返回null可能更有效，因为不会创建新的对象。然而，它也经常需要一个空检查(或异常处理)。

从语义上讲，null和空列表的意思是不同的。这些差异是微妙的，在特定的情况下，一种选择可能比另一种更好。

不管你的选择是什么，记录下来以避免混淆。

大约一周前，我们在开发团队中讨论过这个问题，我们几乎一致同意采用空收集。有人想要返回null，原因与Mike上面指定的相同。

还有一点尚未被提及。考虑下面的代码:

    public static IEnumerable<string> GetFavoriteEmoSongs()
    {
        yield break;
    }

在调用此方法时，c#语言将返回一个空枚举数。因此，为了与语言设计(以及程序员的期望)保持一致，应该返回一个空集合。

我喜欢在这里用适当的例子来解释。

考虑一个案例。

int totalValue = MySession.ListCustomerAccounts()
                          .FindAll(ac => ac.AccountHead.AccountHeadID 
                                         == accountHead.AccountHeadID)
                          .Sum(account => account.AccountValue);

这里考虑我使用的函数..

1. ListCustomerAccounts() // User Defined
2. FindAll()              // Pre-defined Library Function

我可以很容易地使用ListCustomerAccount和FindAll代替。

int totalValue = 0; 
List<CustomerAccounts> custAccounts = ListCustomerAccounts();
if(custAccounts !=null ){
  List<CustomerAccounts> custAccountsFiltered = 
        custAccounts.FindAll(ac => ac.AccountHead.AccountHeadID 
                                   == accountHead.AccountHeadID );
   if(custAccountsFiltered != null)
      totalValue = custAccountsFiltered.Sum(account => 
                                            account.AccountValue).ToString();
}

注意:由于AccountValue不为空，Sum()函数不会为空 返回null。因此我可以直接使用它。

I call it my billion-dollar mistake…At that time, I was designing the first comprehensive type system for references in an object-oriented language. My goal was to ensure that all use of references should be absolutely safe, with checking performed automatically by the compiler. But I couldn’t resist the temptation to put in a null reference, simply because it was so easy to implement. This has led to innumerable errors, vulnerabilities, and system crashes, which have probably caused a billion dollars of pain and damage in the last forty years. – Tony Hoare, inventor of ALGOL W.

See here for an elaborate shit storm about null in general. I do not agree with the statement that undefined is another null, but it is still worth reading. And it explains, why you should avoid null at all and not just in the case you have asked. The essence is, that null is in any language a special case. You have to think about null as an exception. undefined is different in that way, that code dealing with undefined behavior is in most cases just a bug. C and most other languages have also undefined behavior but most of them have no identifier for that in the language.

空集合。如果您正在使用c#，那么假定最大化系统资源并不是必要的。虽然效率较低，但返回Empty Collection对于所涉及的程序员来说要方便得多(出于Will在上面概述的原因)。