Duplicate keys in .NET dictionaries?

Are there any dictionary classes in the .NET base class library which allow duplicate keys to be used? The only solution I've found is to create, for example, a class like:

Dictionary<string, List<object>>

But this is quite irritating to actually use. In Java, I believe a MultiMap accomplishes this, but cannot find an analog in .NET.

Solution 1:

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 :(

Solution 2:

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);
}

Solution 3:

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