ToCharArray vs. String[i]

If I want to loop through the characters in a string, would it matter at all (as far as efficiency is concerned) whether I access the characters with brackets or if I convert the whole thing to a character array first and then access the characters?
ie:

string myStr = "MrPolite rules the world";
for (int i=0; i<myStr.Length; i++)
Console.WriteLine (myStr[i]);

vs.

string myStr = "MrPolite rules the world";
char[] chrs = myStr.ToCharArray();
for (int i=0; i<chrs.Length; i++)
Console.WriteLine (chrs[i]);

:afrog:

I THINK , the first way is more efficient since string is actually char array .So , there's no need to convert it to char array . You just access it by index. At least , this would make sense in C/C++ .