C#分組方式比較

測試方法：

private static void Main(string[] args)
        {
            var list = new List<Person>();
            for (int i = 0; i < 1000000; i++)
            {
                list.Add(new Person() { Age = 18, Name = "老石" });
            }

            var time1 = Time(() =>
            {
                list.GroupBy(t => new { t.Age, t.Name })
                .Select(t => t.FirstOrDefault())
                .ToList();
            });
            Console.WriteLine($"分組耗時：{time1}");


            var time2 = Time(() =>
            {
                  list.Distinct(d => new { d.Age, d.Name }).ToList();
             });
            Console.WriteLine($"HashSet耗時：{time2}");


            var time3 = Time(() =>
            {
                list.Distinct((a, b) => a.Age == b.Age && a.Name == b.Name).ToList();
            });
            Console.WriteLine($"委托耗時：{time3}");
        }

        static long Time(Action action)
        {
            var stopwatch = new Stopwatch();
            stopwatch.Start();
            action();
            stopwatch.Stop();
            return stopwatch.ElapsedMilliseconds;
        }      

　　

public static class DistinctTest
    {

        public static void Test()
        {
            var list = new List<Person>();
            for (int i = 0; i < 1000000; i++)
                list.Add(new Person() { Age = 18, Name = "jeffcky" });

            list = list.GroupBy(t => new { t.Age, t.Name })
                .Select(t => t.FirstOrDefault())
                .ToList();

            list = list.Distinct(d => new { d.Age, d.Name }).ToList();

            list = list.Distinct((a, b) => a.Age == b.Age && a.Name == b.Name && a.count==b.count ).ToList();
        }

        public static IEnumerable<TScource> Distinct<TScource, TKey>(this IEnumerable<TScource> source,
            Func<TScource, TKey> keySelector)
        {
            var hashSet = new HashSet<TKey>();
            foreach (TScource element in source)
            {
                if (hashSet.Add(keySelector(element)))
                    yield return element;
            }
        }
    }


    public class Person
    {
        public string Name { get; set; }
        public int Age { get; set; }

        public int count { get; set; }
    }      

　　委托：

public static class Extensions
    {
        public static IEnumerable<T> Distinct<T>(
        this IEnumerable<T> source, Func<T, T, bool> comparer)
        where T : class
        => source.Distinct(new DynamicEqualityComparer<T>(comparer));


        public sealed class DynamicEqualityComparer<T> : IEqualityComparer<T>
        where T : class
        {
            private readonly Func<T, T, bool> _func;

            public DynamicEqualityComparer(Func<T, T, bool> func)
            {
                _func = func;
            }

            public bool Equals(T x, T y) => _func(x, y);

            public int GetHashCode(T obj) => 0;
        }
    }