using System;

public class Program

{

    public static void Main()

    {

        Console.WriteLine(CustomSorting("Id"));

        Console.WriteLine(CustomSorting("Id desc"));

        Console.WriteLine(CustomSorting("Rating asc"));

        Console.WriteLine(CustomSorting("Name"));

        Console.WriteLine(CustomSorting("Name desc"));

        Console.WriteLine(CustomSorting("Unknown"));

        Console.WriteLine(CustomSorting("Unknown desc"));

    }

    public static string CustomSorting(string sorting)

    {

        var returnSorting = sorting;

        var sortArray = sorting.Split(" ");

        var columnName = sortArray[0];

        var direction = sortArray.Length > 1 ? sortArray[1].ToUpper() : "ASC";

        switch (sortArray[0])

        {

            case nameof(Customer.Id):

                returnSorting = $"CustomerId {direction}";

                break;

            case nameof(Customer.Rating):

                returnSorting = $"Products.Rank {direction}";

                break;

            case nameof(Customer.Name):

            default:

                returnSorting = $"{columnName} {direction}";

                break;  

        }

        return returnSorting;

    }

}

​

public class Customer

{

    public string Id { get; set; }

    public string Name { get; set; }

    public int Rating { get; set; }

}