为什么nullable bools不允许if(nullable)，但允许if(nullable == true)?

This code compiles:

这段代码编译:

    static void Main(string[] args)
    {
        bool? fred = true;

        if (fred == true)
        {
            Console.WriteLine("fred is true");
        }
        else if (fred == false)
        {
            Console.WriteLine("fred is false");
        }
        else
        {
            Console.WriteLine("fred is null");
        }
    }
    static void