using System;

using System.Linq;

public class Program

{

    public static void Main()

    {

        int[] array = { 1, 2, 3, 5, 6, 8, 10 };

        // yes it work but we can do better

        Console.WriteLine(string.Join(",", array.FindMissing()));

        /*

            Consider the next two personal choices

        */

        Console.WriteLine(string.Join(",", array.Missing()));

        Console.WriteLine(string.Join(",", array.Missing1()));

    }

}

​

public static class SequenceExtensions

{

​

    public static int[] FindMissing(this int[] sequence) =>

        sequence.Zip(sequence.Skip(1), (valueLeft, valueRight)

            => Enumerable.Range(valueLeft + 1, (valueRight - valueLeft) - 1))

            .SelectMany(item => item)

            .ToArray();

    // C# 8 and above using hat-operator

    public static int[] Missing(this int[] sequence)

        => Enumerable.Range(1, sequence[^1]).Except(sequence)

        .ToArray(); 

​

    public static int[] Missing1(this int[] sequence)

        => Enumerable.Range(sequence.First(), sequence.Last()).Except(sequence)

        .ToArray();     

​

}