Object initializer surprise.

thingthangthung.cs

void Main()
{
  var t = new Thing(new Thang { Thung = "lol" })
  {
    Thang = { { "OMG" } }
  };

  t.Dump();
}

class Thing
{
  private readonly Thang _thang;

  public Thing(Thang thang)
  {
    _thang = thang;
  }

  public Thang Thang
  {
    get { return _thang; }
  }
}

class Thang : IEnumerable
{
  public string Thung { get; set; }

  public void Add(string s)
  {
    Thung = s;
  }

  IEnumerator IEnumerable.GetEnumerator()
  {
    return new[] { Thung }.GetEnumerator();
  }
}