C# LINQ order by and switch expression

Question

See my code

i am trying to order Quarter. order would look like 1Q 2010 2Q 2010 1H 2010 3Q 2010 4Q 2010 2H 2010 2010 FY

List<string> lstperiods = new List<string>();
 lstperiods.Add("1H 2015");
 lstperiods.Add("1H 2014");
 lstperiods.Add("1Q 2015");
 lstperiods.Add("2Q 2015");
 lstperiods.Add("3Q 2015");
 lstperiods.Add("4Q 2015");
 lstperiods.Add("2015 FY");

 lstperiods.Add("1Q 2014");
 lstperiods.Add("2Q 2014");
 lstperiods.Add("3Q 2014");
 lstperiods.Add("4Q 2014");
 lstperiods.Add("2014 FY");


 lstperiods.Add("2010 FY");
 lstperiods.Add("2011 FY");
 lstperiods.Add("2012 FY");
 lstperiods.Add("2013 FY");
 lstperiods.Add("2H 2015");
 lstperiods.Add("2H 2014");

List<string> allperiods = new List<string>();
 allperiods = lstperiods.OrderBy(x => (x.ToString().Contains("FY")
 ? x.ToString().Substring(0, 4)
 : x.ToString().Substring(3, 4)))
 .ThenBy(y => y switch
 {
 y.Contains("1Q") => 1,
 y.Contains("2Q") => 2,
 y.Contains("1H") => 3,
 y.Contains("3Q") => 4,
 y.Contains("4Q") => 5,
 y.Contains("2H") => 6,
 y.Contains("FY") => 7,
 _ => 8
 }).ToList();

the above code did not compile but when i change the code below way then it worked

List<string> allperiods = new List<string>();
 allperiods = lstperiods.OrderBy(x => (x.ToString().Contains("FY")
 ? x.ToString().Substring(0, 4)
 : x.ToString().Substring(3, 4)))
 .ThenBy(y => y switch
 {
 string a when y.Contains("1Q") => 1,
 string b when y.Contains("2Q") => 2,
 string c when y.Contains("1H") => 3,
 string d when y.Contains("3Q") => 4,
 string e when y.Contains("4Q") => 5,
 string f when y.Contains("2H") => 6,
 string g when y.Contains("FY") => 7,
 _ => 8
 }).ToList();

1) tell me what is the meaning of this line string a when y.Contains("1Q") => 1,?
why do i need to declare multiple variable called a,b,c.....g?

2) also please show me other possible ways to compose the above same code ?

3) i have checked switch expression has multiple variation.

Thanks

Answer 1

pattern match matches the value to the expression. because you need to transform the value before compare, you need to use the var pattern and the when. as you don't need the variable you can just use a placeholder:

y switch 
{ 
  _ when y.Contains("1Q") => 1, 
  _ when y.Contains("2Q") => 2, 
  _ when y.Contains("1H") => 3, 
  _ when y.Contains("3Q") => 4, 
  _ when y.Contains("4Q") => 5, 
  _ when y.Contains("2H") => 6, 
  _ when y.Contains("FY") => 7, 
  _ => 8 
  } 

Answer 2

I think a practical example might help, see the following project and search for when.

And here is a refactor of your code, note the indexer x.ToString()[..4]

using System;
using System.Collections.Generic;
using System.Linq;

namespace ExamplePatternMatching
{
    partial class Program
    {
        static void Main(string[] args)
        {
            List<string> allperiods = 
                Periods().OrderBy(x => x.ToString().Contains("FY")
                    ? x.ToString()[..4]
                    : x.ToString().Substring(3, 4))
                .ThenBy(y => y switch
                {
                    { } a when y.Contains("1Q") => 1,
                    { } b when y.Contains("2Q") => 2,
                    { } c when y.Contains("1H") => 3,
                    { } d when y.Contains("3Q") => 4,
                    { } e when y.Contains("4Q") => 5,
                    { } f when y.Contains("2H") => 6,
                    { } g when y.Contains("FY") => 7,
                    _ => 8
                }).ToList();

            Console.ReadLine();
        }

        public static List<string> Periods() => new()
        {
                "1H 2015",
                "1H 2014",
                "1Q 2015",
                "2Q 2015",
                "3Q 2015",
                "4Q 2015",
                "2015 FY",
                "1Q 2014",
                "2Q 2014",
                "3Q 2014",
                "4Q 2014",
                "2014 FY",
                "2010 FY",
                "2011 FY",
                "2012 FY",
                "2013 FY",
                "2H 2015",
                "2H 2014"
           };
    }
}