C# List definition, parentheses vs curly braces

Question

I've just noticed that when you declare a List in c# you can put parentheses or curly braces at the end.

List<string> myList = new List<string>(); List<string> myList2 = new List<string>{}; 

Both these list appear to have the same functionality. Is there any actual difference caused by declaring them with parentheses or curly braces?

Answer 1

The use of curly braces { } is called a collection initializer. For types that implement IEnumerable the Add method would be invoked normally, on your behalf:

List<string> myList2 = new List<string>() { "one", "two", "three" }; 

Empty collection initializers are allowed:

List<string> myList2 = new List<string>() { }; 

And, when implementing an initializer, you may omit the parenthesis () for the default constructor:

List<string> myList2 = new List<string> { }; 

You can do something similar for class properties, but then it's called an object initializer.

var person = new Person                  {                      Name = "Alice",                      Age = 25                  }; 

And its possible to combine these:

var people = new List<Person>                  {                      new Person                          {                              Name = "Alice",                              Age = 25                          },                      new Person                          {                              Name = "Bob"                          }                  }; 

This language feature introduced in C# 3.0 also supports initializing anonymous types, which is especially useful in LINQ query expressions:

var person = new { Name = "Alice" }; 

They also work with arrays, but you can further omit the type which is inferred from the first element:

var myArray = new [] { "one", "two", "three" }; 

And initializing multi-dimensional arrays goes something like this:

var myArray = new string [,] { { "a1", "b1" }, { "a2", "b2" }, ... }; 

Update

Since C# 6.0, you can also use an index initializer. Here's an example of that:

var myDictionary = new Dictionary<string, int>                        {                            ["one"] = 1,                            ["two"] = 2,                            ["three"] = 3                        };