionary<int, string>();
dict.Add(1, "Miroslav");
dict.Add(2, "Naomi");
dict.Add(2, "Ingrid"); // System.ArgumentException: An item with the same key has already been added.

So you should always consider two at least two things:

1. Do you want to search concrete items in dictionary?
2. Do you want to have some fields non-unique (for example pairs: firstname/lastname).

The List would also be useful when you care about the order of the items.

Further to Phillip Ngan's answer, SOAP or otherwise, you cannot XML serialize objects that implements IDictionary.

> Q: Why can't I serialize hashtables? > > A: The XmlSerializer cannot process classes implementing the IDictionary interface. This was partly due to schedule constraints and partly due to the fact that a hashtable does not have a counterpart in the XSD type system. The only solution is to implement a custom hashtable that does not implement the IDictionary interface.

from here

In SOAP webservices for silverlight, we have found that Dictionary's do not serialize. This would be a situation where you would use a List of KeyValuePair over a Dictionary.

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From http://blogs.msdn.com/bclteam/archive/2004/09/03/225473.aspx:

> KeyValuePair vs. DictionaryEntry
> [Krzysztof Cwalina] > > We discussed a problem with > implementation of IEnumerable on > Dictionary<K,V>. What type should > IEnumerable.GetEnumerator().Current > return? KeyValuePair<K,V> or > DictionaryEntry? Same for > ICollection.CopyTo. Instances of what > type should be copied to the array? > > We decided the following: IEnumerable > and ICollection interface > implementations will use > KeyValuePair<K,V> as the item type. > IDictionary specific members > (GetEnumerator returning > IDictionaryEnumerator) will use > DictionaryEntry as the item type. > > The reason is that we are in a process > of making a change where > IEnumerator<T> would extend > IEnumerator. It would be very strange > if walking the hierarchy from > Dictionary<K,V>->IEnumerable<T>->IEnumerable > we suddenly changed the type of the > item returned from enumerators.