如何在LINQ查询中使用分组依据？

Question

There is linq query in my existing project, something like this:

group new {a.name,a,description,a.number,i.productname}
by new {i.productname} into grpProduct

There is a Group By in my query and its syntax is like above query. Means it's using group new and after that by new . But I am not getting the purpose of this syntax. Can anyone help me out on this?

Answer 1

Let's say you had a class like this:

public class Student
{
    public int Id { get; set; }

    public string Name { get; set; }

    public int Age { get; set; }
}

And you defined a list and added some students to it...

var students = new List<Student>();

And then you created groups from it like this:

var i = (from s in students
         group new { s.Id, s.Name }
         by new { s.Age });

The by new section specifies the key(s) by which you'd like to create the grouping. So when you execute the query, you'll get one group per Age , and Age will be the "key" for each group.

(Since there's only one field specified, you could rewrite that part as by s.Age .)

The group new section specifies what you'd like to have available in each "group". Whereas Age will be the key, when you drill down into each group you'll have a collection of an anonymous type that includes Id and Name .

Answer 2

The by statement declares what will be the key value used to group your data, while the group statement selects what will be "grouped". So your result is going to be a IGrouping on i.productname (it will be the key) and if you fetch each a these grouped items you will access to all the new {a.name,a,description,a.number,i.productname} objects with i.productname corresponding to this key.