C# ??=运算符

PlatformList ??= new List<SnowPlatform>();

等效于

      
if (PlatformList==null)
            {
                PlatformList = new List<SnowPlatform>();
            }

等效于

PlatformList = (PlatformList == null) ? new List<SnowPlatform>(): PlatformList;

也等效于

    PlatformList = PlatformList ?? new List<SnowPlatform>();

终极简化写法:

    PlatformList ??= new List<SnowPlatform>();