Linq中的lambda /方法语法中的左外连接

Question

可能重复:
如何使用linq扩展方法执行左外连接

我找不到Linq lambda的左外连接示例(使用扩展方法),至少不是一个明确的.

假设我有下表:

Parent
{
    PID     // PK
}

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

我想加入Parent with Child,对于每个失踪的孩子,我希望Text的默认值为"[[Empty]]".我怎么能用linq lambda语法做到这一点？

我目前有以下内容:

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

你很亲密 下面将选择PID,CID并Text为每一个孩子,和PID,CID = -1和Text = "[[Empty]]"为没有孩子每个父母:

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]]"
}

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]]" } } );