c# - How to sort a list of objects with IComparable and IComparer

Question

I'm trying to implement the same example of this link but more oriented on the number of dependent kids.

http://www.codeproject.com/Articles/42839/Sorting-Lists-using-IComparable-and-IComparer-Inte

So I have 3 employees with A: 0, B: 0, C: 2 respectively. I want to order them descendently by number of kids. So I would have C:2, B:0, A:0

But my list it's not being sorted. It remains as A: 0, B: 0, C: 2

What I'm doing wrong?

My Comparer

public class EmployeeComparer : IComparer<Employee>
{
    public int Compare(Employee x, Employee y)
    {
        this.CompareNumberOfKids(x, y);
    }

    public int CompareNumberOfKids(Employee x, Employee y)
    {
        if (x.NumberOfKids > y.NumberOfKids)
        {
            return -1;
        }
        else if (x.NumberOfKids < y.NumberOfKids)
        {
            return 1;
        }
        else
        {
            return 0;
        }
    }
}

My Business Entity

public class Employee : IComparable<Employee>
{
    //...//
    Public NumberOfKids { get; set; }

    int IComparable<Employee>.CompareTo(Employee next)
    {
        return new EmployeeComparer().Compare(this, next);
    }

    public override bool Equals(object obj)
    {
        if (obj != null && obj is Emmployee)
        {
            return ((Employee)obj).ID.Equals(this.ID);
        }
        else return base.Equals(obj);
    }

    public override int GetHashCode()
    {
        return base.GetHashCode();
    }
}

Aspx.cs

public List<Employee> GetEmployeeSortedList()
{
    List<Employee> list = new List<Employee>();
    list.Add(new Employee() { Name = "A", NumberOfKids = 0 } );
    list.Add(new Employee() { Name = "B", NumberOfKids = 0 } );
    list.Add(new Employee() { Name = "C", NumberOfKids = 2 } );
    list.Add(new Employee() { Name = "D", NumberOfKids = 1 } );
    list.Add(new Employee() { Name = "E", NumberOfKids = 0 } );
    list.Add(new Employee() { Name = "F", NumberOfKids = 4 } );

    list = list.Take(3).ToList();
    EmployeeComparer comp = new EmployeeComparer();
    list.Sort(comp);
    return list;
}

Answer 1

First, there's no need to have an IComparer<Employee> that sorts by descending if your Employee class implements IComparable<Employee> using the same sort criteria. And it's horribly inefficient for your Employee class to instantiate a new IComparer<Employee> for every comparision.

You should change your Employee class so that its CompareTo looks like this:

int CompareTo(Employee next)
{
    return next.NumberOfKids.CompareTo(this.NumberOfKids);
}

Then you can ditch the EmployeeComparer altogether and sort like this:

list = list.Take(3).ToList();
list.Sort();  // Uses default IComparable for the Employee class
return list;

Typically, you make the IComparable<T> implementation on the class perform the default sorting order. In the case of employees, that'd probably either be by employee ID or perhaps last name, first name. IComparer<T> implementations should be for other sorting criteria.

With List<T>, though, you have another option: use an anonymous function. For example, you could do this by writing:

list.Sort((x, y) => y.NumberOfKids.CompareTo(x.NumberOfKids));

See this List.Sort overload.

Or, you could just ditch the whole idea of IComparer<T> and IComparable<T> and List.Sort altogether and do it the LINQ way:

var result = list.Take(3).OrderByDescending(x => x.NumberOfKids).ToList();