Select Unique Record from List based on Particular value In Linq

Question

I am using LINQ. I have a list in which there some duplicate record based on a particular column value . I want to get unique record based on particular value from a list using Linq . I am using distict method to get the unique record but it is not supported. Here is my code.

        private List<CountOnlineUserModel> GetUserList()
        {

            List<CountOnlineUserModel> Users = new List<CountOnlineUserModel>();
            CountOnlineUserModel p = new CountOnlineUserModel();

            p.UserUniqueID = "IMW3";
            p.Username = "Rahul";
            Users.Add(p);

            p.UserUniqueID = "IMW4";
            p.Username = "Raheem";
            Users.Add(p);

            p.UserUniqueID = "IMW3";
            p.Username = "Rahul";
            Users.Add(p);

            p.UserUniqueID = "IMW4";
            p.Username = "Raheem";
            Users.Add(p);

            return Users;
        }

Answer 1

Try implementing IEquatable<CountOnlineUserModel> in CountOnlineUserModel .

public class CountOnlineUserModel : IEquatable<CountOnlineUserModel> {
    // ...

    public bool Equals(CountOnlineUserModel other) {
        return UserUniqueID == other.UserUniqueID;
        // use the above if you just want to compare the IDs. Use below if you want to compare both the ID and name
        return UserUniqueID == other.UserUniqueID && 
            Username == other.Username;
    }
}

Now you can do Distinct :

        CountOnlineUserModel p = new CountOnlineUserModel();

        p.UserUniqueID = "IMW3";
        p.Username = "Rahul";
        Users.Add(p);

        p = new CountOnlineUserModel();
        p.UserUniqueID = "IMW4";
        p.Username = "Raheem";
        Users.Add(p);

        p = new CountOnlineUserModel();
        p.UserUniqueID = "IMW3";
        p.Username = "Rahul";
        Users.Add(p);

        p = new CountOnlineUserModel();
        p.UserUniqueID = "IMW4";
        p.Username = "Raheem";
        Users.Add(p);
        Console.WriteLine(Users.Distinct());

Answer 2

You can do GroupBy:

return Users.GroupBy(c => c.UserUniqueID).Select(c => c.First()).ToList();

Make sure to add this on your using statements if you want to use GroupBy or Distinct:

using System.Linq;

Answer 3

You can use GroupBy() .

List<CountOnlineUserModel> distinctOnlineUsers = GetUserList()
  .GroupBy(p => p.UserUniqueID)
  .Select(g => g.First())
  .ToList();

You can groups on multiple properties as well, can be done as follows:

List<CountOnlineUserModel> distinctOnlineUsers = GetUserList()
  .GroupBy(p => new {p.UserUniqueID, p.Username} )
  .Select(g => g.First())
  .ToList();

Answer 4

First of all, your sample data is messed up. You have created a list containing four references, all to the same object (whose properties you set once and then overwrite three times). Unless you are doing something pretty unusual, you have to instantiate a new object for each entry:

private List<CountOnlineUserModel> GetUserList()
{
    var users = new List<CountOnlineUserModel>
        {
            new CountOnlineUserModel
                {
                    UserUniqueID = "IMW3",
                    Username = "Rahul"
                },
            new CountOnlineUserModel
                {
                    UserUniqueID = "IMW3",
                    Username = "Rahul"
                },
            new CountOnlineUserModel
                {
                    UserUniqueID = "IMW4",
                    Username = "Raheem"
                },
            new CountOnlineUserModel
                {
                    UserUniqueID = "IMW3",
                    Username = "Rahul"
                }
        }
}

Second, I think you may be overthinking the problem. With the sample data you have provided, if you just want one of the records that matches on a particular value, you don't need distinct or group by. Just take the first record that matches.

var record = users.Where( u => u.UserUniqueID == "IMW3" ).First();