Sorting Complex Type Using IComparer

Question

User1231829591 posted

Hi, I am trying to sort my employees based on ManagementLevel by implementing IComplarer interface. However, for the Compare method of the IComparer interface to work a separate class must be created.

The following is my Employee class definition which implements IComparer's Compare method:

    public class Employee : IComparer<Employee>
    {
        public string Name
        {
            get;
            set;
        }
        public int Salary
        {
            get;
            set;
        }
        public  ManagementLevel ManageLvl
        {
            get; 
            set;
        }

        public int Compare(Employee emp, Employee otherEmp)
        {            
            return emp.ManageLvl.CompareTo(otherEmp.ManageLvl);
        }

    }

The following is my Enum definition

 public enum ManagementLevel
    {
        CEO = 1,
        CFO = 2,
        CTO = 3,
        RegionalManager = 4       
    }

The following adds , sorts, then reverse sorts the list of employees:

List<Employee> employeeList = new List<Employee>();
employee emp = new Employee();            
emp.Name = txtName.Text;
emp.Salary = Convert.ToInt32(txtSalary.Text);
emp.ManageLvl = (ManagementLevel)Enum.Parse(typeof(ManagementLevel), txtManagementLevel.Text);
employeeList.Add(emp);

employeeList.Sort();
employeeList.Reverse();

I am getting the error below

At least one object must implement IComparable.

As stated above the Sort method does not work. I was only able to make the Compare method of the ICompare interface sort the way I want by creating a separate class but why can't I implement it in the Employee class, thanks in advance for your reply.

Answer 1

User-434868552 posted

@ManyTitles

TIMTOWTDI

use IComparable instead of IComparer

this code which works in LINQPad5 is mostly your code from above with fixed typos.   http://linqpad.net  

public enum ManagementLevel
{
    CEO = 1,
    CFO = 2,
    CTO = 3,
    RegionalManager = 4
}

public class Employee : IComparable<Employee>
{
    public string Name{ get; set; }
    public int Salary{ get; set; }
    public ManagementLevel ManageLvl{ get; set; }

    public int CompareTo(Employee employee)
    {
        return this.ManageLvl.CompareTo(employee.ManageLvl);
    }
}

void Main()
{
    List<Employee> employeeList = new List<Employee>();
    
    Employee emp = new Employee();
    emp.Name = "ManyTitles";  emp.Salary = 1234;
    emp.ManageLvl = ManagementLevel.RegionalManager;
    employeeList.Add(emp);
    
    Employee cto = new Employee();
    cto.Name = "CTO";  cto.Salary = 2345;
    cto.ManageLvl = ManagementLevel.CTO;
    employeeList.Add(cto);

    Employee ceo = new Employee();
    ceo.Name = "CEO";  ceo.Salary = 123456;
    ceo.ManageLvl = ManagementLevel.CEO;
    employeeList.Add(ceo);

    Employee cfo = new Employee();
    cfo.Name = "CFO";  cfo.Salary = 99999;
    cfo.ManageLvl = ManagementLevel.CFO;
    employeeList.Add(cfo);
    
    employeeList.Dump("unordered");
    employeeList.Sort();
    employeeList.Dump("ordered");
    employeeList.Reverse();
    employeeList.Dump("reversed");
}

The .Dump method is provided by LINQPad5

output:

unordered

List<Employee> (4 items)
Name	Salary	ManageLvl
ManyTitles	1234	RegionalManager
CTO	2345	CTO
CEO	123456	CEO
CFO	99999	CFO

ordered

List<Employee> (4 items)
Name	Salary	ManageLvl
CEO	123456	CEO
CFO	99999	CFO
CTO	2345	CTO
ManyTitles	1234	RegionalManager

reversed

List<Employee> (4 items)
Name	Salary	ManageLvl
ManyTitles	1234	RegionalManager
CTO	2345	CTO
CFO	99999	CFO
CEO	123456	CEO

References:

https://support.microsoft.com/en-us/kb/320727 "How to use the IComparable and IComparer interfaces in Visual C#"

http://www.codedigest.com/Articles/CSHARP/84_Sorting_in_Generic_List.aspx "Sorting in C# Generic List(List<T>)"

it is not working without creating a separate class

From the first reference above:

The role of IComparer is to provide additional comparison mechanisms. ...

Using IComparer is a two-step process.

First, declare a class that implements IComparer,
and then implement the Compare method

Answer 2

User1231829591 posted

Hi, thanks for replying. At first I wasn't clear as to why a new class had to be created to implement IComparer since I was following a tutorial online which did not explain the details on usage.

Answer 3

User303363814 posted

Alternatively, don't implement IComp* at all just use linq.

var reversedList = employeeList.OrderByDescending(e => e.ManageLvl);