How to remove all records except few records in C# list object?

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C#5.0

I have users list from that list how to remove all records except few records in c#.


The below code is removing all records and count is 0.

Scenario


list: 1,2,3,4,5,6

require only : 3,4,5

What I have tried:

I Have tried with the below code and getting count 0

class Program
    {
        static void Main(string[] args)
        {
            List<Userdetails> allDetails = new List<Userdetails>() { new Userdetails() { UserId=1,UserName="A"},
            new Userdetails() { UserId=2,UserName="B"},
            new Userdetails() { UserId=3,UserName="C"},
            new Userdetails() { UserId=4,UserName="D"},
            new Userdetails() { UserId=5,UserName="E"},
            new Userdetails() { UserId=6,UserName="F"}};

            List<int> newIds = new List<int>();
            newIds.Add(2);
            newIds.Add(3);
            

            foreach (int userId in newIds)
            {
                var result = allDetails.Where(u => u.UserId == userId).ToList();
            }

        }
    }
    public class Userdetails { public int UserId { get; set; } public string UserName { get; set; } }