第()

当您知道结果包含超过1个期望的元素时，您应该只输入序列的第一个元素。

FirstOrDefault ()

FirstOrDefault()与First()类似，不同之处在于，如果没有元素匹配指定的条件，它将返回泛型集合的底层类型的默认值。如果没有找到元素，它不会抛出InvalidOperationException。但如果元素或序列的集合为空，则抛出异常。

第一:

返回序列的第一个元素 抛出异常:结果中没有元素 使用when:当期望有多个元素，而您只想要第一个元素时

FirstOrDefault:

返回序列的第一个元素，如果没有找到元素，则返回默认值 抛出异常:仅当源为空时 使用when:当期望有多个元素，而您只想要第一个元素时。同样，结果为空也是可以的

来自:http://www.technicaloverload.com/linq-single-vs-singleordefault-vs-first-vs-firstordefault/

.First will throw an exception when there are no results. .FirstOrDefault won't, it will simply return either null (reference types) or the default value of the value type. (e.g like 0 for an int.) The question here is not when you want the default type, but more: Are you willing to handle an exception or handle a default value? Since exceptions should be exceptional, FirstOrDefault is preferred when you're not sure if you're going to get results out of your query. When logically the data should be there, exception handling can be considered.

Skip()和Take()通常在设置结果分页时使用。(比如显示前10个结果，接下来的10个在下一页，等等)

someList.First(); // exception if collection is empty.
someList.FirstOrDefault(); // first item or default(Type)

用哪一个? 它应该由业务逻辑决定，而不是担心异常/程序失败。

例如, 如果业务逻辑说我们在任何工作日都不可能有零事务(只是假设)。那么您不应该尝试使用一些聪明的编程来处理这种情况。 我将始终在这样的集合上使用First()，如果有其他东西搞砸了业务逻辑，则让程序失败。

代码:

var transactionsOnWorkingDay = GetTransactionOnLatestWorkingDay();
var justNeedOneToProcess = transactionsOnWorkingDay.First(): //Not FirstOrDefault()

我想看看其他人对此的评论。

Others have very well described the difference between First() and FirstOrDefault(). I want to take a further step in interpreting the semantics of these methods. In my opinion FirstOrDefault is being overused a lot. In the majority of the cases when you’re filtering data you would either expect to get back a collection of elements matching the logical condition or a single unique element by its unique identifier – such as a user, book, post etc... That’s why we can even get as far as saying that FirstOrDefault() is a code smell not because there is something wrong with it but because it’s being used way too often. This blog post explores the topic in details. IMO most of the times SingleOrDefault() is a much better alternative so watch out for this mistake and make sure you use the most appropriate method that clearly represents your contract and expectations.

这种类型的函数属于元素操作符。下面定义了一些有用的元素操作符。

第一/ FirstOrDefault 去年/ LastOrDefault 单/ SingleOrDefault

当需要根据特定条件从序列中选择单个元素时，我们使用元素操作符。这里有一个例子。

  List<int> items = new List<int>() { 8, 5, 2, 4, 2, 6, 9, 2, 10 };

First()操作符返回序列满足条件后的第一个元素。如果没有找到元素，则抛出异常。

Int result = items。Where(item => item == 2).First();

FirstOrDefault()操作符返回满足条件后序列的第一个元素。如果没有找到元素，则返回该类型的默认值。

Int result1 = items。Where(item => item == 2).FirstOrDefault();