Can I use LINQ to check if objects in a list have a unique ID?

Question

say I have a list containing objects like this one:

public class Person
{
    private string _name; 
    private string _id;
    private int _age;

    public Person 
    {
    }

    // Accessors
}

public class ManipulatePerson
{
    Person person = new Person();
    List<Person> personList = new List<Person>;

    // Assign values

    private void PopulateList();
    {
        // Loop 
        personList.Add(person);

        // Check if every Person has a unique ID
    }
}

and I wanted to check that each Person had a unique ID. I would like to return a boolean true/false depending on whether or not the IDs are unique. Is this something I can achieve with LINQ?

Use group by http://stackoverflow.com/questions/7325278/group-by-in-linq — edc65, Feb 19 '16 at 15:13
Or else, use `Distinct` and `Count` if the number differs. But firstly, put your ID to be a property rather than private field. — Ian, Feb 19 '16 at 15:13

score 10 · Accepted Answer · answered Feb 19 '16 at 15:25

10

Note that you can even leverage directly an HashSet<>:

var hs = new HashSet<string>();
bool areAllPeopleUnique = personList.All(x => hs.Add(x.Id));

(and is the code that I normally use)

It has the advantage that on the best case (presence of some duplicates) it will stop before analyzing all the personList collection.

answered Feb 19 '16 at 15:25

xanatos

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Intersection, please! :D – Matías Fidemraizer Feb 19 '16 at 15:27
This is fine, but even better would be to override the hashcode and then you could do it directly var hs = new HashSet(); – user853710 Feb 19 '16 at 15:51
@user853710 Or you could create a separate `IEqualityComparer<>` with the special `GetHashCode`. – xanatos Feb 19 '16 at 15:53
true, both are valid thigs. – user853710 Feb 19 '16 at 15:58
Hey, I was wondering if you could answer another question, continuing on from your original answer? – Jack Feb 26 '16 at 15:58

TheLethalCoder · Answer 2 · 2016-02-20T12:06:52.307

I would use Distinct and then check against the counts for example:

bool bAreAllPeopleUnique = (personList.Distinct(p => p.ID).Count == personList.Count);

However as @Ian commented you will need to add a property to the Person class so that you can access the Id like so:

public string ID
{
    get { return _id; }
}

A 'nicer' way to implement this would be to add a method like so:

private bool AreAllPeopleUnique(IEnumerable<Person> people)
{
    return (personList.Distinct(p => p.ID).Count == personList.Count);
}

NOTE: The method takes in an IEnumerable not a list so that any class implementing that interface can use the method.

score 1 · Answer 3 · edited May 23 '17 at 11:54

One of best ways to do so is overriding Equals and GetHashCode, and implementing IEquatable<T>:

public class Person : IEquatable<Person>
{
     public string Id { get; set; }

     public override bool Equals(object some) => Equals(some as Person);
     public override bool GetHashCode() => Id != null ? Id.GetHashCode() : 0;
     public bool Equals(Person person) => person != null && person.UniqueId == UniqueId;
}

Now you can use HashSet<T> to store unique objects and it will be impossible that you store duplicates. And, in addition, if you try to add a duplicated item, Add will return false.

NOTE: My IEquatable<T>, and Equals/GetHashCode overrides are very basic, but this sample implementation should give you a good hint on how to elegantly handle your scenario.

You can check this Q&A to get an idea on how to implement GetHashCode What is the best algorithm for an overridden System.Object.GetHashCode?

Maybe this other Q&A might be interesitng for you: Why is it important to override GetHashCode when Equals method is overridden?

Sirwan Afifi · Answer 4 · 2016-02-19T15:21:45.043

0

You can use GroupBy for getting unique items:

var result = personList.GroupBy(p=> p.Id)
                   .Select(grp => grp.First())
                   .ToList();

edited Feb 19 '16 at 15:21

answered Feb 19 '16 at 15:16

Sirwan Afifi

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You can use the aggregation Key. On 100 K records it is a significant improvement Select(grp => grp.Key) – user853710 Feb 19 '16 at 15:57

Can I use LINQ to check if objects in a list have a unique ID?

4 Answers4