Select items with a frequency of 1 from a List<T> using LINQ

Question

I have a list of Point classes.

Two of the points are repeated only once in the list and the rest are repeated twice.

How can I find the points that have been repeated once using LINQ?

Answer 1

This solution will group identical points together, allowing you to find the groups with only one member, and return that member.

I haven't checked the actual runtime, but there's a good chance that it's better, performance-wise, than a solution which involves running a Count() operation inside a Where, since that would probably run at O(n^2) time, whereas the GroupBy implementation probably does it more elegantly.

var result = points
 .GroupBy(p => p)
 .Where(group => group.Count() == 1)
 .Select(group => group.First());

Answer 2

尝试这个 ：

var result = points.Where(p1 => points.Count(p2 => p1.Contains(p2)) == 1);

Answer 3

using System;
using System.Collections.Generic;
using System.Linq;

class Point
{
    int x, y;
    public Point(int x, int y)
    {
        this.x = x;
        this.y = y;
    }
    public int X
    {
        get { return x; }
        set { x = value; }
    }
    public int Y
    {
        get { return y; }
        set { y = value; }
    }
}
class Test
{
    static void Main()
    {
        var collection = new List<Point>
        {
            new Point(1,1),
            new Point(1,2),
            new Point(1,1),
            new Point(1,2),
            new Point(3,3),
            new Point(4,5),
        };
        var result = collection.Where(a => collection.Count(b => b.X == a.X && b.Y == a.Y) == 1);
        foreach (var val in result)
            Console.WriteLine(val.X + "," + val.Y);
    }
}
//output:
3,3
4,5