C# object initialization of read only collection properties

Question

For the life of me, I cannot figure out what is going on in the example piece of C# code below. The collection (List) property of the test class is set as read only, but yet I can seemingly assign to it in the object initializer.

** EDIT: Fixed the problem with the List 'getter'

using System;
using System.Collections.Generic;
using NUnit.Framework;

namespace WF4.UnitTest
{
    public class MyClass
    {
        private List<string> _strCol = new List<string> {"test1"};

        public List<string> StringCollection 
        {
            get
            {
                return _strCol;
            }
        }
    }

    [TestFixture]
    public class UnitTests
    {
        [Test]
        public void MyTest()
        {
            MyClass c = new MyClass
            {
                // huh?  this property is read only!
                StringCollection = { "test2", "test3" }
            };

            // none of these things compile (as I wouldn't expect them to)
            //c.StringCollection = { "test1", "test2" };
            //c.StringCollection = new Collection<string>();

            // 'test1', 'test2', 'test3' is output
            foreach (string s in c.StringCollection) Console.WriteLine(s);
        }
    }
}

Answer 1

This:

MyClass c = new MyClass
{
    StringCollection = { "test2", "test3" }
};

is translated into this:

MyClass tmp = new MyClass();
tmp.StringCollection.Add("test2");
tmp.StringCollection.Add("test3");
MyClass c = tmp;

It's never trying to call a setter - it's just calling Add on the results of calling the getter. Note that it's also not clearing the original collection either.

This is described in more detail in section 7.6.10.3 of the C# 4 spec.

EDIT: Just as a point of interest, I was slightly surprised that it calls the getter twice. I expected it to call the getter once, and then call Add twice... the spec includes an example which demonstrates that.

Answer 2

You aren't calling the setter; you are essentially calling c.StringCollection.Add(...) each time (for "test2" and "test3") - it is a collection initializer. For it to be the property assignment, it would be:

// this WON'T work, as we can't assign to the property (no setter)
MyClass c = new MyClass
{
    StringCollection = new StringCollection { "test2", "test3" }
};