I got some large XML files that had invalid characters in them and would not process. I ran a script on them to replace non-ASCII characters with empty strings. Here is the snippet.
string source = File.ReadAllText(@"C:\SourceFile.xml");
string dest = Regex.Replace(source, @"s/[^\x20-\x7E|\n|\ |\t]//g", string.Empty);
For the record, I got the RegEx here…