在*上看到关于这个问题的讨论，打算记录下来。
关于全连接(Full Outer Join)，Linq并不支持，所以就需要使用其它方法来实现，就像不支持全连接的数据库一样，先使用内连接找出公共的部分，然后分别找出左连接和右连接的部分，把这三部分的结果UNION一下，即可得到全连接的效果。
看问题：
ID FirstName
-- ---------
1 John
2 Sue
ID LastName
-- --------
1 Doe
3 Smith显示下面的结果：ID FirstName LastName
-- --------- --------
1 John Doe
2 Sue
3 Smith
实现方法一：
先找出左连接的结果，再找出右连接的结果，然后把这两个结果UNION即可，UNION时会自动把重复的数据过滤掉。
var firstNames = new[]
{
new { ID = 1, Name = "John" },
new { ID = 2, Name = "Sue" },
};
var lastNames = new[]
{
new { ID = 1, Name = "Doe" },
new { ID = 3, Name = "Smith" },
};
var leftOuterJoin = from first in firstNames
join last in lastNames
on first.ID equals last.ID
into temp
from last in temp.DefaultIfEmpty(new { first.ID, Name = default(string) })
select new
{
first.ID,
FirstName = first.Name,
LastName = last.Name,
};
var rightOuterJoin = from last in lastNames
join first in firstNames
on last.ID equals first.ID
into temp
from first in temp.DefaultIfEmpty(new { last.ID, Name = default(string) })
select new
{
last.ID,
FirstName = first.Name,
LastName = last.Name,
};
var fullOuterJoin = leftOuterJoin.Union(rightOuterJoin);
方法二：
得到左连接的数据，再从右连接的数据中把左连接的数据排除掉，两者的数据Concat一下即可
var firstNames = new[]
{
new { ID = 1, Name = "John" },
new { ID = 2, Name = "Sue" },
};
var lastNames = new[]
{
new { ID = 1, Name = "Doe" },
new { ID = 3, Name = "Smith" },
};
var leftData = (from first in firstNames
join last in lastNames on first.ID equals last.ID into temp
from last in temp.DefaultIfEmpty(new { first.ID, Name = default(string) })
select new
{
first.ID,
FirstName = first.Name,
LastName = last.Name,
});
var rightRemainingData = (from r in lastNames
where !(from a in leftData select a.ID).Contains(r.ID)
select new
{
r.ID,
FirstName = default(string),
LastName = r.Name
});
var fullOuterjoinData = leftData.Concat(rightRemainingData);
以上两种方法，其实是同一种方式来实现全连接的，只是第二次拿出的数据有所不同。
以上，希望对大家有所帮助。