How to get a distinct list from a List of objects?

I have a List<MyClass> someList.

class MyClass
{
    public int Prop1...
    public int Prop2...
    public int Prop3...
}

I would like to know how to get a new distinct List<MyClass> distinctList from List<MyClass> someList, but only comparing it to Prop2.

Solution 1:

You can emulate the effect of DistinctBy using GroupBy and then just using the first entry in each group. Might be a bit slower that the other implementations though.

someList.GroupBy(elem=>elem.Prop2).Select(group=>group.First());

Solution 2:

Unfortunately there's no really easy built-in support for this in the framework - but you can use the DistinctBy implementation I have in MoreLINQ.

You'd use:

var distinctList = someList.DistinctBy(x => x.Prop2).ToList();

(You can take just the DistinctBy implementation. If you'd rather use a Microsoft implementation, I believe there's something similar in the System.Interactive assembly of Reactive Extensions.)

Solution 3:

you need to use .Distinct(..); extension method. Here's a quick sample:

public class Comparer : IEqualityComparer<Point>
    {
        public bool Equals(Point x, Point y)
        {
            return x.X == y.X;
        }

        public int GetHashCode(Point obj)
        {
            return (int)obj.X;
        }
    }

Do not forget about GetHashCode.

Usage:

List<Point> p = new List<Point>();
// add items
p.Distinct(new Comparer());