C# - Linq with two Lists

Question

I've got 2 classes which are connected together via an ID:

public class A {
    public int AID;
    public string Name;
}

public class B {
    public int BID;
    public int AID;
}

Now I want to filter my listB to get all B where A's Name is euqals the parameter name:

List<A> listA = ...;
List<B> listB = ...;

public List<B> Filter(string name) {
    var list = listB.Where(**... A.Name == name ...**).ToList();
    return list;
}

I don't know how to filter listB with the parameter I defined above. Maybe someone can help me figuring it out.

Thanks!

Answer 1

Use Join and Where. I prefer the query syntax with joins:

IEnumerable<B> bs = 
    from a in listA
    join b in listB on a.AID equals b.AID
    where a.Name == name
    select b;
List<B> list = bs.ToList();

---

Another way which avoids duplicate B's (if it's a 1-to-many relationship) is using a lookup:

ILookup<int, string> aNameLookup = listA.ToLookup(a => a.AID, a => a.Name);
List<B> list = listB.Where(b => aNameLookup[b.AID].Contains(name)).ToList();

Answer 2

Although you could filter a list of Bs using b.AID linear search, it would be inefficient:

return listB.Where(b => listA.FirstOrDefault(a => a.AID == b.AID)?.Name == name);

A better approach would be to place entries from listA into a Dictionary<int,A>, and use dictionary lookup by b.AID:

IDictionary<int,A> dictA = listA.ToDictionary(a => a.AID);
return listB.Where(b => dictA.TryGetValue(b.AID, out var a) && a.Name == name);

I would highly recommend keeping As in dictA rather than listA, because it would let you avoid rebuilding the dictionary for each query.