using System;

using System.Text;

public class Program

{

    public static void Main()

    {

        var first = "Has period at end";

        var second = "Has period at end.";

        var sb = new StringBuilder(first);

        if(sb.ToString() != second)

        {

            Console.WriteLine("Strings are not equal because of missing period.");

        }

        AssureEndsWithPeriod(sb);

        if(sb.ToString() == second)

        {

            Console.WriteLine("Strings are now equal, period was added.");

        }

    }

    static void AssureEndsWithPeriod(StringBuilder sb)

    {

        if(sb.Length <= 0)

            return;

        if('.' != sb[^1])

        {

            sb.Append('.');

        }

    }

}