C#: 如何获取列表中出现次数最多的元素？

Question

我有一个“Activity”类型的列表，需要找到出现最多的元素。例如：

Activity a = new Activity();
a.Location = "USA";

Activity b = new Activity();
b.Location = "Sri Lanka";

Activity b = new Activity();
b.Location = "USA";

List<Activity> activityList = new List<Activity>();
activityList.add(a);
//......adding the rest of the objects as well.

现在我需要找到这个列表中出现最多的位置。例如，在上面的例子中，出现最多的位置是：美国。

我尝试过这样做：

            String currentLocation = "";
            String mostOccurring = "";
            int currentCount = 0;
            int highest = 0;

            for (int i = 0; i < activityList.Count; i++)
            {
                currentLocation = activityList[i].Location;

                foreach (Activity activity in activityList)
                {
                    if (activity.Location.Equals(currentLocation))
                    {
                        currentCount++;
                        highest = currentCount;
                        //compare the highest count
                    }
                }
            }

但是我卡住了，看起来不是很高效。这是在一个ASP.NET Web项目中，因此效率非常重要。您认为最有效的方法是什么？

- MWUser

---

也可以在这里查看：http://stackoverflow.com/questions/17180139/find-max-count-of-a-list-of-custom-types - Kavindu Dodanduwa

Answer 1

使用Linq非常简单。

var query = activityList.GroupBy(x => x.Location)
    .Select(group => new {Location = group.Key, Count = group.Count()})
    .OrderByDescending(x => x.Count);

var item = query.First();

var mostfrequent = item.Location;
var mostfrequentcount = item.Count;

- Erik Funkenbusch

Answer 2

activityList.GroupBy(a => a.Location)
            .OrderByDescending(g => g.Count())
            .First().Key;

- Victor Hurdugaci

Answer 3

我曾经遇到过类似的问题，我使用了一个 字典。

Dictionary<Location, int>  LocationCounter; 
. . .
public void CountLocations(Location thisPlace) {
    if (!LocationCounter(thisPlace)
        LocationCounter.Add(thisPlace, 1);
    else 
        LocationCounter[thisPlace]++;
}

这里是一个关于按字典值排序的SO链接

- radarbob