C# - Sorting using Extension Method

Question

I want to sort a list of person say

List<Person> persons=new List<Person>();
persons.Add(new Person("Jon","Bernald",45000.89));
persons.Add(new Person("Mark","Drake",346.89)); 
persons.Add(new Person("Bill","Watts",456.899));

based on

public enum CompareOptions
 {
    ByFirstName,
    ByLastName,
    BySalary
 }

 public enum SortOrder
 {
   Ascending,
   Descending
 }

using lambda expression what is the way to go for sorting?

    public static List<Person> SortPeople(this List<Person> lst, 
   CompareOptions opt1,SortOrder ord)

        {
           lst.Sort((p,op1,op2)=>{ how to apply lambda expression here});
        }

Answer 1

It looks like you are attempting to call the Sort method on List<T> which takes a Comparison<T> delegate. This will require a bit of work because you first have to define a compatible comparison function.

First step is to write a comparison function based on the CompareOptions value

private static Comparison<Person> Create(CompareOptions opt) {
  switch (opt) {
    case CompareOptions.ByFirstName: (x,y) => x.FirstName.CompareTo(y.FirstName);
    case CompareOptions.ByLastName: (x,y) => x.LastName.CompareTo(y.LastName);
    case CompareOptions.BySalary: (x,y) => x.Salary - y.Salary;
    default: throw new Exception();
  }
}

By default this function will sort in ascending order. If you want it to be descending simply negate the value. So now writing SortPeople can be done by the following

public static List<Person> SortPeople(
   this List<Person> list, 
   CompareOptions opt1,
   SortOrder ord) )
   var original = Create(opt1);
   var comp = original;
   if( ord == SortOrder.Descending ) {
     comp = (x,y) => -(orig(x,y));
   }
   list.Sort(comp);
}

EDIT

Version which is done 100% in a lambda

public static List<Person> SortPeople(
   this List<Person> list, 
   CompareOptions opt1,
   SortOrder ord) )

   list.Sort( (x,y) => {
     int comp = 0;
     switch (opt) {
       case CompareOptions.ByFirstName: comp = x.FirstName.CompareTo(y.FirstName);
       case CompareOptions.ByLastName: comp = x.LastName.CompareTo(y.LastName);
       case CompareOptions.BySalary: comp = x.Salary.CompareTo(y.Salary);
       default: throw new Exception();
     }
     if ( ord == SortOrder.Descending ) {
       comp = -comp;
     }
     return comp;
   });
}