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

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 ""; } ...} 

其他回答

我个人返回该对象的默认实例。原因是我希望该方法返回0到多或0到1(取决于该方法的目的)。使用这种方法,它将是任何类型的错误状态的唯一原因是,如果方法不返回任何对象,并且总是期望返回(就一对多或单数返回而言)。

至于假设这是一个业务领域的问题——我只是没有从等式的那一边看到它。返回类型的规范化是一个有效的应用程序体系结构问题。至少,它是编码实践标准化的主题。我怀疑是否有业务用户会说“在场景X中,只给他们一个null”。

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。

在我们的业务对象中,我们有两个主要的Get方法:

为了让事情在语境中变得简单,或者你的问题是:

// Returns null if user does not exist
public UserEntity GetUserById(Guid userId)
{
}

// Returns a New User if user does not exist
public UserEntity GetNewOrExistingUserById(Guid userId)
{
}

第一种方法用于获取特定的实体,第二种方法用于在网页上添加或编辑实体。

这使我们能够在使用它们的上下文中两全其美。

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.

我会说返回null而不是空对象。

但你在这里提到的具体例子, 您正在通过用户id搜索用户,即排序 那个用户的键,在这种情况下,我可能想要 如果没有用户实例,则抛出异常 发现。

这是我通常遵循的规则:

如果通过主键查找操作没有找到结果, 把ObjectNotFoundException。 如按任何其他标准没有发现结果, 返回null。 如果通过非关键条件查找未找到结果,则可能返回多个对象 返回一个空集合。