Copy distinct id's to a list

Question

I have 4 Lists, like this:

List <Emp> l1 = new List<Emp>();
List <Emp> l2 = new List<Emp>();
List <Emp> l3 = new List<Emp>();
List <Emp> l4 = new List<Emp>();

Emp has a property called id , How can I get a List of all distinct id's present in all the four Lists?

Answer 1

You'll need MoreLinq for the DistinctBy :

 IEnumrable<Emp> uniqueEmps = l1.Concat(l2).Concat(l3).Concat(l4)
                                .DistinctBy(e => e.id);

for just the Id's :

 IEnumrable<int> uniqueIds = l1.Concat(l2).Concat(l3).Concat(l4)
                               .Select(e => e.id).Distinct();

Answer 2

How can I get a List of all distinct id's present in all the four Lists?

If you are looking for a list of IDs (as opposed to Emp objects that have these IDs) you can do it with a simple query:

var ids = l1.Concat(l2).Concat(l3).Concat(l4).Select(e => e.Id).Distinct();

Answer 3

If your emp class implements equality comparison then you can just union the lists together:

IEnumrable<int> uniqueIds = l1.Union(l2).Union(l3).Union(l4).Select(e => e.id);

To implement equality in your class, you simply override Equals and GetHashCode :

public override bool Equals(object obj)
{
    var compareTo = obj as Emp;
    return Id.Equals(CompareTo.Id);
}

public override int GetHashCode()
{
    return Id.GetHashCode();
}

The benefit of doing it this way is that anywhere you need to compare two emp instances, it will leverage this implementation.