How do you use object initializers for a list of key value pairs?

Question

I can't figure out the syntax to do inline collection initialization for:

var a = new List<KeyValuePair<string, string>>();

score 93 · Answer 1 · answered Jul 27 '12 at 20:10

93

Note that the dictionary collection initialization { { key1, value1 }, { key2, value2 } } depends on the Dictionary's Add(TKey, TValue) method. You can't use this syntax with the list because it lacks that method, but you could make a subclass with the method:

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

public class Program
{
    public static void Main()
    {
        var list = new KeyValueList<string, string>
                   {
                       { "key1", "value1" },
                       { "key2", "value2" },
                       { "key3", "value3" },
                   };
    }
}

answered Jul 27 '12 at 20:10

phoog

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+1 for describing exactly what the dictionary initializer sugar translates to. – Shibumi May 09 '13 at 18:00
You might want make that 'base.Add' because as I understand C# you're simply calling the Add method of the `KeyValueList` object you've defined. – Richard Barker Oct 14 '16 at 03:35
@RichardBarker you are correct that `base.Add` would be less ambiguous. I prefer as a matter of style to avoid `base.` and `this.` unless they are necessary, because in my view they just create so much clutter. But others disagree, and those people would indeed want to use the `base.` keyword. – phoog Oct 14 '16 at 03:43

digEmAll · Accepted Answer · 2012-07-27T20:15:11.443

89

Pretty straightforward:

var a = new List<KeyValuePair<string, string>>()
{
    new KeyValuePair<string, string>("A","B"),
    new KeyValuePair<string, string>("A","B"),
    new KeyValuePair<string, string>("A","B"),
};

Note that you can leave the trailing comma after the last element (probably because .net creators wanted to make automatic code-generation easier), or remove the () brackets of the list contructor and the code still compiles.

edited Jul 27 '12 at 20:15

answered Jul 27 '12 at 20:04

digEmAll

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You might want to remove that trailing comma – Stefan H Jul 27 '12 at 20:05
12

@StefanH The trailing comma is valid and makes future additions easier – Daniel DiPaolo Jul 27 '12 at 20:05
@StefanH same apply to semicolon at the end of the type definition. – Lukasz Madon Jul 27 '12 at 20:08
1

@lukas could you explain what you mean by that? – Stefan H Jul 27 '12 at 20:09
@StefanH `class Foo {};` <- it's optional in C# – Lukasz Madon Jul 27 '12 at 20:15
2

@lukas That has nothing to do with object initializers - I didn't realize we had moved to the topic of all optional syntax in C#... – Stefan H Jul 27 '12 at 20:16
2

What is more notable is that the `;` is not optional after the collection initializer's `}` – Stefan H Jul 27 '12 at 20:21
2

@StefanH It's not all that notable: The collection initializer is a statement; a class declaration is not a statement. – phoog May 29 '15 at 16:48

elRobbo · Answer 3 · 2017-02-06T05:06:20.863

13

Another alternative that doesn't require making a subclass:

List<KeyValuePair<String, String>> list = new Dictionary<String, String>
    {
        {"key1", "value1"},
        {"key2", "value2"},
    }.ToList();

As mentioned in the comments: drawbacks with this method include possible loss of ordering and inability to add duplicate keys.

edited Feb 06 '17 at 05:06

answered May 04 '15 at 05:34

elRobbo

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hmm.. this might not preserve the ordering you have specified in your initializer, but i am not exactly sure. maybe you can initialize a sorted list and then convert ToList? – Can Bud Sep 07 '15 at 11:47
3

You also cannot store duplicate entries like this. One of the reasons for using List... – ComeIn Feb 05 '17 at 23:51
1

It also involves hashing all the keys and a lot more allocations. – Edward Brey Mar 01 '18 at 12:46

user764754 · Answer 4 · 2017-10-27T11:45:14.570

Since collection initializers (C# 6.0) have not been mentioned here:

Implementation

public static class InitializerExtensions
{
   public static void Add<T1, T2>(this ICollection<KeyValuePair<T1, T2>> target, T1 item1, T2 item2)
   {
       if (target == null)
            throw new ArgumentNullException(nameof(target));

        target.Add(new KeyValuePair<T1, T2>(item1, item2));
    }
}

Usage

var list = new List<KeyValuePair<string, string>> { {"ele1item1", "ele1item2"}, { "ele2item1", "ele2item2" } };

How to make it work

Just include the right using statements in your file so that InitializerExtensions is available (meaning you could call InitializerExtensions.Add explicitly) and the special collection initializer syntax will become available if you are using VS 2015 or higher.

This extension must be a built-in part of C#, why we can use compact initilaizers for `Dictionary` but not `List` ?! — S.Serpooshan, Jun 04 '23 at 19:21

score -1 · Answer 5 · answered Jul 27 '12 at 20:08

-1

Any initialization is done by object(params_if_exist) { any public member or instance};

f.e. var list = new List<int> {1,2,3};

var bw = new BackgroundWorker() {WorkerSupportsCancellation = true};

answered Jul 27 '12 at 20:08

burning_LEGION

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This does not answer the OPs question. The OP is looking for collection initialization specifically for KeyValue pairs. – Stefan H Jul 27 '12 at 20:10
i think that he must understand as it work, but not use ready solution, and ask again and again same question – burning_LEGION Jul 27 '12 at 20:12
Which "public member or instance" is set by `1,2,3` in your first example? – phoog Jul 27 '12 at 20:13
@phoog I think that it is syntactic sugar for grabbing the Add(T) method of the list, same applies with all collections, that's why digEmAll's solution works – Stefan H Jul 27 '12 at 20:14
@StefanH you are correct. I was trying to point out *why* the answer "does not answer the OP's question", as you said, because "any public member or instance" is not an adequate description of initialization: it describes object initializers, but not collection initializers. – phoog Jul 27 '12 at 21:02

How do you use object initializers for a list of key value pairs?

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