String Reverse using Substring

using System;
using System.Collections.Generic;
using System.Text;

namespace SB_Sub
{
class Program
{
static void Main(string[] args)
{
string str1="123456789";
string str2;
if (str1 != "")
{
str2 = StringReverse(str1);
Console.Write(str2);

}
else
{
Console.WriteLine("Please enter a value string!");
}
Console.ReadLine();
}
public static string StringReverse(string str)
{
int len = str.Length;
string str5;
StringBuilder sb = new StringBuilder();
for (int i = 0; i <= len-1; i++)
{
sb.Append(str.Substring(len - i-1,1));

}
str5=sb.ToString();
//Console.Write(sb);
//Console.ReadLine();
return str5;
}
}
}