List<T> is dead, long live LINQ

Well, ok, maybe not completely dead. It has it’s uses but for the most part I find myself using an actual list less and less. I do end up using ObservableCollection<T> for ViewModels still, of course, but that can’t be avoided.

I do find myself slowly banishing it from my daily coding habits however, thanks to LINQ. I am finding that the covariant interface IEnumerable<T> is almost always a better choice as a return value for methods and properties and with the help of the higher order set functions provided along with LINQ there is nothing you can’t do without List<T> that you could have done with it.

Also, I would like to mention that when I am talking about LINQ I am talking exclusively about the extension methods found in System.Linq.Enumerable not the actual internal DSL syntax in C# / VB. I almost never use that syntax since it can’t express all of the functions I need and is sometimes hard to debug.

I did have to add one extension method to help ease general use of LINQ however.

public static class EnumerableExtensions
    public static IEnumerable<T> ToEnumerable<T>(this T @object)
        if (@object == null)
            return System.Linq.Enumerable.Empty<T>();

        var enumerable = @object as IEnumerable;
        if (enumerable == null)
            return new[] { (T)@object };

        return enumerable.Cast<T>();

This allows you to ensure than any object can be used as an IEnumerable<T>. With this in hand you can replace any method on List<T>. Here is an example of a bunch of methods on List<T> and their equivalent LINQ translations.

var a = 1;
var b = 2;
var c = 3;

var list = new List<int>();
var set = Enumerable.Empty<int>();

set = set.Concat(a.ToEnumerable());

set = set.Where(i => i != a);

list.AddRange(new[] { a, b });
set = set.Concat(a.ToEnumerable()).Concat(b.ToEnumerable());

set = set.OrderBy(i => i);

set.Any(i => i == a);

var count1 = list.Count;
var count2 = set.Count();

list.FindAll(i => i == a);
set.Where(i => i == a);

set.TakeWhile(i => i == a).Count();

list.FindLast(i => i == a);
set.Last(i => i == a);

list.GetRange(0, 2);

list.Insert(1, c);
set = set.Take(1).Concat(c.ToEnumerable()).Concat(set.Skip(1));

set = set.Reverse();

list.TrueForAll(i => i == 0);
set.All(i => i == 0);

Note that in order to get some of the features of List<T> you have to compose various LINQ functions together, such as Insert. At first this may seem overly verbose but the benefit is an incredible flexibility. There is nearly no limit on what you can express with LINQ because of this composability (also you could create extensions that simply wrap up the complex composition with a neater signature). List<T> has a few more methods that I will leave up to you to translate but LINQ has many more functions and function composition possibilities that I would highly recommend discovering.

Executing the above code and printing list and set results in this output.

list: { 2 3 1 }
set: { 2 3 1 }

When you use LINQ your set is immutable at every point in time, which is very helpful in case you would like to try to use the Parallel Extensions (aka PLINQ). One other interesting aspect of the LINQ extensions is deferred execution. This means that none of my set operations above are even executed until I iterate over the set in my print function. This allows me to change the value of ‘a’, reiterate and receive a different result. This can be confusing if you’re unaware of it but it can be a huge win if you understand it well. This is all possible because of the functional nature of LINQ and (as best as I can understand) is an example of a Monad in .NET. And, of course, coroutines in C# compliment LINQ very nicely as well (aka yield return). I think it would be very interesting to experiment with a language where every variable is essentially treated as a set, NULL is not possible but empty sets are. An object is simply a set of 1. It would be interesting.

Another very cool aspect of LINQ is Lambda Expressions and how they are related to Expression Trees and the DLR. It’s extremely powerful and they are intimately tied together.

Also, do check out IEnumerable<T>’s mirrored twin: IObservable<T>.


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