How to get the ASCII value of a string

Given your comment, it sounds like all you need is:

char[] chars = str.ToCharArray();
Array.Sort(chars);

A char value in .NET is actually a UTF-16 code unit, but for all ASCII characters, the UTF-16 code unit value is the same as the ASCII value anyway.

You can create a new string from the array like this:

string sortedText = new string(chars);
Console.WriteLine(chars);

As it happens, "Hello" is already in ascending ASCII order...