从函数返回数据的最佳实践是什么?是返回Null对象好还是返回空对象好?为什么要选择一种而不是另一种呢?

考虑一下:

public 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 null; 
        //Should I be doing this here instead? 
        //return new UserEntity();  
     else
        return existingUserEntity;
}

让我们假设在这个程序中存在有效的情况,即数据库中没有具有该GUID的用户信息。我认为在这种情况下抛出异常是不合适的??另外,我的印象是异常处理会损害性能。


当前回答

更多的肉要磨:让我们说我的DAL返回一个NULL的GetPersonByID,正如一些建议。我的(相当薄)BLL应该做什么,如果它收到一个NULL?传递NULL,并让最终消费者担心它(在这种情况下,一个ASP。网络页面)?让BLL抛出一个异常怎么样?

BLL可能正在被ASP使用。Net和Win App,或者其他类库——我认为期望最终消费者本质上“知道”GetPersonByID方法返回null是不公平的(除非使用空类型,我猜)。

My take (for what it's worth) is that my DAL returns NULL if nothing is found. FOR SOME OBJECTS, that's ok - it could be a 0:many list of things, so not having any things is fine (e.g. a list of favourite books). In this case, my BLL returns an empty list. For most single entity things (e.g. user, account, invoice) if I don't have one, then that's definitely a problem and a throw a costly exception. However, seeing as retrieving a user by a unique identifier that's been previously given by the application should always return a user, the exception is a "proper" exception, as in it's exceptional. The end consumer of the BLL (ASP.Net, f'rinstance) only ever expects things to be hunky-dory, so an Unhandled Exception Handler will be used instead of wrapping every single call to GetPersonByID in a try - catch block.

如果我的方法有明显的问题,请让我知道,因为我总是渴望学习。正如其他帖子所说,异常是代价高昂的事情,“先检查”的方法是好的,但异常应该只是例外。

我很喜欢这篇文章,很多关于“视情况而定”的好建议:-)

其他回答

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.

我们使用CSLA。NET,并且它认为失败的数据获取应该返回一个“空”对象。这实际上是相当烦人的,因为它要求检查obj。IsNew而不是obj == null。

正如前面提到的,null返回值将导致代码立即失败,从而降低了由空对象引起的隐形问题的可能性。

就我个人而言,我认为null更优雅。

这是一种非常常见的情况,我很惊讶这里的人似乎对此感到惊讶:在任何web应用程序中,数据通常是使用querystring参数获取的,这显然会被破坏,因此要求开发人员处理“未找到”的情况。

你可以这样处理:

if (User.Exists(id)) {
  this.User = User.Fetch(id);
} else {
  Response.Redirect("~/notfound.aspx");
}

...但是这每次都是对数据库的额外调用,这在高流量页面上可能是一个问题。而:

this.User = User.Fetch(id);

if (this.User == null) {
  Response.Redirect("~/notfound.aspx");
}

...只需要一个呼叫。

把别人说的话用更简洁的方式来表达……

例外情况只适用于特殊情况

如果这个方法是纯数据访问层,我会说,给定一些参数,被包含在一个选择语句中,它将期望我可能找不到任何行,从中构建一个对象,因此返回null将是可接受的,因为这是数据访问逻辑。

另一方面,如果我希望我的参数反映一个主键,我应该只返回一行,如果我返回了不止一行,我就会抛出异常。0可以返回null, 2则不行。

现在,如果我有一些登录代码检查LDAP提供程序,然后检查DB以获得更多详细信息,并且我希望它们始终保持同步,那么我可能会抛出异常。正如其他人所说,这是商业规则。

Now I'll say that is a general rule. There are times where you may want to break that. However, my experience and experiments with C# (lots of that) and Java(a bit of that) has taught me that it is much more expensive performance wise to deal with exceptions than to handle predictable issues via conditional logic. I'm talking to the tune of 2 or 3 orders of magnitude more expensive in some cases. So, if it's possible your code could end up in a loop, then I would advise returning null and testing for it.

我个人会返回null,因为这是我所期望的DAL/Repository层的操作方式。

如果它不存在,不要返回任何可以被解释为成功获取对象的东西,null在这里工作得很好。

最重要的是在你的DAL/Repos层保持一致,这样你就不会对如何使用它感到困惑。

这取决于什么对你的案子最有意义。

返回null是否有意义?“不存在这样的用户”?

或者创建一个默认用户有意义吗?当您可以安全地假设如果用户不存在,则调用代码在请求时希望用户存在时,这是最有意义的。

或者,如果调用代码要求使用无效ID的用户,抛出异常(如“FileNotFound”)是否有意义?

然而,从分离关注点/SRP的角度来看,前两点更为正确。从技术上讲,第一个是最正确的(但只差一点点)——GetUserById应该只负责一件事——获取用户。通过返回其他内容来处理自己的“用户不存在”情况可能违反SRP。如果您确实选择抛出异常,则分隔为不同的check - bool DoesUserExist(id)是合适的。

根据下面大量的评论:如果这是一个api级别的设计问题,这个方法可以类似于“OpenFile”或“readwholfile”。我们正在从某个存储库中“打开”一个用户,并从结果数据中补充对象。在这种情况下,一个例外可能是合适的。也许不是,但也有可能。

所有的方法都是可以接受的——这取决于API/应用程序的上下文。