What is this syntax using new followed by a list inside braces?

Question

Ive just seen a piece of code that uses a generic list class to instantiate itself in the following manner:

var foo = new List<string>(){"hello", "goodbye"}; 

The curly braces after the contructor are especially confusing. It reminds me somewhat of

var bar = new string[]{"hi","bye"}; 

but in the past i've wouldve always used:

var foo = new List<string>(new []{"hello", "goodbye"}); 

Has anybody got a link explaining the syntax in the first line of code? I wouldnt even know where to begin with googling it.

Answer 1

As others have pointed out, that is a collection initializer. Some other features you might not be aware of that were added to C# 3:

• A collection initializer constructor may omit the parentheses if the argument list is empty. So new List<int> { 10, 20, 30 } is fine.
• An array initialized with an array initializer may in some cases omit the type. For example, var myInts = new[] { 10, 20, 30}; infers that myInts is int[].
• Objects may be initialized using a similar object initializer syntax. var c = new Customer() { Name = "Fred" }; is the same as var temp = new Customer(); temp.Name = "Fred"; var c = temp;

The point of these features is to (1) make more things that used to require statements into things that require only expressions; LINQ likes things to be expressions, and (2) to enable richer type inference, particularly for anonymous types.

Finally: there has been some confusion in some of the answers and comments regarding what is required for a collection initializer. To be used with a collection initializer the type must (1) implement IEnumerable (so that we know it is a collection) and (2) have an Add method (so that we can add stuff to it.)

See

http://blogs.msdn.com/b/madst/archive/2006/10/10/what-is-a-collection_3f00_.aspx

for additional thoughts on the design of the feature.