.Net List ForEach Item

Question

I have 2 objects:

public class ClassA
{
   public int Id
   public string name;

   public ClassB myObjectB;

}
public class ClassB
{
   public int Id
   public string name
}

Having 2 Lists for <ClassA> <ClassB> Some items from List1 match by Id with an item on List2... I want to set the objectB foreach item...

foreach(ClassA item in List1)
{
   ClassB obj = (from b in List2 where b.Id == item.Id select b).SingleOrDefault()
   if(obj != null)
   {
       item.myObjectB = obj; 
       ////////break;    <- ignore this
   }
}

This solution works for me, but I'm just wondering if there is a better way to do this, instead of Foreach

Thanks everyone for your help!!!

You should probably remove break; to make it work for each item of List1 — horgh
– horgh, Commented Dec 7, 2012 at 0:17
That is really bad code. you could've done List2.SingleOrDefault(x=>x.Id == item.Id); instead of all that query syntax nonsense with a select. — caesay
– caesay, Commented Dec 7, 2012 at 0:24

Reed Copsey · Accepted Answer · 2012-12-07 00:14:31Z

8

I think that a foreach, in this case, is actually the appropriate approach, as you're mutating your list. You could potentially simplify your code a bit, however:

foreach(ClassA item in List1)
{
    item.myObjectB = List2.FirstOrDefault(b => b.Id == item.Id);
}

This will set the item every time, though it will be set to null if there is no match. If you already have items in myObjectB and setting them to null is inappropriate, you could use:

foreach(ClassA item in List1)
{
    item.myObjectB = List2.FirstOrDefault(b => b.Id == item.Id) ?? item.myObjectB;
}

answered Dec 7, 2012 at 0:14

Reed Copsey

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2 Comments

Sergey Kalinichenko Over a year ago

It looks like the OP is breaking after the first match, so perhaps he does not need a foreach?

Osvier Over a year ago

Oh, when I was writing the simplified version of my code, I put the break there, but good catch, the break is not supposed to be there...

caesay · Accepted Answer · 2013-09-06 23:38:03Z

1

Expanding on Reed's answer.. You can actually do this in a one-liner, because a list has a ForEach method.

List1.ForEach(item => item.myObjectB = List2.FirstOrDefault(b => b.Id == item.Id) ?? item.myObjectB);

edited Sep 6, 2013 at 23:38

answered Dec 7, 2012 at 0:19

caesay

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Comments

Aaron Anodide · Accepted Answer · 2012-12-07 00:40:51Z

0

List<ClassA> list1 = new List<ClassA>();
List<ClassB> list2 = new List<ClassB>();

list1.Add(new ClassA { Id = 2, name = "a2" });
list1.Add(new ClassA { Id = 3, name = "a3" });
list1.Add(new ClassA { Id = 4, name = "a4" });
list1.Add(new ClassA { Id = 5, name = "a5" });

list2.Add(new ClassB { Id = 1, name = "b1" });
list2.Add(new ClassB { Id = 2, name = "b2" });
list2.Add(new ClassB { Id = 4, name = "b4" });
list2.Add(new ClassB { Id = 5, name = "b5" });

// Goal is to set ClassA::myObjectB from List1 to 
// matching instance (if any) of ClassB from List2
var query = 
    from a in list1
    from b in list2
    where a.Id == b.Id
    select Tuple.Create(a, b);  
foreach (var element in query)
    element.Item1.myObjectB = element.Item2;

Update:

Or if you really don't want a for loop, I just realized you can use the fact that assignments return a value and at the same time make an entry in an obfuscated code contest :)

(from a in list1
 from b in list2
 where a.Id == b.Id
 select a.myObjectB = b).ToList();

Update2:

I just thought of an alternate approach - depending on your scenario, a lazy mechanism might work for you?

public class ClassA
{
   public int Id
   public string name;

   private ClassB myObjectB;
   public ClassB MyObjectB { 
      get { return myObjectB ?? (myObjectB = list2.FirstOrDefault(x => this.Id == x.Id)); } 
   }
}

edited Dec 7, 2012 at 0:40

answered Dec 7, 2012 at 0:22

Aaron Anodide

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2 Comments

caesay Over a year ago

keep in mind that you can use anonymous types with linq, and would be clearer than using a turple.

Aaron Anodide Over a year ago

i personally feel that's debatable because in this kind of situation the anonymous type ends up having member names with redundant information - but I'm not saying your wrong, thats just thought I've had about it...

Dave Cousineau · Accepted Answer · 2013-09-07 00:17:56Z

With classes defined as follows:

class ClassA {
   public int Id { get; private set; }
   public string name { get; private set; }

   public ClassB myObjectB { get; set; }

   public ClassA(int pId, string pName) {
      Id = pId;
      name = pName;
   }
}

class ClassB {
   public int Id { get; private set; }
   public string name { get; private set; }

   public ClassB(int pId, string pName) {
      Id = pId;
      name = pName;
   }
}

You can do the following using the LINQ Join method:

var listA = new List<ClassA> {
   new ClassA(1, "OneA"),
   new ClassA(2, "TwoA"),
   new ClassA(3, "ThreeA")
};

var listB = new List<ClassB> {
   new ClassB(1, "OneB"),
   new ClassB(2, "TwoB"),
   new ClassB(4, "FourB")
};

listA
.Join(
   listB,
   itemA => itemA.Id,
   itemB => itemB.Id,
   (itemA, itemB) => new { ItemA = itemA, ItemB = itemB }
).ForEach(pair => pair.ItemA.myObjectB = pair.ItemB);

listA.ForEach(itemA => Console.WriteLine(
   "{0} maps to {1}",
   itemA == null
      ? "null"
      : itemA.name,
   (itemA == null || itemA.myObjectB == null)
      ? "null"
      : itemA.myObjectB.name
));

Output is:

OneA maps to OneB
TwoA maps to TwoB
ThreeA maps to null

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