How to Use LINQ to Get a Count of Duplicates in a List

Often we have to get a count of duplicate items in a list. An easy way to do this is to group the list on the property we want to count, then use LINQ's GroupBy's Count feature. Below is a quick example using an employee list.

 

private static void Main()
{
    List<Employee> empList = new List<Employee>();

    empList.Add(new Employee() { ID = 1, FName = "John", Age=23, Sex='M'  });
    empList.Add(new Employee() { ID = 2, FName = "Mary", Age = 25, Sex = 'F' });
    empList.Add(new Employee() { ID = 3, FName = "Amber", Age = 23, Sex = 'M' });
    empList.Add(new Employee() { ID = 4, FName = "Kathy", Age=25, Sex='M'});
    empList.Add(new Employee() { ID = 5, FName = "Lena", Age=27, Sex='F'});
    empList.Add(new Employee() { ID = 6, FName = "John", Age = 28, Sex = 'M' });
    empList.Add(new Employee() { ID = 7, FName = "Kathy", Age = 27, Sex = 'F' });
    empList.Add(new Employee() { ID = 8, FName = "John", Age = 28, Sex = 'M' });

    var dup = empList
        .GroupBy(x => new { x.FName })
        .Select(group => new { Name = group.Key, Count = group.Count() })
        .OrderByDescending(x => x.Count);

    foreach (var x in dup)
    {
        Response.Write(x.Count + " " + x.Name);
    }
}


class Employee
{
    public int ID { get; set; }
    public string FName { get; set; }
    public int Age { get; set; }
    public char Sex { get; set; }
}