C# 如何将这些linq查询组合成一个查询？

作为LINQ的新手，我创建了几个查询，并希望将它们合并为一个查询，但我不知道如何实现。这是：

var u = dc.Users.Where(w => w.UserName == userName).SingleOrDefault();

var m = dc.Memberships.Where(w => w.UserId == u.UserId).SingleOrDefault();

m.PasswordQuestion = securityQuestion;
m.PasswordAnswer = securityAnswer;

dc.SubmitChanges();

dc.Users

是

aspnet\u用户表

dc.Membership
是
aspnet\u会员资格表

另外，
SingleOrDefault
和
FirstOrDefault
之间有什么区别？
如果序列包含多个元素，SingleOrDefault将抛出错误，FirstOrDefault则不会。在这种情况下，您可能需要FirstOrDefault，因为用户可以有多个成员资格

var m = dc.MemberShips where(w => w.UserId.UserName == userName).FirstOrDefault();

SingleOrDefault的意思是“如果没有元素，给我默认值，如果有，给我默认值，否则抛出异常”。FirstOrDefault的意思是“如果没有元素，那么给我默认值，否则给我第一个”

至于你关于组合查询的问题——为什么？如果代码写得很好，为什么要修改它

回答一个您没有问过的问题：组合查询的常用方法是使用查询延续。一种常见的模式是：

var q1 = from y in something 
         somequeryclauses;
var q2 = from x in q1 
         someotherquerqyclauses;

您可以将其编写为一个大查询，如下所示：

var q2 = from x in (from y in something 
                    somequeryclauses) 
         someotherqueryclauses;

var m = dc.Memberships.Where(w => w.Users.UserName == userName).SingleOrDefault(); 

m.PasswordQuestion = securityQuestion; 
m.PasswordAnswer = securityAnswer; 

dc.SubmitChanges(); 

var membership = dc.Memberships.Where(x => x.User.UserName == userName).SingleOrDefault();

var membership = (from m in dc.Membership join u in dc.Users on u.UserId equals m.UserId
                  where u.UserName == userName
                  select u).SingleOrDefault();

这很难理解。您可以将这两个查询与查询延续相结合：

var q2 = from y in something 
         somequeryclauses 
         into x
         someotherqueryclauses;

有意义吗？
我前面没有Visual Studio，但它应该是这样的：

var q2 = from x in (from y in something 
                    somequeryclauses) 
         someotherqueryclauses;

var m = dc.Memberships.Where(w => w.Users.UserName == userName).SingleOrDefault(); 

m.PasswordQuestion = securityQuestion; 
m.PasswordAnswer = securityAnswer; 

dc.SubmitChanges(); 

var membership = dc.Memberships.Where(x => x.User.UserName == userName).SingleOrDefault();

var membership = (from m in dc.Membership join u in dc.Users on u.UserId equals m.UserId
                  where u.UserName == userName
                  select u).SingleOrDefault();

w、 用户允许您跟踪成员资格表和用户表之间的外键链接

如果返回多个结果，SingleOrDefault将引发异常。FirstOrDefault只返回第一条记录。
不确定它们是否有关系（应该有关系）。如果是这样，那么LINQtoSQL设计器将为您提供成员身份对象的用户属性（或者反过来）。然后你可以这样写：

var q2 = from x in (from y in something 
                    somequeryclauses) 
         someotherqueryclauses;

var m = dc.Memberships.Where(w => w.Users.UserName == userName).SingleOrDefault(); 

m.PasswordQuestion = securityQuestion; 
m.PasswordAnswer = securityAnswer; 

dc.SubmitChanges(); 

var membership = dc.Memberships.Where(x => x.User.UserName == userName).SingleOrDefault();

var membership = (from m in dc.Membership join u in dc.Users on u.UserId equals m.UserId
                  where u.UserName == userName
                  select u).SingleOrDefault();

如果他们没有关系，你可以这样写：

var q2 = from x in (from y in something 
                    somequeryclauses) 
         someotherqueryclauses;

var m = dc.Memberships.Where(w => w.Users.UserName == userName).SingleOrDefault(); 

m.PasswordQuestion = securityQuestion; 
m.PasswordAnswer = securityAnswer; 

dc.SubmitChanges(); 

var membership = dc.Memberships.Where(x => x.User.UserName == userName).SingleOrDefault();

var membership = (from m in dc.Membership join u in dc.Users on u.UserId equals m.UserId
                  where u.UserName == userName
                  select u).SingleOrDefault();

SingleOrDefault（）和FirstOrDefault（）之间的区别在于SingleOrDefault（）假定与查询匹配的项不超过一个。如果两个项与查询匹配，则将引发异常。如果您使用FirstOrDefault（），并且有两项与查询匹配，则会选择第一项。
您的第二个问题是重复的：