LINQ Single vs First

本文介绍了LINQ Single vs First的处理方法，对大家解决问题具有一定的参考价值，需要的朋友们下面随着小编来一起学习吧！

问题描述

LINQ:

当我确定查询将返回单条记录时，使用 Single() 运算符而不是 First() 是否更有效?/strong>?

Is it more efficient to use the Single() operator over First() when ever I know for certain that the query will return a single record?

有区别吗?

如果您期待单条记录，那么在您的代码中明确表示总是好的.

我知道其他人已经写过你为什么使用其中一个，但我想我会说明为什么你不应该使用一个，当你指的是另一个.

注意:在我的代码中，我通常会使用 FirstOrDefault() 和 SingleOrDefault() ，但这是一个不同的问题.

Note: In my code, I will typically use FirstOrDefault() and SingleOrDefault() but that's a different question.

以使用复合键( ID, Lang )以不同语言存储 Customers 的表为例:

Take, for example, a table that stores Customers in different languages using a Composite Key ( ID, Lang ):

DBContext db = new DBContext();
Customer customer = db.Customers.Where( c=> c.ID == 5 ).First();

上面的这段代码引入了一个可能的逻辑错误(难以追踪).它将返回多个记录(假设您有多种语言的客户记录)但它总是只返回第一个......有时可能有效......但其他人无效.这是不可预测的.

This code above introduces a possible logic error ( difficult to trace ). It will return more than one record ( assuming you have the customer record in multiple languages ) but it will always return only the first one... which may work sometimes... but not others. It's unpredictable.

由于您的意图是返回单个 Customer，请使用 Single();

Since your intent is to return a Single Customer use Single();

以下将引发异常(在本例中正是您想要的):

The following would throw an exception ( which is what you want in this case ):

DBContext db = new DBContext();
Customer customer = db.Customers.Where( c=> c.ID == 5 ).Single();

然后，您只需敲击自己的额头，然后对自己说……哎呀！我忘了语言领域！以下是正确的版本:

Then, you simply hit yourself on the forehead and say to yourself... OOPS! I forgot the language field! Following is the correct version:

DBContext db = new DBContext();
Customer customer = db.Customers.Where( c=> c.ID == 5 && c.Lang == "en" ).Single();

First() 在以下场景中很有用:

First() is useful in the following scenario:

DBContext db = new DBContext();
NewsItem newsitem = db.NewsItems.OrderByDescending( n => n.AddedDate ).First();

它将返回一个对象，并且由于您正在使用排序，它将是返回的最新记录.

It will return ONE object, and since you're using sorting, it will be the most recent record that is returned.

当您认为应该明确返回 1 条记录时使用 Single() 将帮助您避免逻辑错误.

Using Single() when you feel it should explicitly always return 1 record will help you avoid logic errors.

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