List<T> Ordering

Question

I have an issue, I am allowing a user to select the criterea for ordering a List

Lets say my list is called

List<Cars> AllCars = new List<Cars>;
allCars = //call the database and get all the cars

I now want to order this list

allCars.orderBy(registrationDate)

I understand the above doesn't work but i haven't anyidea what i should be putting in the brackets.

Answer 1

allCars.OrderBy(c => c.RegistrationDate);

Answer 2

I understand the above doesn't work but i haven't anyidea what i should be putting in the brackets.

The declaration of Enumerable.OrderBy is

public static IOrderedEnumerable<TSource> OrderBy<TSource, TKey>(
    this IEnumerable<TSource> source,
    Func<TSource, TKey> keySelector
)

and, as it's an extension method it can be invoked as

source.OrderBy(keySelector).

Your List<Car> is playing the role of source as List<T> : IEnumerable<T>. The second parameter is the more interesting one and the one that you are confused by. It's declared as being of type

Func<TSource, TKey>

This means that it is a delegate that eats instances of TSource (in your case Car) and returns an instance of TKey; it's up to you to decide what TKey is. You have stated that you want to order by Car.registrationDate so it sounds like TKey is DateTime. Now, how do we get one of these delegates?

In the old days we could say

DateTime GetRegistrationDate(Car car) {
    return car.registrationDate;
}

and use OrderBy like so:

allCars.OrderBy(GetRegistrationDate).

In C# 2.0 we gained the ability to use anonymous delegates; these are delegates that don't have a name and are defined in-place.

allCars.OrderBy(delegate(Car car) { return car.registrationDate; });

Then, in C# 3.0 we gained the ability to use lambda expressions which are very special anonymous delegates with a compact notation

allCars.OrderBy(car => car.registrationDate);

Here, c => c.registrationDate is the lambda expression and it represents a Func<Car, DateTime> than can be used the second parameter in Enumerable.OrderBy.

allCars.orderBy(registrationDate)

The reason this doesn't work is because registrationDate is not a delegate. In fact, without any context at all registrationDate is meaningless to the compiler. It doesn't know if you mean Car.registrationDate or maybe you mean ConferenceAttendee.registrationDate or who knows what. This is why you must give additional context to the compiler and tell it that you want the property Car.registrationDate. To do this, you use a delegate in one of the three ways mentioned above.