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手动将LINQ查询语法转换为Lambda表示法

linqpad lambda linq c#

2

Peter Smith · 技术社区 · 12 年前

我通常使用lambda表示法进行选择,但对连接感到失望。我用LINQPad给自己做了一个简单的练习。LINQ查询为:

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 = 
        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();

产生预期的结果集 2,4,6,8 .

然而,当我用lambda表示法将其写成:

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();

我得到一组结果 1,2,3,4,5,6,7,8,9

AND LINQPad不显示lambda转换。

两个问题:

lambda查询有什么问题?
如何让LINQPad显示lambda转换?

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。

2 回复 | 直到 12 年前

1

3

Wasif Hossain 12 年前

第二个问题的答案是打电话 .AsQueryable() 在源集合上。这允许LINQPad显示C#到lambda语法的转换:

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();

2

voidengine 12 年前

这是如何编写联接的:

var q = allStudents
    .GroupJoin(studentsIdList, a => a, b => b, (a, b) => new { Id = a, Present = b })
    .Where(join => !join.Present.Any())
    .Select(join => join.Id);

当然,对于这种情况,使用 Except 会简单得多。

无法帮助解决Linqpad问题,因为我自己不会使用它。