Collection classes may need to be populated directly from constant data.

In C# 2.0 you were forced to either write multiple lines of Add statements or transform it from an array. C# 3.0 allows you to specify an initializer using curly brackets {} to provide necessary initial data.

Code

List now

var phonetics = new List<string>()
    { "Alpha", "Bravo", "Charlie", "Delta", "Echo", "Foxtrot", "Golf" };

List before

var phonetics = new List<string>();
phonetics.Add("Alpha");
phonetics.Add("Bravo");
phonetics.Add("Charlie");
phonetics.Add("Delta");
phonetics.Add("Echo");
phonetics.Add("Foxtrot");
phonetics.Add("Golf");

Dictionary now

var phonetics = new Dictionary<string, string>()
{
    { "A", "Alpha" },
    { "B", "Bravo" },
    { "C", "Charlie" },
    { "D", "Delta" },
    { "E", "Echo" }
};

Dictionary before

var phonetics = new Dictionary<string, string>();
phonetics.Add("A", "Alpha");
phonetics.Add("B", "Bravo");
phonetics.Add("C", "Charlie");
phonetics.Add("D", "Delta");
phonetics.Add("E", "Echo");

Notes

• When creating a dictionary if there is something on the value that can derive the key create an array of the values and then ToDictionary() with the key specification
• The compiler replaces collection initializers with multiple calls to any method named Add on the class
• Index initializers ^{C# 6.0} added support for indexers to be used in initializers too

More information

• Fun with Custom C# Collection Initializers @ Marius Schulz