c# - How to Count Duplicates in List with LINQ

Question

I have a list of items

• John ID
• Matt ID
• John ID
• Scott ID
• Matt ID
• John ID
• Lucas ID

I want to shove them back into a list like so which also means I want to sort by the highest number of duplicates.

• John ID 3
• Matt ID 2
• Scott ID 1
• Lucas ID 1

Let me know how I can do this with LINQ and C#.

Thanks All

EDIT 2 Showing Code:

    List<game> inventory = new List<game>();
    drinkingforDataContext db = new drinkingforDataContext();
    foreach (string item in tbTitle.Text.Split(' '))
    {

        List<game> getItems = (from dfg in db.drinkingfor_Games
                               where dfg.game_Name.Contains(tbTitle.Text)
                               select new game
                               {
                                   gameName = dfg.game_Name,
                                   gameID = Boomers.Utilities.Guids.Encoder.EncodeURLs(dfg.uid)
                               }).ToList<game>();

        for (int i = 0; i < getItems.Count(); i++)
        {
            inventory.Add(getItems[i]);
        }
    }

    var items = (from xx in inventory
                 group xx by xx into g
                 let count = g.Count()
                 orderby count descending
                 select new
                    {
                        Count = count,
                        gameName = g.Key.gameName,
                        gameID = g.Key.gameID
                    });

    lvRelatedGames.DataSource = items;
    lvRelatedGames.DataBind();

This query displays these results:

• 1 hello world times
• 1 hello world times
• 1 Hello World.
• 1 hello world times
• 1 hello world times
• 1 hello world times
• 1 Hello World.
• 1 hello world times

It gives me the count and name, but it doesn't give me the ID of the game....

It should display:

• 6 hello world times 234234
• 2 Hello World. 23432432

Answer 1

You can use "group by" + "orderby". See LINQ 101 for details

var list = new List<string> {"a", "b", "a", "c", "a", "b"};
var q = from x in list
        group x by x into g
        let count = g.Count()
        orderby count descending
        select new {Value = g.Key, Count = count};
foreach (var x in q)
{
    Console.WriteLine("Value: " + x.Value + " Count: " + x.Count);
}

In response to this post (now deleted):

If you have a list of some custom objects then you need to use custom comparer or group by specific property.

Also query can't display result. Show us complete code to get a better help.

Based on your latest update:

You have this line of code:

group xx by xx into g

Since xx is a custom object system doesn't know how to compare one item against another. As I already wrote, you need to guide compiler and provide some property that will be used in objects comparison or provide custom comparer. Here is an example:

Note that I use Foo.Name as a key - i.e. objects will be grouped based on value of Name property.

There is one catch - you treat 2 objects to be duplicate based on their names, but what about Id ? In my example I just take Id of the first object in a group. If your objects have different Ids it can be a problem.

//Using extension methods
var q = list.GroupBy(x => x.Name)
            .Select(x => new {Count = x.Count(), 
                              Name = x.Key, 
                              ID = x.First().ID})
            .OrderByDescending(x => x.Count);

//Using LINQ
var q = from x in list
        group x by x.Name into g
        let count = g.Count()
        orderby count descending
        select new {Name = g.Key, Count = count, ID = g.First().ID};

foreach (var x in q)
{
    Console.WriteLine("Count: " + x.Count + " Name: " + x.Name + " ID: " + x.ID);
}