我同意这里的大多数帖子，它们趋向于零。

我的理由是，生成一个具有非空属性的空对象可能会导致错误。例如，具有int ID属性的实体的初始值为ID = 0，这是一个完全有效的值。如果这个对象，在某些情况下，被保存到数据库中，这将是一件坏事。

对于任何带有迭代器的东西，我总是使用空集合。类似的

foreach (var eachValue in collection ?? new List<Type>(0))

在我看来是代码的味道。集合属性永远不应该为空。

An edge case is String. Many people say, String.IsNullOrEmpty isn't really necessary, but you cannot always distinguish between an empty string and null. Furthermore, some database systems (Oracle) won't distinguish between them at all ('' gets stored as DBNULL), so you're forced to handle them equally. The reason for that is, most string values either come from user input or from external systems, while neither textboxes nor most exchange formats have different representations for '' and null. So even if the user wants to remove a value, he cannot do anything more than clearing the input control. Also the distinction of nullable and non-nullable nvarchar database fields is more than questionable, if your DBMS is not oracle - a mandatory field that allows '' is weird, your UI would never allow this, so your constraints do not map. So the answer here, in my opinion is, handle them equally, always.

Concerning your question regarding exceptions and performance: If you throw an exception which you cannot handle completely in your program logic, you have to abort, at some point, whatever your program is doing, and ask the user to redo whatever he just did. In that case, the performance penalty of a catch is really the least of your worries - having to ask the user is the elephant in the room (which means re-rendering the whole UI, or sending some HTML through the internet). So if you don't follow the anti-pattern of "Program Flow with Exceptions", don't bother, just throw one if it makes sense. Even in borderline cases, such as "Validation Exception", performance is really not an issue, since you have to ask the user again, in any case.

有趣的问题，我认为没有“正确”的答案，因为它总是取决于你的代码的职责。您的方法是否知道没有找到的数据是否存在问题?在大多数情况下，答案是“不”，这就是为什么返回null并让调用者处理他的情况是完美的。

也许区分抛出方法和返回null方法的一个好方法是在您的团队中找到一个约定:如果没有得到任何东西，表示它们“得到”某些东西的方法应该抛出异常。可能返回null的方法可以以不同的方式命名，可能是“Find…”。

这是一个业务问题，取决于具有特定Guid Id的用户的存在是否是该函数的预期正常用例，还是会阻止应用程序成功完成该方法向用户对象提供的任何功能的异常情况……

如果它是一个“异常”，因为缺少具有该Id的用户将阻止应用程序成功完成它正在做的任何功能，(假设我们正在为我们已向其发货的客户创建发票……)，那么这种情况应该抛出ArgumentException(或其他一些自定义异常)。

如果没有用户(调用此函数的潜在正常结果之一)，则返回空值....

编辑:(处理亚当在另一个回答中的评论)

If the application contains multiple business processes, one or more of which require a User in order to complete successfully, and one or more of which can complete successfully without a user, then the exception should be thrown further up the call stack, closer to where the business processes which require a User are calling this thread of execution. Methods between this method and that point (where the exception is being thrown) should just communicate that no user exists (null, boolean, whatever - this is an implementation detail).

但是如果应用程序中的所有进程都需要一个用户，我仍然会在这个方法中抛出异常……

如果您打算表示没有可用的数据，返回null通常是最好的主意。

空对象表示已返回数据，而返回null则明确表示没有返回任何数据。

此外，如果试图访问对象中的成员，返回null将导致null异常，这对于突出显示有bug的代码很有用——试图访问没有任何成员的成员是没有意义的。访问空对象的成员不会失败，这意味着bug不会被发现。

对于集合类型，我将返回一个空集合，对于所有其他类型，我更喜欢使用NullObject模式来返回实现与返回类型相同接口的对象。有关该模式的详细信息，请查看链接文本

使用NullObject模式这将是:-

public UserEntity GetUserById(Guid userId)

｛ //假设这里有一些代码来访问数据库.....

 //Check if data was returned and return a null if none found
 if (!DataExists)
    return new NullUserEntity(); //Should I be doing this here instead? return new UserEntity();  
 else
    return existingUserEntity;

}

class NullUserEntity: IUserEntity { public string getFirstName(){ return ""; } ...} 

如果用户没有被找到的情况经常出现，你想要根据情况以各种方式处理(有时抛出异常，有时替换一个空用户)，你也可以使用接近f#的Option或Haskell的Maybe类型，它显式地将“无值”情况与“发现了一些东西!”数据库访问代码看起来像这样:

public Option<UserEntity> GetUserById(Guid userId)
{
 //Imagine some code here to access database.....

 //Check if data was returned and return a null if none found
 if (!DataExists)
    return Option<UserEntity>.Nothing; 
 else
    return Option.Just(existingUserEntity);
}

并且像这样使用:

Option<UserEntity> result = GetUserById(...);
if (result.IsNothing()) {
    // deal with it
} else {
    UserEntity value = result.GetValue();
}

不幸的是，每个人似乎都有自己喜欢的类型。