如何使用 LAMBDA 表达式在 LINQ 中执行 IN 或 CONTAINS？

Question

我有以下 Transact-Sql，我正在尝试将其转换为 LINQ ...并且很挣扎。

SELECT * FROM Project
WHERE Project.ProjectId IN (SELECT ProjectId FROM ProjectMember Where MemberId = 'a45bd16d-9be0-421b-b5bf-143d334c8155')

任何帮助将不胜感激......如果可能的话，我想用 Lambda 表达式来实现。

提前致谢！

Answer 1

您可能需要 Any() 运算符：

var q = db.Projects
          .Where(p => db.ProjectMembers
                        .Where(pm => pm.MemberId == memberId)
                        .Any  (pm => p.ProjectId == pm.ProjectId)); 

由于 db.ProjectMembers.Where(...) 的结果集始终相同，因此您可以将其拆分，以便它只执行一次：

var projectMembers = db.ProjectMembers.Where(pm => pm.MemberId == memberId);
var q              = db.Projects
                       .Where(p  => projectMembers
                                    .Any(pm => p.ProjectId == pm.ProjectId));

Answer 2

GFrizzle 比我先一步。 但这里是一个 C# 版本

var projectsMemberWorkedOn = from p in Projects
                    join projectMember in ProjectMembers on
                        p.ProjectId equals projectMember.ProjectId
                    where projectMember.MemberId == "a45bd16d-9be0-421b-b5bf-143d334c8155"
                    select p;

，作为奖励，还有一个纯 LINQ 方法链版本：

var projectsMemberWorkedOn =
            Projects.Join( ProjectMembers, p => p.ProjectId, projectMember => projectMember.ProjectId,
                ( p, projectMember ) => new { p, projectMember } )
                .Where( @t => @t.projectMember.MemberId == "a45bd16d-9be0-421b-b5bf-143d334c8155" )
                .Select(@t => @t.p );

Answer 3

在这种情况下，您可以只使用 .Contains()，如下所示：

var projects = 
from p in db.Projects
where db.ProjectMembers.Where(m => m.MemberId == "a45bd16d-9be0-421b-b5bf-143d334c8155").Select(pp => pp.ProjectID).Contains(p.ProjectID)
select p;

Answer 4

不需要 lambda，这里只需一个简单的 LINQ 连接：

From p in db.Projects _
Join m in db.ProjectMember On p.ProjectId Equals m.ProjectId _
Where m.MemberId = "a45bd16d-9be0-421b-b5bf-143d334c8155" _
Select p