LeftAndRightShiftStrings, C#

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//Rextester.Program.Main is the entry point for your code. Don't change it.
//Compiler version 4.0.30319.17929 for Microsoft (R) .NET Framework 4.5

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text.RegularExpressions;
using System.Text;

namespace Rextester
{
    public class Program
    {
        public static void Main(string[] args)
        {
            String str = "Hello world!";
            leftshift(str,3);
            rightshift(str,3);
        }
        public static void leftshift(String str, int n)
        {
            StringBuilder sb = new StringBuilder();
            String temp = reverse(str.Substring(0,n));
            sb.Append(str.Substring(n-1,str.Length-n)+ temp.ToString());
            Console.WriteLine("Left shifted " +sb.ToString());
        }
        
           public static void rightshift(String str, int n)
        {
            StringBuilder sb = new StringBuilder();
            String temp = reverse(str.Substring(str.Length-n,n));
            sb.Append(temp.ToString() + str.Substring(0,str.Length-n));
            Console.WriteLine("Right shifted " + sb.ToString());
        }
        
        public static String reverse(String st)
        {
            StringBuilder sb = new StringBuilder();
            int i=0;
            for(i= st.Length-1; i>=0; i--)
            {
                sb.Append(st[i]);
            }
            return sb.ToString();
        }
                                
        
     
        
        
    }
}

Show compiler warnings [ + ] Show input
Compilation time: 0,14 sec, absolute running time: 0,11 sec, cpu time: 0,09 sec, average memory usage: 12 Mb, average nr of threads: 2, absolute service time: 0,27 sec