ToList()-- Does it Create a New List?

Question

Let's say I have a class

public class MyObject
{
   public int SimpleInt{get;set;}
}

And I have a List<MyObject>, and I ToList() it and then change one of the SimpleInt, will my change be propagated back to the original list. In other words, what would be the output of the following method?

public void RunChangeList()
{
  var objs = new List<MyObject>(){new MyObject(){SimpleInt=0}};
  var whatInt = ChangeToList(objs );
}
public int ChangeToList(List<MyObject> objects)
{
  var objectList = objects.ToList();
  objectList[0].SimpleInt=5;
  return objects[0].SimpleInt;

}

Why?

P/S: I'm sorry if it seems obvious to find out. But I don't have compiler with me now...

Answer 1

ToList will always create create a new list, which will not reflect any subsequent changes to the collection.

However, it will reflect changes to the objects themselves (Unless they're mutable structs).

In other words, if you replace an object in the original list with a different object, the ToList will still contain the first object.
However, if you modify one of the objects in the original list, the ToList will still contain the same (modified) object.

Answer 2

A new list is created but the items in it are references to the orginal items (just like in the original list). Changes to the list itself are independent, but to the items will find the change in both lists.

Answer 3

Yes, it creates a new list. This is be design.

The list will contain the same results as the original enumerabnle sequence, but materialized into a persistent (in memory) collection. This allows you to consume the results multiple times without incurring the cost of recomputing the sequence.

THe beauty of LINQ sequences is that they are composable. Often, the IEnumerable<T> you get is the result of combining multiple filtering, ordering, and/or projection operations. Extension methods like ToList() and ToArray() allow you to convert the computed sequence into a standard collection.

Answer 4

Yes, ToList will create a new list, but because in this case MyObject is a reference type then the new list will contain references to the same objects as the original list.

Updating the SimpleInt property of an object referenced in the new list will also affect the equivalent object in the original list.

(If MyObject was declared as a struct rather than a class then the new list would contain copies of the elements in the original list, and updating a property of an element in the new list would not affect the equivalent element in the original list.)

Answer 5

ToList will create a brand new list.

If the items in the list are value types, they will be directly updated, if they are reference types, any changes will be reflected back in the referenced objects.

Answer 6

Why is this line needed. var objectList = objects.ToList(); 'objects' is already a list. The ChangeToList would return 5. because u r modifying the SimpleInt property of the first element in the list.

I am not sure what your question is.

Answer 7

 var objectList = objects.ToList();
  objectList[0].SimpleInt=5;

This will update the original object as well. The new list will contain references to the objects contained within it, just like the original list. You can change the elements either and the update will be reflected in the other.

Now if you update a list (adding or deleting an item) that will not be reflected in the other list.

Answer 8

I think that this is equivalent to asking if ToList does a deep or shallow copy. As ToList has no way to clone MyObject, it must do a shallow copy, so the created list contains the same references as the original one, so the code returns 5.

Answer 9

From the Reflector'd source:

public static List<TSource> ToList<TSource>(this IEnumerable<TSource> source)
{
    if (source == null)
    {
        throw Error.ArgumentNull("source");
    }
    return new List<TSource>(source);
}

So yes, your original list won't be updated (i.e. additions or removals) however the referenced objects will.