C# Linq 中 lambda/方法语法中的左外连接
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left outer join in lambda/method syntax in Linq
提问by NomenNescio
Possible Duplicate:
How do you perform a left outer join using linq extension methods
可能的重复：
如何使用 linq 扩展方法执行左外连接
I can't find a left outer join example of Linq lambda (with extension methods), at least, not a clear one.
我找不到 Linq lambda 的左外连接示例（带有扩展方法），至少，不是一个清晰的示例。
Let's say I have the following table:
假设我有下表：
Parent
{
PID // PK
}
Child
{
CID // PK
PID // FK
Text
}
I want to join Parent with Child, and for every child missing, I want the default value for Text to be "[[Empty]]". How can I do this with linq lambda syntax?
我想将 Parent 与 Child 一起加入，对于每个丢失的孩子，我希望 Text 的默认值为“[[Empty]]”。我怎样才能用 linq lambda 语法做到这一点？
I currently have the following:
我目前有以下几点：
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?
});
采纳答案by Rawling
You're close. The following will select PID, CIDand Textfor each child, and PID, CID = -1and Text = "[[Empty]]"for each parent with no children:
你很接近。以下将为每个孩子选择PID,CID和,以及为每个没有孩子的父母选择：TextPIDCID = -1Text = "[[Empty]]"
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);
回答by Amiram Korach
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]]"
}
With lambda:
使用 lambda：
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]]" } } );
