Question

In C# i can write

var y = new List<string>(2) { "x" , "y" };

to get a List with “x” and “y” initialized.

How do I declare a class to accept this initialization syntax?

I mean, I want to write:

var y = new MyClass(2, 3) { "x" , "y" };

Answer 1

Look at section 7.6.10.3 of the C# spec:

The collection object to which a collection initializer is applied
must be of a type that implements System.Collections.IEnumerable or a
compile-time error occurs. For each specified element in order, the
collection initializer invokes an Add method on the target object with
the expression list of the element initializer as argument list,
applying normal overload resolution for each invocation. Thus, the
collection object must contain an applicable Add method for each
element initializer.

A very simple example of this:

   class AddIt : IEnumerable
   {
      public void Add(String foo) { Console.WriteLine(foo); }

      public IEnumerator GetEnumerator()
      {
         return null; // in reality something else
      }
   }

   class Program
   {
      static void Main(string[] args)
      {
         var a = new AddIt() { "hello", "world" };

         Console.Read();
      }
   }

This will print “hello”, followed by “world” to the console.