Duplicate keys in .NET dictionaries?

Question

If you're using .NET 3.5, use the Lookup class.

EDIT: You generally create a Lookup using Enumerable.ToLookup. This does assume that you don't need to change it afterwards - but I typically find that's good enough.

If that doesn't work for you, I don't think there's anything in the framework which will help - and using the dictionary is as good as it gets :(

The List class actually works quite well for key/value collections containing duplicates where you would like to iterate over the collection. Example:

List<KeyValuePair<string, string>> list = new List<KeyValuePair<string, string>>();

// add some values to the collection here

for (int i = 0;  i < list.Count;  i++)
{
    Print(list[i].Key, list[i].Value);
}

Here is one way of doing this with List< KeyValuePair< string, string > >

public class ListWithDuplicates : List<KeyValuePair<string, string>>
{
    public void Add(string key, string value)
    {
        var element = new KeyValuePair<string, string>(key, value);
        this.Add(element);
    }
}

var list = new ListWithDuplicates();
list.Add("k1", "v1");
list.Add("k1", "v2");
list.Add("k1", "v3");

foreach(var item in list)
{
    string x = string.format("{0}={1}, ", item.Key, item.Value);
}

Outputs k1=v1, k1=v2, k1=v3

If you are using strings as both the keys and the values, you can use System.Collections.Specialized.NameValueCollection, which will return an array of string values via the GetValues(string key) method.

I just came across the PowerCollections library which includes, among other things, a class called MultiDictionary. This neatly wraps this type of functionality.

Very important note regarding use of Lookup:

You can create an instance of a Lookup(TKey, TElement) by calling ToLookup on an object that implements IEnumerable(T)

There is no public constructor to create a new instance of a Lookup(TKey, TElement). Additionally, Lookup(TKey, TElement) objects are immutable, that is, you cannot add or remove elements or keys from a Lookup(TKey, TElement) object after it has been created.

(from MSDN)

I'd think this would be a show stopper for most uses.