对于LINQ -> SQL:

SingleOrDefault

将生成查询，如“select TOP 2 * from users where userid = 1” 选择匹配的记录，如果发现多个记录则抛出异常 如果基于主键/唯一键列获取数据，则使用

FirstOrDefault

将生成类似于“select userid = 1的用户中的top 1 *”的查询 选择第一个匹配的行 如果基于非主键/唯一键列获取数据，则使用

没有人提到SQL中转换的FirstOrDefault执行TOP 1记录，而SingleOrDefault执行TOP 2记录，因为它需要知道是否有多个记录。

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.

无论何时使用SingleOrDefault，都明确声明查询最多只能产生一个结果。另一方面，当使用FirstOrDefault时，查询可以返回任意数量的结果，但您声明只想要第一个结果。

我个人认为语义非常不同，根据预期结果使用适当的语义可以提高可读性。

两者都是元素操作符，用于从序列中选择单个元素。但是它们之间有一个微小的区别。SingleOrDefault()操作符将在多个元素满足条件时抛出异常，而as FirstOrDefault()将不会对相同元素抛出任何异常。下面是一个例子。

List<int> items = new List<int>() {9,10,9};
//Returns the first element of a sequence after satisfied the condition more than one elements
int result1 = items.Where(item => item == 9).FirstOrDefault();
//Throw the exception after satisfied the condition more than one elements
int result3 = items.Where(item => item == 9).SingleOrDefault();

SingleOrDefault:您的意思是“最多”有一个项与查询或默认值匹配 FirstOrDefault:您的意思是有“至少”一项与查询或默认值匹配

下次你需要选择的时候大声说出来，你可能会做出明智的选择。：）