加入Linq

Question

When i declare

 int[] a = { 1, 2 ,10,30};

 int[] b = { 10, 20, 30 };

  var q = from item1 in a
                     join item2 in b
                     on item1 equals item2
                     into g
                     select g

1)What is actually getting selected into g ? It is difficult to understand the into keyword. if you give example to explain the keyword "into" ,i will be pleased.

2)

 How do the following code actually projected ?

1    from a in db.coll1
2    join b in db.coll2 on a.PK equals b.PK into b_join
3    from b in b_join.DefaultIfEmpty()
4    where
5      b.PK == null 
6   select new {
7      a.PK,
8      a.Value,
9      Column1 = (System.Int32?)b.PK,
10      Column2 = b.Value
    }

in line 2 we are using "b' to select the item in line 3 also we are using the same b ,does it mean we are overriding the data we selected at line 2 ?

Answer 1

1) You are doing a group join . g will be an IGrouping for each element in a . g.Key will be the element from a , and g (which implements IEnumerable ) will contain all of the matching elements from b . To understand this fully, try it out in LINQPad . Also, read the documentation .

2) The into keyword merges the first b into b_join , which will be an IGrouping as I described above. After that, the first b is no longer in scope.

Answer 2

If you use .NET Reflector on your compiled code, you can see that your first query is equivalent to the following:

IEnumerable<IEnumerable<int>> q = a.GroupJoin(b,
   item1 => item1,
   item2 => item2,
   (item1, group) => group);

Note that you are performing a group join, but only returning the groups. Compare this with an ordinary join:

IEnumerable<int> q = a.Join(b,
    item1 => item1,
    item2 => item2,
    (item1, item2) => item2);

This returns the elements in b that matches to each a, if any such element exists.