名单 <T> id匹配时加到T

Question

I have 2 lists of 2 different objects:

public class A {
    string Id;
    string Name;
    List<B> listofB;
}

public class B {
    string Id;
    string Test;
}

The Id in B is the same as the Id in A

now I have 2 lists

var listA = new List<A>();
var listB = new List<B>();

How would you add every B object where the Id's match to the list of A via linq?

for now I did:

foreach(var a in listA){
   a.listofB = listB.FindAll(i => i.Id == a.Id);
}

But I would think there is a better way of doing this

(This always returns null for now, still trying to figure out why)

Answer 1

But I would think there is a better way of doing this

If that's working for you, why do you think that there's something better? It's readable and probably efficient.

However, you could use Enumerable.Join + GroupBy :

var idGrps = from a in listA
             join b in listB
             on a.Id equals b.Id into bGroup
             select new { a, bGroup };
foreach (var grp in idGrps)
{
    grp.a.listofB = grp.bGroup.ToList();
}

But even if Join is more efficient if the lists are large, it's not as readable. Maybe it's better to initialize the list from the A -constructor from the start instead of maintaining two lists.

Answer 2

Use GroupJoin

void Main()
{
    var alist = new List<A>();
    var blist = new List<B>();

    result = alist.GroupJoin(
        blist,
        a=>a.Id,    //Pick group key from A
        b=>b.Id,    //Pick group key from B
        (singleA,manyB)=>
            new{TheA=singleA,AllTheBs=manyB.ToList()}); //Create a resulting object.
}

Answer 3

You could use a GroupBy clause:

var theBsById = listB.GroupBy(b => b.Id);

and then:

foreach(IGrouping<int, B> grouping in theBsById)
{
    A theA = listA.Where(a => a.Id == grouping.Key).FirstOrDefault();
    if (theA != null)
    {
        theA.listofB = grouping.ToList();
    }
}

(This would only update the listofBs in the As that have Ids in listB)

or:

foreach(var a in listA)
{
    a.listofB = theBsById
      .Where(grouping => grouping.Key == a.Id)
      .SelectMany(grouping => grouping)
      .ToList();
}

(this would update the listofB for all As, which is what your code above does, too.)