If you business rule is simply to reverse the order of every two chars of a string you could use this simple code:
string input = ""123456789;
string output = string.Empty;
for (int i = 0; i < input.Length - 1; i += 2)
output = string.Format("{0}{1}{2}", output, input[i + 1], input[i]);
If your string is really long I would use StringBuilder like this:
string input = "hdfshfjvsnghvdf4fh12gy5hh2g2h4rtf5htf5bh1xtfh45tfh5f5hf445th46tfh4646t486t3f1htyh84f";
StringBuilder output = new StringBuilder();
for (int i = 0; i < input.Length - 1; i += 2)
output.AppendFormat("{0}{1}", input[i + 1], input[i]);