c# - 手动将 LINQ 查询语法转换为 Lambda 表示法

Question

我通常使用 lambda 表示法进行选择，但对连接感到沮丧。我使用 LINQPad 为自己设置了一个简单的练习。 LINQ 查询是:

List<int> allStudents = new List<int> {1,2,3,4,5,6,7,8,9};
List <int> studentsIdList = new List<int> {1,3,5,7,9};

var q = 
        from c in allStudents 
        join p in studentsIdList on c equals p into ps 
        from p in ps.DefaultIfEmpty() 
        where p == 0
        select new { Student = c}; 


q.Dump();

它会产生预期的结果集2,4,6,8。

然而，当我用 lambda 表示法将其写为:

List<int> allStudents = new List<int> {1,2,3,4,5,6,7,8,9};
List <int> studentsIdList = new List<int> {1,3,5,7,9};

var q = 
        allStudents
        .GroupJoin(
                   studentsIdList, 
                   m => allStudents,
                   n => studentsIdList, 
                   (m, n) => new  {allS = m, excS = n.DefaultIfEmpty(0)})
        .Where(x => x.excS.SingleOrDefault () == 0)
        .Select (x => x.allS);

q.Dump();

我得到的结果集为1,2,3,4,5,6,7,8,9

并且 LINQPad 不显示 lambda 转换。

两个问题:

• 我的 lambda 查询出了什么问题？
• 如何让 LINQPad 显示 lambda 翻译？

更新

使用下面的答案我能够纠正我的尝试

List<int> allStudents = new List<int> {1,2,3,4,5,6,7,8,9};
List <int> studentsIdList = new List<int> {1,3,5,7,9};

var q = allStudents
    .GroupJoin(studentsIdList, 
        a => a, b => b, 
        (a, b) => new { Id = a, Present = b.DefaultIfEmpty() })
    .Where(x => x.Present.Single() == 0)
    .Select(x => x.Id);

q.Dump();

非常感谢。

Answer 1

第二个问题的答案是在源集合上调用 .AsQueryable() 。这允许 LINQPad 显示 C# 到 lambda 语法的翻译:

List<int> allStudents = new List<int> {1,2,3,4,5,6,7,8,9};
List <int> studentsIdList = new List<int> {1,3,5,7,9};
var q = from c in allStudents.AsQueryable()
        join p in studentsIdList on c equals p into ps 
        from p in ps.DefaultIfEmpty() 
        where p == 0
        select new { Student = c}; 

q.Dump();