using System;
using System.Text;
class Program
{
static string replace_char(string s, int idx, char ch) {
int len = s.Length;
if (idx < 0 || idx > len) return s;
StringBuilder sb = new StringBuilder(s);
sb[idx] = ch;
return sb.ToString();
}
static void Main() {
string s = "c# programming";
s = replace_char(s, 3, 'G');
Console.Write(s);
}
}
/*
run:
c# Grogramming
*/