CON, PRN, AUX, CLOCK$, NUL
COM0, COM1, COM2, COM3, COM4, COM5, COM6, COM7, COM8, COM9
LPT0, LPT1, LPT2, LPT3, LPT4, LPT5, LPT6, LPT7, LPT8, and LPT9.

 public static string MakeValidFileName(string text, string replacement = "_")
  {
   StringBuilder str=new StringBuilder();
   var invalidFileNameChars = System.IO.Path.GetInvalidFileNameChars();
   foreach (var c in text)
   {
    if (invalidFileNameChars.Contains(c))
    {
     str.Append(replacement??"");
    }
    else
    {
     str.Append(c);
    }
   }
   return str.ToString();
  }

public static string GetSafeFilename(string arbitraryString)
{
 var invalidChars = System.IO.Path.GetInvalidFileNameChars();
 var replaceIndex = arbitraryString.IndexOfAny(invalidChars, 0);
 if (replaceIndex == -1) return arbitraryString;
 var r = new StringBuilder();
 var i = 0;
 do
 {
  r.Append(arbitraryString, i, replaceIndex - i);
  switch (arbitraryString[replaceIndex])
  {
   case '"':
    r.Append("''");
    break;
   case '<':
    r.Append('\u02c2'); // '˂' (modifier letter left arrowhead)
    break;
   case '>':
    r.Append('\u02c3'); // '˃' (modifier letter right arrowhead)
    break;
   case '|':
    r.Append('\u2223'); // '∣' (divides)
    break;
   case ':':
    r.Append('-');
    break;
   case '*':
    r.Append('\u2217'); // '∗' (asterisk operator)
    break;
   case '\\':
   case '/':
    r.Append('\u2044'); // '⁄' (fraction slash)
    break;
   case '\0':
   case '\f':
   case '?':
    break;
   case '\t':
   case '\n':
   case '\r':
   case '\v':
    r.Append(' ');
    break;
   default:
    r.Append('_');
    break;
  }
  i = replaceIndex + 1;
  replaceIndex = arbitraryString.IndexOfAny(invalidChars, i);
 } while (replaceIndex != -1);
 r.Append(arbitraryString, i, arbitraryString.Length - i);
 return r.ToString();
}