左外連接中的LINQ

Question

可能重複：
How do you perform a left outer join using linq extension methods

我不能找到的LINQ拉姆達的左外連接的例子（與擴展方法），至少，不是一個明確的。

比方說，我有如下表：

Parent 
{ 
    PID  // PK 
} 

Child 
{ 
    CID  // PK 
    PID  // FK 
    Text 
} 

我想參加家長與兒童，併爲每一個孩子不見了，我要爲文本設置爲默認值「[空]」。我怎樣才能用linq lambda語法來做到這一點？

目前，我有以下幾點：

var source = lParent.GroupJoin(
    lChild, 
    p => p.PID, 
    c => c.PID, 
    (p, g) => 
     new // ParentChildJoined 
     { 
      PID = p.PID; 
      // How do I add child values here? 
     }); 

Answer 1

你靠近。下面將選擇PID，爲每個父CID和Text爲每一個孩子，和PID，CID = -1和Text = "[[Empty]]"沒有孩子：

var source = lParent.GroupJoin(
    lChild, 
    p => p.PID, 
    c => c.PID, 
    (p, g) => g 
     .Select(c => new { PID = p.PID, CID = c.CID, Text = c.Text }) 
     .DefaultIfEmpty(new { PID = p.PID, CID = -1, Text = "[[Empty]]" })) 
    .SelectMany(g => g); 

Answer 2

from p in Parent 
join c in Child on p.PID equals c.PID into g 
from c in g.DefaultIfEmpty 
select new 
{ 
    p.PID, 
    CID = c != null ? (int?)c.CID : null, // Could be null 
    Text = c != null ? c.Text : "[[Empty]]" 
} 

和λ：

class ChildResult 
{ 
    public int PID { get; set; } 
    public int? CID { get; set; } 
    public string Text { get; set; } 
} 

lParent.SelectMany(p => p.Childs.Any() ? 
    p.Childs.Select(c => new ChildResult() { PID = c.PID, CID = c.CID, Text = c.Text }) : 
    new [] { new ChildResult() { PID = p.PID, CID = null, Text = "[[Empty]]" } });