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void Main()
{
var listA=new List<A>(){new A{ ID=1, Content="aaaaaaaaaa"}};
var listB=new List<B>()
{
new B{ ID=1, AID=1, UserID=2012},
new B{ ID=2, AID=1, UserID=2013},
new B{ ID=3, AID=1, UserID=2014},
};
var query=from a in listA
join b in listB
on a.ID equals b.AID into t
select new
{
ID=a.ID,
Content=a.Content,
UserIDs=string.Join(",",t.Select(x=>x.UserID.ToString()).ToArray())
};
}
public class A
{
public int ID{get;set;}
public string Content{get;set;}
}
public class B
{
public int ID{get;set;}
public int AID{get;set;}
public int UserID{get;set;}
}
var query=listA.GroupJoin(listB,
a=>a.ID,
b=>b.AID,
(a,t)=>new
{
ID=a.ID,
Content=a.Content,
UserIDs=string.Join(",",t.Select(x=>x.UserID.ToString()).ToArray())
});