string yazi = "csharp kitabı";
for (int i = yazi.Length-1; i >= 0; i--)
{
Console.Write(yazi[i]);
}
Console.ReadKey();

string yazi = "abc";
char[] ters = new char[yazi.Length];
for (int i = 0; i < yazi.Length;i++)
{
ters[i] = yazi[yazi.Length -i- 1];
}
Console.WriteLine(ters);