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;}
}

------解决方案--------------------
当然，如果你喜欢用lamda表达式，也可以这么写：

C# code

    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())
                });