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

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.

其他回答

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。

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……

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.

请原谅我的伪php/代码。

我认为这真的取决于结果的预期用途。

如果你想编辑/修改返回值并保存它,那么返回一个空对象。这样,您就可以使用相同的函数在新对象或现有对象上填充数据。

假设我有一个函数,它接受一个主键和一个数据数组,用数据填充行,然后将结果记录保存到db。因为我打算用我的数据填充对象,所以从getter返回一个空对象可能是一个巨大的优势。这样,我可以在两种情况下执行相同的操作。无论如何都要使用getter函数的结果。

例子:

function saveTheRow($prim_key, $data) {
    $row = getRowByPrimKey($prim_key);

    // Populate the data here

    $row->save();
}

在这里,我们可以看到相同的一系列操作操作了该类型的所有记录。

但是,如果返回值的最终目的是读取数据并对数据做一些事情,那么我将返回null。这样,我可以非常快速地确定是否没有返回数据,并向用户显示适当的消息。

通常,我将在检索数据的函数中捕获异常(因此我可以记录错误消息等),然后从捕获中直接返回null。对于最终用户来说,问题是什么通常并不重要,因此我发现最好将错误记录/处理直接封装在获取数据的函数中。如果你在任何大公司维护一个共享的代码库,这是特别有益的,因为你可以强制适当的错误记录/处理,即使是最懒的程序员。

例子:

function displayData($row_id) {
    // Logging of the error would happen in this function
    $row = getRow($row_id);
    if($row === null) {
        // Handle the error here
    }

    // Do stuff here with data
}

function getRow($row_id) {
 $row = null;
 try{
     if(!$db->connected()) {
   throw excpetion("Couldn't Connect");
  }

  $result = $db->query($some_query_using_row_id);

  if(count($result) == 0 ) {
   throw new exception("Couldn't find a record!");
  }

  $row = $db->nextRow();

 } catch (db_exception) {
  //Log db conn error, alert admin, etc...
  return null; // This way I know that null means an error occurred
 }
 return $row;
}

这是我的一般原则。到目前为止,它运行得很好。

还有一种方法涉及传入一个回调对象或委托,它将对值进行操作。如果没有找到值,则不调用回调。

public void GetUserById(Guid id, UserCallback callback)
{
    // Lookup user
    if (userFound)
        callback(userEntity);  // or callback.Call(userEntity);
}

当您希望避免在整个代码中进行空检查时,以及当找不到值并不是错误时,这种方法非常有效。如果需要任何特殊处理,还可以在没有找到对象时提供回调。

public void GetUserById(Guid id, UserCallback callback, NotFoundCallback notFound)
{
    // Lookup user
    if (userFound)
        callback(userEntity);  // or callback.Call(userEntity);
    else
        notFound(); // or notFound.Call();
}

使用单个对象的相同方法如下所示:

public void GetUserById(Guid id, UserCallback callback)
{
    // Lookup user
    if (userFound)
        callback.Found(userEntity);
    else
        callback.NotFound();
}

从设计的角度来看,我真的很喜欢这种方法,但是它的缺点是在不支持第一类函数的语言中使调用站点变得更庞大。