OOC: What is the difference between ToList() and casting to List<T> in .NET?

c#, casting, collections

Solution

`ToList()` is not the same as (casting) to List.

`ToList()` takes any `IEnumerable` (Lists, Arrays, Dictionaries, Sets, etc) and turns it into a `List`.

Casting to `List` takes an object that is already a list of some sort, and labels it as a list. example:

// fail -- arrays are not lists
var not_a_list = (List<int>)int[];
// success: arrays *are* IEnumerable, so you can convert them to a list.
var list_from_array = new [] { 1,2,3,4,5 }.ToList();
// success: WorkflowRoleCollection derives from List<WorkflowRole>
var derived_from_list = (List<WorkflowRole>) new WorkflowRoleCollection();

In your case, `Concat()` returns an `IEnumerable`, and not a `List`. Remember that it has to support generators (which are lazy-evaluated), so it doesn't make sense for it to be anything like a list underneath.

Btw, have you taken a look at the built-in function `Enumerable.Range` ?

Problem

OOC: Out Of Curiosity So, as a little exercise and for the sake of learning, I decided to check if I was able to implement a very basic recursive function that would return a `List<int>`, but with the following restrictions: 1- The result should be returned by the function itself (as opposed to passed as an argument to a `void` function). 2 - No local "named" variables declared in the function's body. I came up with the solution below (BTW: can this be improved in any way?) While doing this, I learnt that `ToList()` is not the same thing as casting to `List<T>` (see example below) - Anyone out there who can explain what happens under the hood and what the difference between the two is? Thanks! PS - I'm using version 4.0 (in case it matters). EDIT: the runtime error is `Unable to cast object of type '<ConcatIterator>d__71'1[System.Int32]' to type 'System.Collections.Generic.List'1[System.Int32]'` ``` public static List<int> SomeIntegers(int min, int max) { //assume max >= min for simplicity if (min == max) return new List<int>() { min }; // runtime error //return (List<int>)(SomeIntegers(min, max - 1).Concat(new List<int>() { max })); //works return (SomeIntegers(min, max - 1).Concat(new List<int>() { max })).ToList(); } ```

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