Reverse a string in C#

using System;
using System.Data;
using System.Collections;
using System.Linq;
using System.Text;

public class checkString
{
public static void Main()
{
string strSource = "ABCacd12";

ReverseString(strSource);
}

public static void ReverseAString(string str)
{
int counter =0;
counter=str.Length;
StringBuilder strReversed = new StringBuilder();
while(counter!=0)
{
counter--;
strReversed.Append(str.ElementAt(counter));
}
Console.WriteLine(strReversed.ToString());
}
}