Language: C++

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

class Program
{
    public static void Main()
    {
        var mod = 1000000007L;
        var n = Int32.Parse(Console.ReadLine());
        var a = Console.ReadLine().Split(' ').Select(x => long.Parse(x)).ToArray();
        var dic = new Dictionary<long, long>();
        for (int i = 0; i < n; ++i)
        {
            long value;
            if (dic.TryGetValue(a[i], out value))
            {
                dic[a[i]]++;
            }
            else
            {
                dic.Add(a[i], 1);
            }
        }
        var keyList = dic.Keys.OrderBy(x=>x).ToArray();
        long ans = 0;
        for (int i = 0;i < keyList.Length - 1; ++i)
        {
            for (int j = i + 1; j < keyList.Length; ++j)
            {
                var num = (((((keyList[i] * keyList[j]) % mod) * dic[keyList[i]]) % mod) * dic[keyList[j]]) % mod;
                ans += num;
                ans %= mod;
            }
        }

        Console.WriteLine(ans);
    }
}


Compiler output:
a.cc:1:7: error: expected nested-name-specifier before 'System'
    1 | using System;
      |       ^~~~~~
a.cc:2:7: error: expected nested-name-specifier before 'System'
    2 | using System.Collections.Generic;
      |       ^~~~~~
a.cc:3:7: error: expected nested-name-specifier before 'System'
    3 | using System.Linq;
      |       ^~~~~~
a.cc:7:11: error: expected ':' before 'static'
    7 |     public static void Main()
      |           ^~~~~~~
      |           :
a.cc:39:2: error: expected ';' after class definition
   39 | }
      |  ^
      |  ;
a.cc: In static member function 'static void Program::Main()':
a.cc:9:9: error: 'var' was not declared in this scope
    9 |         var mod = 1000000007L;
      |         ^~~
a.cc:10:13: error: expected ';' before 'n'
   10 |         var n = Int32.Parse(Console.ReadLine());
      |             ^
a.cc:11:13: error: expected ';' before 'a'
   11 |         var a = Console.ReadLine().Split(' ').Select(x => long.Parse(x)).ToArray();
      |             ^
a.cc:12:13: error: expected ';' before 'dic'
   12 |         var dic = new Dictionary<long, long>();
      |             ^~~
a.cc:13:29: error: 'n' was not declared in this scope
   13 |         for (int i = 0; i < n; ++i)
      |                             ^
a.cc:16:17: error: 'dic' was not declared in this scope
   16 |             if (dic.TryGetValue(a[i], out value))
      |                 ^~~
a.cc:16:33: error: 'a' was not declared in this scope
   16 |             if (dic.TryGetValue(a[i], out value))
      |                                 ^
a.cc:16:39: error: 'out' was not declared in this scope
   16 |             if (dic.TryGetValue(a[i], out value))
      |                                       ^~~
a.cc:25:13: error: expected ';' before 'keyList'
   25 |         var keyList = dic.Keys.OrderBy(x=>x).ToArray();
      |             ^~~~~~~
a.cc:27:28: error: 'keyList' was not declared in this scope
   27 |         for (int i = 0;i < keyList.Length - 1; ++i)
      |                            ^~~~~~~
a.cc:31:21: error: expected ';' before 'num'
   31 |                 var num = (((((keyList[i] * keyList[j]) % mod) * dic[keyList[i]]) % mod) * dic[keyList[j]]) % mod;
      |                     ^~~
a.cc:32:24: error: 'num' was not declared in this scope; did you mean 'enum'?
   32 |                 ans += num;
      |                        ^~~
      |                        enum
a.cc:33:24: error: 'mod' was not declared in this scope
   33 |                 ans %= mod;
      |                        ^~~
a.cc:37:9: error: 'Console' was not declared in this scope
   37 |         Console.WriteLine(ans);
      |         ^~~~~~~