At least one object must implement IComparable

Question

using System; using System.Xml; using System.Collections.Generic; using System.Linq; using System.Text;  namespace ConsoleApplication1 {     class Program     {         static void Main(string[] args)         {             SortedSet<Player> PlayerList = new SortedSet<Player>();              while (true)             {                 string Input;                 Console.WriteLine("What would you like to do?");                 Console.WriteLine("1. Create new player and score.");                 Console.WriteLine("2. Display Highscores.");                 Console.WriteLine("3. Write out to XML file.");                 Console.Write("Input Number: ");                 Input = Console.ReadLine();                 if (Input == "1")                 {                     Player player = new Player();                     string PlayerName;                     string Score;                      Console.WriteLine();                     Console.WriteLine("-=CREATE NEW PLAYER=-");                     Console.Write("Player name: ");                     PlayerName = Console.ReadLine();                     Console.Write("Player score: ");                     Score = Console.ReadLine();                      player.Name = PlayerName;                     player.Score = Convert.ToInt32(Score);                      //====================================                     //ERROR OCCURS HERE                     //====================================                     PlayerList.Add(player);                       Console.WriteLine("Player \"" + player.Name + "\" with the score of \"" + player.Score + "\" has been created successfully!" );                     Console.WriteLine();                 }                 else                 {                     Console.WriteLine("INVALID INPUT");                 }             }         }     } } 

So i keep getting the "

At least one object must implement IComparable.

" when trying to add a second player, the first one works, but the second one doesn't. I also MUST use SortedSet because that is the requirement for the work, it's school work.

Answer 1

Well, you're trying to use SortedSet<>... which means you care about the ordering. But by the sounds of it your Player type doesn't implement IComparable<Player>. So what sort order would you expect to see?

Basically, you need to tell your Player code how to compare one player with another. Alternatively, you could implement IComparer<Player> somewhere else, and pass that comparison into the constructor of SortedSet<> to indicate what order you want the players in. For example, you could have:

public class PlayerNameComparer : IComparer<Player> {     public int Compare(Player x, Player y)     {         // TODO: Handle x or y being null, or them not having names         return x.Name.CompareTo(y.Name);     } } 

Then:

// Note name change to follow conventions, and also to remove the // implication that it's a list when it's actually a set... SortedSet<Player> players = new SortedSet<Player>(new PlayerNameComparer()); 

Answer 2

This is a more general answer to this error i suppose.

This line will fail with the error you got:

Items.OrderByDescending(t => t.PointXYZ); 

However you can specify how to compare it directly:

Items.OrderByDescending(t => t.PointXYZ.DistanceTo(SomeOtherPoint)) 

Then you dont need the IComparable interface. Depends on the API you are using. In my case i have a Point and a DistanceTo-method. (Revit API) But an integer should be even easier to determine the "size/position" of.