“兼容包”用于向后移植新的.NET Framework功能？

For various reasons I often find it desirable to write code that is compatible with .NET framework 2.0 or 3.5 or compatible with the .NET Compact Framework, but it is a problem that there are numerous "small" features in new .NET frameworks that are not available in the older frameworks or Compact Framework.For various reasons I often find it desirable t