Quickest Method to Reverse in String in C#.net

I'm currently writing a quick solution for Euler Problem #4 where one must find the largest palindromic number from the product of two 3-digit numbers.

To identify if a number is palindromic, you would obviously compare a reverse of the number with the original.

Since C# doesn't have a built in String.Reverse() method, what is the quickest way to reverse a string?

I will be testing all the suggested solution in a loop with 100,000,000 iterations. The correct answer will be given to the person who submitted the fastest solution.

I will be testing the solution in a C#.Net 3.5 console application