Question

I am going to be working with a collection that has about 500,000 items in it and am looking for a decent way of getting all the duplicates. After looking through this post I see that the most popular solution is to use a hashed set. But what if I want to get all Cars that have the color red not just Car4 and Car5?

Car1.Color = Red;

Car2.Color = Blue;

Car3.Color = Green;

Car4.Color = Red;

Car5.Color = Red;

Given the problem what would be a reasonably fast way to do this?

EDIT:
I saw in that post that the code below could easily be changed to fit my needs. And I’m not sure there is really a better way to solve the problem but I will leave the post up just to see.

var duplicates = from car in cars
                 group car by car.Color into grouped
                 from car in grouped
                 select car;

Answer 1

You can use the Enumerable.ToLookup Extension Method to group the cars by color and retrieve all cars of one color:

var cars = new List<Car> { car1, car2, car3, car4, car5 };
var lookup = cars.ToLookup(car => car.Color);
var redCars = lookup[Red];
// redCars == { car1, car4, car5 }