Copy a list of objects to another typed list in one line of code

Question

I have a List of MyType1 objects. I want to copy all these objects in a new List of MyType2 objects, having a constructor for MyType2 that takes a MyType1 object as argument.

For now, I do it this way:

List<MyType1> type1objects = new List<MyType1>();
// Fill type1objects
// ...
List<MyType2> type2objects = new List<MyType2>();
foreach (MyType1 type1obj in type1objects) {
    MyType2 type2obj = new MyType2(type1obj);
    type2objects.Add(type2obj);
}

Is there a way to do this one-line (I'm thinking maybe it is possible with Linq)?

Answer 1

var type2objects = type1objects.Select(o => new MyType2(o)).ToList();

Answer 2

You can use a simple projection using Select() with Linq, then make a list out of this enumeration using ToList() :

var type2objects = type1objects.Select( x => new MyType2(x))
                               .ToList();

Answer 3

var type2objects = type1objects.Select(type1obj => new MyType2(type1obj)).ToList();

Answer 4

你应该可以做这样的事情：

List<MyType2> list2 = new List<MyType2>(list1.Select(x => new MyType2(x)));

Answer 5

If you just copy all elements from list1 to list2 you will not get true copies of those elements. You will just have another list with the same elements!!!

Answer 6

List<MyType2> type2objects = new List<MyType2>();
type2objects.AddRange(type1objects.Select(p => new MyType2(p)));

In this way you can even preserve the values contained in type2objects .

I'll add that you know the size of type1objects , so you can speedup a little:

List<MyType2> type2objects = new List<MyType2>(type1objects.Count);

Here the capacity of type2objects is pre-setted.

or, if you are re-using an "old2" type2objects

if (type2objects.Capacity < type1objects.Count + type2objects.Count)
{
    type2objects.Capacity = type1objects.Count + type2objects.Count;
}

In this way you know the List won't need intermediate resizes.

Someone will call this premature optimization, and it is. But you could need it.

Answer 7

You might also get some mileage out of Convert.ChangeType() depending on what you're converting:

List<MyType2> type2objects = type1objects.Select(
    type1object => Convert.ChangeType(type1object, typeof(MyType2))
    ).ToList();