HashSet<object> objs = new HashSet<object>();
objs.Add(new { a = 1, b = 2 }); // true
objs.Add(new { a = 1, b = 2 }); // false
objs.Add(new Man { id = 1, name = "A" }); // true
objs.Add(new Man { id = 1, name = "A" }); // true

Console.WriteLine(new { a = 1, b = 2 } == new { a = 1, b = 2 }); // false
Console.WriteLine(new Man { id = 1, name = "A" } == new Man { id = 1, name = "A" }); // false

Console.WriteLine(object.Equals(new { a = 1, b = 2 }, new { a = 1, b = 2 })); // true
Console.WriteLine(object.Equals(new Man { id = 1, name = "A" }, new Man { id = 1, name = "A" })); // false

Console.WriteLine(object.ReferenceEquals(new { a = 1, b = 2 }, new { a = 1, b = 2 })); // false
Console.WriteLine(object.ReferenceEquals(new Man { id = 1, name = "A" }, new Man { id = 1, name = "A" })); // false

Console.WriteLine(new { a = 1, b = 2}.GetHashCode());
Console.WriteLine(new { a = 1, b = 2}.GetHashCode()); // 内容相同的，HashCode都相同
Console.WriteLine(new { a = 1, b = 3}.GetHashCode());

Console.WriteLine(new Man { id = 1, name = "A" }.GetHashCode()); // 每次都不同
Console.WriteLine(new Man { id = 1, name = "A" }.GetHashCode()); // 每次都不同



// 结论：
//   匿名对象的 GetHashCode 和 Equals 是相同的，以便于方便地用作LINQ连接和分组中的散列键，这样的设计是合理的；
//   HashSet.Add 方法判断对象是否已经存在，是根据 GetHashCode 和 Equals 进行的，单纯的 HashCode 相同没用，还是会添加；
//   所以，下面的 DistinctBy 扩展方法 .DistinctBy(p => new { p.Id, p.Name }) 或 .DistinctBy(p => p.Id) 都可以正常工作；
//   参考[https://stackoverflow.com/questions/12123512/why-does-the-equals-implementation-for-anonymous-types-compare-fields]
//   There is also a very practical reason to do this: Anonymous types are convenient to use as hash keys in LINQ joins and groupings. 



public class Man
{
	public int id { get; set; }
	public string name { get; set; }
	
  //public override int GetHashCode()
  //{
  //  return 123;
  //}

  //public override bool Equals(object obj)
  //{
  //	return ((Man)obj).id == this.id && ((Man)obj).name == this.name;
  //}
}

public static class Extensions
{
    public static IEnumerable<TSource> DistinctBy<TSource, TKey>(this IEnumerable<TSource> source, Func<TSource, TKey> keySelector)
    {
        HashSet<TKey> seenKeys = new HashSet<TKey>();
        foreach (TSource element in source)
        {
            if (seenKeys.Add(keySelector(element)))
            {
                yield return element;
            }
        }
    }
}