LambB表达式中的GroupBy

Question

from x in myCollection
    group x by x.Id into y
    select new { 
       Id = y.Key, 
       Quantity = y.Sum(x => x.Quantity)
    };

你如何将上面的内容写成lambda表达式？我被困在了group into一边.

Answer 1

查询连续(select ... into和group ... into,但不是 join ... into)等同于仅拆分查询表达式.所以我想把你的例子想象成:

var tmp = from x in myCollection
          group x by x.Id;
var result = from y in tmp
             select new { 
               Id = y.Key, 
               Quantity = y.Sum(x => x.Quantity)
             };

将这些更改为点符号:

var tmp = myCollection.GroupBy(x => x.Id);
var result = tmp.Select(y => new { 
               Id = y.Key, 
               Quantity = y.Sum(x => x.Quantity)
             });

然后你可以把它们组合起来:

var tmp = myCollection.GroupBy(x => x.Id)
                      .Select(y => new { 
                                Id = y.Key, 
                                Quantity = y.Sum(x => x.Quantity)
                              });

一旦你弄清楚C#编译器对查询表达式的作用,剩下的就相对简单了:)

Answer 2

myCollection.GroupBy(x => x.Id)
            .Select(y => new {
                                 Id = y.Key,
                                 Quantity = y.Sum(x => x.Quantity)
                             });

Answer 3

myCollection
    .GroupBy(x => x.Id)
    .Select(x => 
        new 
        { 
          Id = x.Key, 
          Quantity = x.Sum(y => x.Quantity
        });