Distinct by property of class with LINQ [duplicate

Question

This question already has answers here:

LINQ's Distinct() on a particular property (20 answers)

Closed 5 months ago.

I have a collection:

List<Car> cars = new List<Car>();

Cars are uniquely identified by their property CarCode.

I have three cars in the collection, and two with identical CarCodes.

How can I use LINQ to convert this collection to Cars with unique CarCodes?

Answer 1

You can use grouping, and get the first car from each group:

List<Car> distinct =
  cars
  .GroupBy(car => car.CarCode)
  .Select(g => g.First())
  .ToList();

Answer 2

Use MoreLINQ, which has a DistinctBy method :)

IEnumerable<Car> distinctCars = cars.DistinctBy(car => car.CarCode);

(This is only for LINQ to Objects, mind you.)

Answer 3

You can check out my PowerfulExtensions library. Currently it's in a very young stage, but already you can use methods like Distinct, Union, Intersect, Except on any number of properties;

This is how you use it:

using PowerfulExtensions.Linq;
...
var distinct = myArray.Distinct(x => x.A, x => x.B);

Answer 4

Same approach as Guffa but as an extension method:

public static IEnumerable<T> DistinctBy<T, TKey>(this IEnumerable<T> items, Func<T, TKey> property)
{
    return items.GroupBy(property).Select(x => x.First());
}

Used as:

var uniqueCars = cars.DistinctBy(x => x.CarCode);

Answer 5

Another way to accomplish the same thing...

List<Car> distinticBy = cars
    .Select(car => car.CarCode)
    .Distinct()
    .Select(code => cars.First(car => car.CarCode == code))
    .ToList();

It's possible to create an extension method to do this in a more generic way. It would be interesting if someone could evalute performance of this 'DistinctBy' against the GroupBy approach.

Answer 6

You can't effectively use Distinct on a collection of objects (without additional work). I will explain why.

The documentation says:

It uses the default equality comparer, Default, to compare values.

For objects that means it uses the default equation method to compare objects (source). That is on their hash code. And since your objects don't implement the GetHashCode() and Equals methods, it will check on the reference of the object, which are not distinct.