Question

I have a generic List (of Foo) which contains n objects of Type Foo. One of the properties of Foo is PropertyA. PropertyA can be one of ValueA, ValueB or ValueC. Is there an easy way of splitting this into three seperate Lists, one for ValueA, one for ValueB and one for ValueC?

I can write some code which loops the original list and adds each item to a new list based on the property value but this does not seem to be very maintainable (what if I suddenly get a ValueD for example?)

**EDIT. I should have mentioned that I’m using version 2.0 of the framework.

Answer 1

In C# with .Net 2.0, I have written (too many times):

 //if PropertyA is not int, change int to whatever that type is
Dictionary<int, List<foo>> myCollections =
  new Dictionary<int, List<foo>>();
//
foreach(Foo myFoo in fooList)
{
  //if I haven't seen this key before, make a new entry
  if (!myCollections.ContainsKey(myFoo.PropertyA))
  {
    myCollections.Add(myFoo.PropertyA, new List<foo>());
  }
  //now add the value to the entry.
  myCollections[myFoo.PropertyA].Add(myFoo);
}
//
// now recollect these lists into the result.
List<List<Foo>> result = new List<List<Foo>>();
foreach(List<Foo> someFoos in myCollections.Values)
{
  result.Add(someFoos);
}

---

Nowadays, I just write:

List<List<foo>> result = fooList
  .GroupBy(foo => foo.PropertyA)
  .Select(g => g.ToList())
  .ToList();

Or

 ILookup<TypeOfPropertyA, foo>> result = fooList.ToLookup(foo => foo.PropertyA);