using System;

using System.Linq;

public class Program

{

    public static void Main()

    {

        var userId = 1;

        var users = new [] { "1,Joao", "2,Marcos", "3,joaquim123", "31,Jonas" };

        var usuario = users.First(u => u.Split(',')[0] == userId.ToString());

        var usuarios = users.Where(u => u.Split(',')[0].Contains(userId.ToString()));

        Console.WriteLine($"Usuário com Id = {userId}");

        Console.WriteLine($"\t{usuario}");

        Console.WriteLine($"\n\nUsuário onde o Id contém {userId}");

        foreach(var u in usuarios)

            Console.WriteLine($"\t{u}");

    }

}