Framework Tips VIII: Initializing Dictionaries and Collections

In .NET < 3.5 the only collections you could initialize inline were arrays. So this was legal:

public class CollectionTest

{

    public static readonly ICollection<string> _list = 

        new string[]{"one","two","three"};

}

However if you wanted to have List instead of array, you had to use a trick and pass array as constructor parameter:

public static readonly ICollection<string> _list = 

    new List<string>(new string[]{"one","two","three"});

Not the most elegant piece of code, but at least it works. So far so good. What if you wanted to have a IDictionary instead of ICollection? Well… in this case you’re out of luck, at least partially. To have a dictionary initialized, you’d have to use explicit static constructor, and it means – performance hit. Still, it’s better than nothing.

public class CollectionTest

{

    public static readonly IDictionary<string,int> _dictionary = 

        new Dictionary<string,int>();

    static CollectionTest()

    {

        _dictionary.Add("one",1);

        _dictionary.Add("two",2);

        _dictionary.Add("three",3);

    }

}

But this all was in medieval timessmile_wink. Now, with object and collection initializers you can initialize any ICollection, the way you could with Arrays.

private static readonly ICollection<string> _list = 

    new List<string> {"one", "two", "three"};

This is neat, but even better stuff, is that you can do similar thing with Dictionaries, which means, no explicit static constructor required anymore.

private static readonly IDictionary<string, int>

    _data = new Dictionary<string, int>

            {

                {"one", 1},

                {"two", 2},

                {"three", 3}

            }:

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  • ribeeye27

    anyway to make this work for .net 3.5 and VS 2005?

  • GeeK

    Bingo – right question… the described behavior is independent of .Net 3.5 as mentioned in this article, u need VS 2008. The "collection initializers" also work for project u develope for .Net 2.0 (i used it) and probably for .Net 1.1 (? but who would use .Net 1 at all ;))

  • Prakash

    Any way to make such dictionary case insensitive? So that _data["ONE"] also give 1 value.

  • Prakash – you can make it such by using appropriate constructor