C#字符串默认值

using System;
class MYTestX
{
    class CT
    {
    }
    class CO
    {
        public CT ott;  //默认是null
        public string strx;//默认也是null，而不是空串""
        public virtual void Test()
        {
            Console.WriteLine("co-testJ");
        }
    }
    static void Main(string[] args)
    {
        CO oo = new CO();
        CO ot = null;

        Console.WriteLine(oo.strx + "ttt");
        Console.WriteLine(null + "ttt"); //null可以与字符串连接，不会出现异常
    }
}