Question

I need to sort an employee list based on predefined uniqueIds.

In simple words, consider a list of employee Ids 1 to 10 in random order.

I have a predefined rule that says order employee objects in 2, 8, 1, 4, 6 And if any employee UId is not in range [1,10] put them at the end of list…(any order).

I wrote following code using IComparer<Employee>.

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

    class Comparision : IComparer<Employee>
    {
        List<int> referenceKeys = new List<int> { 2, 8, 1, 4, 6 };
        public int Compare(Employee thisOne, Employee otherOne)
        {
            var otherIndex = referenceKeys.IndexOf(otherOne.UId);
            var thisIndex = referenceKeys.IndexOf(thisOne.UId);
            if (thisIndex > otherIndex)
            {
                return 1;
            }
            else if (thisIndex < otherIndex)
            {
                return -1;
            }
            else
            {
                //if uid not found in reference list treat both employee obj as equal
                return 0;
            }
        }
    }
    class CustomSorting
    {
        public static 
        List<Employee> employees = new List<Employee>
        {
            new Employee{UId=1, Name="Ram"},
            new Employee{UId=2 , Name="Shyam"},
            new Employee{UId=3 , Name="Krishna"},
            new Employee{UId=4 , Name="Gopal"},
            new Employee{UId=5 , Name="Yadav"},
            new Employee{UId=6 , Name="Vishnu"},
            new Employee{UId=7 , Name="Hari"},
            new Employee{UId=8 , Name="Kanha"},
        };

        void sort()
        {
            employees.Sort(new Comparision());
        }

        static void Main()
        {
            new CustomSorting().sort();
        }
    }

I have been able to sort the list, with following result-

(5, 7, 3), 2, 8, 1, 4, 6 ==> 5, 7, 3 are not listed in reference key, so should appear in last, any order..

But items not found in my reference keys, are sorted first. I need to put them at the end.

For such a scenario, is IComparer, best way to go for ?

Answer 1

var otherIndex = referenceKeys.IndexOf(otherOne.UId); will return -1 if the item isn’t found, which will be less than any found value.

You want all not found items to be greater than any found value, so just add:

if(otherIndex == -1) otherIndex = int.MaxValue;
if(thisIndex == -1) thisIndex = int.MaxValue;

On a side note, you can simplify the remainder of the method by just using:

return thisIndex.CompareTo(otherIndex);