Orderby doesn't call Compare() of the provided comparer

Question

Can anyone please explain why .net framework doesn't call compare method of my comparer when I use Enumerable.OrderBy. Whereas it does get called when I use List.Sort().

// The code below has been taken from another post on StackOverFlow.com

class Employee
{
    public string Name { get; set; }
    public int Salary { get; set; }
}


class Employee_SortBySalaryByAscendingOrder : IComparer<Employee>
{
    #region IComparer<Employee> Members

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

    #endregion
}


    private void TestSort(object sender, EventArgs e)
    {

        List<Employee> empList = new List<Employee>() 
                                {       
                                    new Employee { Name = "a", Salary = 14000 },
                                    new Employee { Name = "b", Salary = 13000 } 
                                };
        Employee_SortBySalaryByAscendingOrder eAsc = 
                    new Employee_SortBySalaryByAscendingOrder();
        // Sort Employees by salary by ascending order.   

        // Does not work
        IOrderedEnumerable<Employee> orderedEmployees = empList.OrderBy(x => x, eAsc);

        // Works
        empList.Sort(eAsc);
    }

Answer 1

It doesn't work because you're not actually evaluating the orderedEmployees sequence. You need to force the evaluation using ToList or ToArray.

Linq uses deferred execution so defining your ordering query in:

IOrderedEnumerable<Employee> orderedEmployees = empList.OrderBy(x => x, eAsc);

does not do any work to actually order the input sequence. Only when you try to use the result of the query will the ordering be done.