How to use a LINQ statement to check if a variable already exists for any object in an existing list before adding that object to the list?

Question

I am trying to implement an Add method which adds a car (object) to the fleet (list of objects)(i.e. adds the car to the list of cars).

The car to add should be a parameter of the method, and before adding a car to the list, I should perform check to see if the car exists in the list already using a LINQ statement to check if the registration already exist for any car in the existing list and if the car already exists in the list then I don’t add it.

Below is what I have done so far. If anyone can help, I would greatly appreciate it. Thanks.

        public void Add(Car carToAdd)
        {
            var regQuery = Cars.Select(car => new { car.Registration });


            foreach (var car in regQuery)
            {
                if (!regQuery.Contains(Car carToAdd.Registration))
                {
                    Cars.Add(carToAdd);
                }
                else
                {
                    Console.WriteLine("Reg already exists!");
                }
            }

please reduce your code to a minimum example that shows your problem — thumbmunkeys, Oct 23 '15 at 13:36

Jatin Parmar · Accepted Answer · 2015-10-23T13:57:28.507

3

Using FirstOrDefault method you can check whether object exist in your list or not, If exist it's first element of a sequence, else return default value in below example it's return null.

You can learn more about LINQ from here

public void Add(Car carToAdd)
{
        var qry = (from car in Cars
                   where car.Registration == carToAdd.Registration 
                   select car).FirstOrDefault();

        if(qry == null)
        {
           Cars.Add(carToAdd);
        }
        else 
        {
           Console.WriteLine("Reg already exists!");
        }
}

edited Oct 23 '15 at 13:57

answered Oct 23 '15 at 13:44

Jatin Parmar

647
4
14

1

Thank you Jatin. I still I have a bit of the way to go but this has definitely helped. – Gavin McCoy Oct 23 '15 at 14:26
@GavinMcCoy Good to know that, Happy coding !! – Jatin Parmar Oct 23 '15 at 14:33

Rik · Answer 2 · 2015-10-26T10:15:43.407

1

public void Add(Car carToAdd)
{
    if ( !Cars.Any(c => c.Registration == carToAdd.Registration) )
    {
        Cars.Add(carToAdd);
    }
    else 
    {
        // handle case where car is already in the list
    }
}

edited Oct 26 '15 at 10:15

answered Oct 23 '15 at 13:36

Rik

28,507
14
48
67

score 1 · Answer 3 · edited May 23 '17 at 12:23

1

Most of answers suggest using Contains() method, I would suggest to use Any() as it's more flexible:

if ( !Cars.Any(c => c.Registration == carToAdd.Registration) )
    Cars.Add(carToAdd);

You can also use SingleOrDefault() to additionaly ensure that Your Cars' list is still satisfying this implicit requirement to have only a single car per registration:

var existing = Cars.SingleOrDefault(c => c.Registration == carToAdd.Registration) )

if(existing == null)
    Cars.Add(carToAdd);

SingleOrDefault() will throw if there are two or more elements satisfying the condition.

edited May 23 '17 at 12:23

Community

1
1

answered Oct 23 '15 at 14:08

Grzegorz Sławecki

1,727
14
27

1

Don't u think u made a blunder in the last if condition in ur code block?I guess there will be duplication! – Amey Khadatkar Oct 23 '15 at 14:20

How to use a LINQ statement to check if a variable already exists for any object in an existing list before adding that object to the list?

3 Answers3