Lambda LINQ 查询带有多个 from 子句

Question

I've got this query

var myobjectList = from g in repository.SpeedGoalsRepository.DbSet.Where(e => e.ID == null)
                   from d in daysOfWeek
                   select new myObject
                   {
                     DayID = (short?)d,
                     GoalA = g.GoalA,
                     GoalB = g.GoalB
                   };

where daysOfWeek is an enum array.

private readonly DayOfWeek[] daysOfWeek = new DayOfWeek[]
{
    DayOfWeek.Sunday,
    DayOfWeek.Monday,
    DayOfWeek.Tuesday,
    DayOfWeek.Wednesday,
    DayOfWeek.Thursday,
    DayOfWeek.Friday,
    DayOfWeek.Saturday
};

so, I need to convert this query sintax to lamda expression. I try with this, but its wrong:(

var defaultSpeedGoals= repository.SpeedGoalsRepository.DbSet.Where(e => e.ID == null);
var myobjectList = SpeedGoals.Cast<SpeedGoalsRepository>()
                            .SelectMany(g => g.DbSet.Cast<DayOfWeek>().Select(sg => new myObject
                            {
                                DayID = (short?)sg,
                                GoalA= g.DbSet.FirstOrDefault().GoalA,
                                GoalB = g.DbSet.FirstOrDefault().GoalB
                            }));

Answer 1

The page you want to look at is here:

https://docs.microsoft.com/en-us/dotnet/csharp/language-reference/language-specification/expressions

It's a lot of text. The bit you want specifically is:

A query expression with a second from clause followed by a select clause

from x1 in e1
from x2 in e2
select v

is translated into

( e1 ) . SelectMany( x1 => e2 , ( x1 , x2 ) => v )

Let's apply that to your example:

from g in repository.SpeedGoalsRepository.DbSet.Where(e => e.ID == null)
from d in daysOfWeek
select new myObject
{
  DayID = (short?)d,
  GoalA = g.GoalA,
  GoalB = g.GoalB
};

• x1 is g
• e1 is repository.SpeedGoalsRepository.DbSet.Where(e => e.ID == null) .
• x2 is d
• e2 is daysOfWeek
• v is new myObject...

So put it all together:

repository.SpeedGoalsRepository.DbSet
  .Where(e => e.ID == null) 
  .SelectMany(
    g => daysOfWeek, 
    (g, d) => new myObject { ... } )

and we're done.