C# replace characters in string by another string

Split the arguments into a list or table and then apply them one at a time. You will also need to take care that a replace does not corrupt one that has already been applied. For example if you have "H=D/A=Q/D=Z", then it would convert all the H characters to Z.

The solution by TheRealSteveJudge is correct in principle, but it doesn't really tell you how to take your args string to build the replacementDictionary and worse it suffers from the iterative processing of the replacementDictionary values giving a substitution cascade which Richard MacCutchan pointed out.

If your replacements are all single character to single character then the replacementDictionary should be:

Dictionary<char, char> replacementDictionary = new Dictionary<char, char>();

and then you can use LINQ to map through the text to generate the output:

string output = string.Join(string.Empty, text.Select(c => {
    char rep;
    return replacementDictionary.TryGetValue(c, out rep) ? rep : c;
  }));

Now to populate the replacementDictionary from the args:
first you need to split args on the '/'
then for each of those strings, split again on the '=' into a "key" and "value"
set the key/value pairs into the replacementDictionary.
If this is upper/lowercase independent, then first insert the key/value pair with both values as uppercase then repeat with both of them as lowercase. (Use the char.ToUpper(c) or char.ToUpperInvariant(c) as apropriate, and likewise, char.ToLower(c)/char.ToLowerInvariant(c))