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

在这种情况下,如果没有这样的用户,则最好返回“null”。还要使你的方法是静态的。

编辑:

通常这样的方法是一些“User”类的成员,不能访问它的实例成员。在这种情况下,方法应该是静态的,否则你必须创建一个“User”的实例,然后调用GetUserById方法,该方法将返回另一个“User”实例。我同意这很令人困惑。但是如果GetUserById方法是某个“DatabaseFactory”类的成员,那么将它作为实例成员是没有问题的。

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.

我不喜欢从任何方法返回null,而是使用Option函数类型。不返回结果的方法返回空选项,而不是null。

而且,不能返回结果的方法应该通过名称来指明。我通常把Try或TryGet或TryFind放在方法名称的开头,以表明它可能返回一个空结果(例如TryFindCustomer, TryLoadFile等)。

这让调用者可以对结果应用不同的技术,比如收集管道(参见Martin Fowler的收集管道)。

下面是另一个使用返回Option而不是null来降低代码复杂性的示例:如何降低圈复杂度:选项函数类型

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。

我更喜欢null,因为它与空合并操作符(??)兼容。

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……