KeyValuePair vs. NameValueCollection

Question

There are other questions such as KeyValuePair vs IDictionary, but I feel this one differs slightly.

NameValueCollection takes a string key and string value.

KeyValuePair is like a dictionary, you tell it what type the key and value is.

I don't understand why NameValueCollection exists. Initializing a KeyValuePair with string types seems sufficient. I also noticed that NameValueCollection has some more methods available to it, but again why not merge both classes into one?

A `KeyValuePair` not like a dictionary. It is simply a Tuple containing the Key and the Value. — , May 27 '11 at 01:32
@pst, a dictionary takes a list of key/value pairs. I'm failing to see a difference. — The Muffin Man, May 27 '11 at 01:39
@Nick `var kvp = new KeyValuePair("Hello", "World!")` -- [KeyValuePair](http://msdn.microsoft.com/en-us/library/5tbh8a42.aspx) — , May 27 '11 at 01:44
I believe my misunderstanding was that a dictonary holds key value pairs, where as a key value pair is singular, you can't keep adding to pst's `kvp` example above for instance. — The Muffin Man, May 27 '11 at 02:02
@Nick: @pst means that A dictionary is a collection of kvps where you can't have duplicate keys, while a kvp is just one instance of a kvp. seems fairly straightforward — bevacqua, May 27 '11 at 02:03
A KeyValuePair stores a single key and a single value. That's it. Not multiple keys vs multiple values... just one of each. It's not a collection, but simply a container for a pair of objects. — spender, May 27 '11 at 02:04
@Nico, @spender, I understand now, thank you for the clearer explanation. — The Muffin Man, May 27 '11 at 17:03

score 19 · Accepted Answer · 2013-01-21T03:35:08.483

A KeyValuePair not like a dictionary. It is simply a Tuple containing the Key and the Value.

NameValueCollection is wrapper over what amounts to a IList<KeyValuePair<string,IList<string>>> (note that NameValueCollection predates generics) - operations like Get(string) are O(n) and items can be fetched by index and each Key maps to one or more Values (this differs from a Dictionary<string,string>).

A reason for this is explained in the NameValueCollection documentation:

This class can be used for headers, query strings and form data.

The newer ~~"replacement"~~ data-structure with some similar behavior for NameValueCollection is Lookup<string,string>. (However, it doesn't directly support the same operations as is immutable as spender notes.)

Happy coding.

The principal problem with Lookup in this scenario is that it is immutable, unlike NaveValueCollection, which can be added to. As such, it's not really a "replacement", but has similar behaviour. — spender, May 27 '11 at 02:07

score 4 · Answer 2 · answered May 27 '11 at 01:35

NameValueCollection existing in .NET 1.0 and 1.1, KeyValuePair is a generic type and wasn't added to .NET until 2.0. All the classes in System.Collections.Specialized all predates the addition of generics; it contains certain strongly typed (specialized if you will) for use when that's exactly what you need to users don't have to cast from object to string.

score 0 · Answer 3 · answered May 27 '11 at 01:39

KeyValuePair is the component you use to iterate a Dictionary

var dictionary = new Dictionary<int,long>

foreach(var kvp in dictionary)
{
    // kvp is KeyValuePair<int,long>. kvp.Key is the int key and kvp.Value is the long value for the key
}

NameValueCollection is indexable.

KeyValuePair vs. NameValueCollection

3 Answers3