Is there a an easier way to initialize a List>, like a Dictionary?

Question

Actually I need something like List> but I want to be able to initialize it like dictionary (i.e. without writing new KeyVa

Answer 1

EDIT: It turns out .NET does have a combination list/dictionary type already: OrderedDictionary. Unfortunately this is a non-generic type, making it rather less attractive in my view. However, it retains the insertion order if you just call Add repeatedly.

It's a little strange as calling Add does not affect entries where a key already exists, whereas using the indexer to add a key/value pair will overwrite a previous association. Basically it doesn't seem like a terribly nice API, and I would personally still avoid it unless your use case exactly matches its behaviour.

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No, .NET doesn't have any insertion-order-preserving dictionaries. You could always write your own list-based type with the relevant Add method. This might even be one of the few places I'd consider extending an existing type:

public class KeyValueList<TKey, TValue> : List<KeyValuePair<TKey, TValue>>
{
    public void Add(TKey key, TValue value)
    {
        Add(new KeyValuePair<TKey, TValue>(key, value));
    }
}

Then:

var list = new KeyValueList<string, string>
{
    { "key1", "value1"}, 
    { "key2", "value2"}
};

An alternative is to use composition, but my gut feeling is that this is a reasonable use of inheritance. I can't place why I'm happy in this case but not usually, mind you...

Answer 2

Because you do not have a dictionary you cannot use a dictionary initiailzer. You have a list so you could use a list initializer which will be the closest you could get:

var l = new List<KeyValuePair<string, string>> 
{
    new KeyValuePair<string, string>("key1", "value1"),
    new KeyValuePair<string, string>("key2", "value2"),
};

Here's the minimum requirement for you to use dictionary initializer: the class must implement IEnumerable and the class must have a public method Add which takes 2 arguments (where the first argument represents the key and the second argument the value). So you could write a custom class which satisfies those requirements and you will be able to use the syntax you have shown in your question.

Answer 3

The code you have typed works fine - further to Mads Togersen's post about the implementation of collection initialisers, the compiler maps the brace-delimited entries ({"key1", "value1"} above) to an Add method on the collection with the same signature - in this case Dictionary.Add(TKey, TValue).