C# | Collection Initializers

Collection initializers

Initialize a collection type with values:

var stringList = new List<string>
{
    "foo",
    "bar",
};

Collection initializers are syntactic sugar for Add() calls. Above code is equivalent to:

var temp = new List<string>();
temp.Add("foo");
temp.Add("bar");
var stringList = temp;

Note that the intialization is done atomically using a temporary variable, to avoid race conditions.

For types that offer multiple parameters in their Add() method, enclose the comma-separated arguments in curly braces:

var numberDictionary = new Dictionary<int, string>
{
    { 1, "One" },
    { 2, "Two" },
};

This is equivalent to:

var temp = new Dictionary<int, string>();
temp.Add(1, "One");
temp.Add(2, "Two");
var numberDictionarynumberDictionary = temp;

C# 6 Index Initializers

Starting with C# 6, collections with indexers can be initialized by specifying the index to assign in square brackets, followed by an equals sign, followed by the value to assign.

Dictionary Initialization

An example of this syntax using a Dictionary:

var dict = new Dictionary<string, int>
{
    ["key1"] = 1,
    ["key2"] = 50
};

This is equivalent to:

var dict = new Dictionary<string, int>();
dict["key1"] = 1;
dict["key2"] = 50

The collection initializer syntax to do this before C# 6 was:

var dict = new Dictionary<string, int>
{
    { "key1", 1 },
    { "key2", 50 }
};

Which would correspond to:

var dict = new Dictionary<string, int>();
dict.Add("key1", 1);
dict.Add("key2", 50);

So there is a significant difference in functionality, as the new syntax uses the indexer of the initialized object to assign values instead of using its Add() method. This means the new syntax only requires a publicly available indexer, and works for any object that has one.

public class IndexableClass
{
    public int this[int index]
    {
        set
        {
            Console.WriteLine("{0} was assigned to index {1}", value, index);
        }
    }
}

var foo = new IndexableClass
{
    [0] = 10,
    [1] = 20
}

This would output:

`10 was assigned to index 0`
`20 was assigned to index 1`

Collection initializers in custom classes

To make a class support collection initializers, it must implement IEnumerable interface and have at least one Add method. Since C# 6, any collection implementing IEnumerable can be extended with custom Add methods using extension methods.

class Program
{
    static void Main()
    {
        var col = new MyCollection {
            "foo",
            { "bar", 3 },
            "baz",
            123.45d,
        };
    }
}

class MyCollection : IEnumerable
{
    private IList list = new ArrayList();

    public void Add(string item)
    {
        list.Add(item)
    }

    public void Add(string item, int count)
    {
        for(int i=0;i< count;i++) {
            list.Add(item);
        }
    }

    public IEnumerator GetEnumerator()
    {
        return list.GetEnumerator();
    }
}

static class MyCollectionExtensions
{
    public static void Add(this MyCollection @this, double value) =>
        @this.Add(value.ToString());
}

Using collection initializer inside object initializer

public class Tag
{
    public IList<string> Synonyms { get; set; }
}

Synonyms is a collection-type property. When the Tag object is created using object initializer syntax, Synonyms can also be initialized with collection initializer syntax:

Tag t = new Tag
{
    Synonyms = new List<string> {"c#", "c-sharp"}
};

The collection property can be readonly and still support collection initializer syntax. Consider this modified example (Synonyms property now has a private setter):

public class Tag
{
    public Tag()
    {
        Synonyms = new List<string>();
    }

    public IList<string> Synonyms { get; private set; }
}

A new Tag object can be created like this:

Tag t = new Tag
{
    Synonyms = {"c#", "c-sharp"}
};

This works because collection initializers are just syntatic sugar over calls to Add(). There’s no new list being created here, the compiler is just generating calls to Add() on the exiting object.

Collection Initializers with Parameter Arrays

You can mix normal parameters and parameter arrays:

public class LotteryTicket : IEnumerable{
    public int[] LuckyNumbers;
    public string UserName;

    public void Add(string userName, params int[] luckyNumbers){
        UserName = userName;
        Lottery = luckyNumbers;
    }
}

This syntax is now possible:

var Tickets = new List<LotteryTicket>{
    {"Mr Cool"  , 35663, 35732, 12312, 75685},
    {"Bruce"    , 26874, 66677, 24546, 36483, 46768, 24632, 24527},
    {"John Cena", 25446, 83356, 65536, 23783, 24567, 89337}
}

Remarks

The only requirement for an object to be initialized using this syntactic sugar is that the type implements System.Collections.IEnumerable and the Add method. Although we call it a collection initializer, the object does not have to be an collection.