LINQ get list from list by comparing enum

Question

I am trying to get a list of objects from a list by comparing an enum that is passed into the method.

public List<IAnimal> GetListOfAnimalsByType(IAnimal.AnimalType animalType)
{
    List<IAnimal> animalTypeList = animalList.SelectMany(ani => ani.Type == animaleType);
    if(animalTypeList != null)
    {
        return animalTypeList;
    }
    else
    {
        return null;
    }
}

also, you should reconsider your naming of IAnimal, it sounds like an interface but obviously it's not since you can't define a nested type within an interface. — Eren Ersönmez
– Eren Ersönmez, Commented Nov 13, 2014 at 12:33

Jon Skeet · Accepted Answer · 2014-11-13 12:20:23Z

3

It looks like you really just want Where instead of SelectMany:

public List<IAnimal> GetListOfAnimalsByType(IAnimal.AnimalType animalType)
{
    return animalList.Where(ani => ani.Type == animaleType).ToList();
}

SelectMany is used to extract one sequence from each element within an original sequence, and usually "flatten" the resulting sequences together... whereas Where is used for filtering.

Additionally:

The ToList() call is necessary because LINQ returns IEnumerable<T> or IQueryable<T>, not List<T>
Your if statement is unnecessary as sequence-producing LINQ operators (e.g. Where, Select etc never return null; they'll return an empty sequence if necessary
Even if the call could return null, you're returning the value of animalTypeList in both cases... "if the value is null, return null, otherwise return the value"... so you could still just return the result of the call

answered Nov 13, 2014 at 12:20

Jon Skeet

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3 Comments

Paweł Reszka Over a year ago

But if he want to return null, he can change condition to animalTypeList.Any(), because this list will never be null.

Patrick Hofman Over a year ago

@PawełReszka: Then just check if there is any item. Why would you want to return null anyway?

Paweł Reszka Over a year ago

No idea;) I just wanted to point that your function will never return null, but OP in some cases wanted to return null. I just wanted to clear out to OP, that he won't get null.

Patrick Hofman · Accepted Answer · 2014-11-13 12:20:05Z

2

You should use ToList to get a list out of the SelectMany. Also, a Where would suffice.

The methods SelectMany and Where return a IEnumerable<TSource>, which isn't a List<T> of course. That's why you need to call ToList.

List<IAnimal> animalTypeList = animalList
                               .Where(ani => ani.Type == animaleType)
                               .ToList();

answered Nov 13, 2014 at 12:20

Patrick Hofman

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