Hi Guys before couple of days only i came to know that the Sort methods used by the .NET collections are unstable. These sort methods, which include System.Array.Sort and System.Collections.Generic.List<T>.Sort, use the Quick Sort algorithm, which is relatively fast but in this case, unstable. However, there may be instances where you require a stable sort, so a custom solution is required. Let me explain you by an example:
Program.CS
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Collections;
namespace GenericSortingDemo
{
class Program
{
static void Main(string[] args)
{
// 19 - May - 1977
Employee empHemang = new Employee(5, "Hemang Shah", DateTime.Parse("05/19/1977"));
// 19 - May - 1987
Employee empDarshak = new Employee(2, "Darshak Shah", DateTime.Parse("05/19/1985"));
// 30 - Jan - 1987
Employee empDevang = new Employee(1, "Devang Udeshi", DateTime.Parse("01/30/1987"));
// 30 - Jan - 1987
Employee empNilesh = new Employee(4, "Nilesh Thakkar", DateTime.Parse("01/30/1987"));
// 19 - Aug - 1987
Employee empKiran = new Employee(3, "Kiran Patil", DateTime.Parse("05/19/1987"));
Console.WriteLine("********************ArrayList***********************");
ArrayList empArrayList = new ArrayList();
empArrayList.Add(empHemang);
empArrayList.Add(empDevang);
empArrayList.Add(empNilesh);
empArrayList.Add(empDarshak);
empArrayList.Add(empKiran);
Console.WriteLine("ArrayList - Before sorting");
foreach (Employee emp in empArrayList)
{
Console.WriteLine(emp.ToString());
}
empArrayList.Sort(new MySorter());
Console.WriteLine(Environment.NewLine);
Console.WriteLine("ArrayList - After sorting");
foreach (Employee emp in empArrayList)
{
Console.WriteLine(emp.ToString());
}
Console.WriteLine(Environment.NewLine);
Console.WriteLine("********************Generics***********************");
// Genercis
List<Employee> lstEmployees = new List<Employee>();
lstEmployees.Add(empHemang);
lstEmployees.Add(empDevang);
lstEmployees.Add(empNilesh);
lstEmployees.Add(empDarshak);
lstEmployees.Add(empKiran);
Console.WriteLine("List<Employee> - Before sorting");
foreach (Employee emp in lstEmployees)
{
Console.WriteLine(emp.ToString());
}
// sort employees in asc order
lstEmployees.Sort(delegate(Employee emp1, Employee emp2)
{ return emp1.EmployeeBirthDate.CompareTo(emp2.EmployeeBirthDate); });
Console.WriteLine(Environment.NewLine);
Console.WriteLine("List<Employee> - After sorting");
foreach (Employee emp in lstEmployees)
{
Console.WriteLine(emp.ToString());
}
}
}
public class Employee
{
public Employee(int employeeID,string employeeName,DateTime employeeBirthDate)
{
EmployeeID = employeeID;
EmployeeName = employeeName;
EmployeeBirthDate = employeeBirthDate;
}
public int EmployeeID { get; set; }
public string EmployeeName { get; set; }
public DateTime EmployeeBirthDate { get; set; }
public override string ToString()
{
return "Employee ID : " + EmployeeID + " Employee Name : " + EmployeeName + " Employee BirthDate : " + EmployeeBirthDate;
}
}
public class MySorter : IComparer
{
#region IComparer Members
public int Compare(object x, object y)
{
Employee emp1 = x as Employee;
Employee emp2 = y as Employee;
DateTime dtEmpl = emp1.EmployeeBirthDate;
DateTime dtEmp2 = emp2.EmployeeBirthDate;
int num = dtEmpl.CompareTo(dtEmp2);//by default ascending order
return num;
}
#endregion
}
}
Output
In their original order [Before sorting], Devang (1/30/1987) appears before Nilesh (also 1/30/1987). But because both objects have the same BirthDate, and the List<T>.Sort and Array.Sort method is unstable, the order of equal objects is reversed. Nilesh appears before Devang after sorting.
Q. Why it happens technically?
Ans: The Sort methods used by the .NET collections are unstable. These sort methods, which include System.Array.Sort and System.Collections.Generic.List<T>.Sort, use the Quick Sort algorithm [We all must have learnt in college days 
DS lectures], which is relatively fast but in this case, unstable. However, there may be instances where you require a stable sort
[Source: http://msdn.microsoft.com/en-us/library/w56d4y5z.aspx], so a custom solution is required. – And the custom solution is “Stable Insertion Sort” – is the best way to do stable sorting [Source: http://www.csharp411.com/c-stable-sort/]
Happy Stable Sorting!
