Escape invalid XML characters in C

Question

I have a string that contains invalid XML characters  How can I escape  or remove  invalid XML characters before I parse the string

User · Answer

Replace invalid characters with empty strings     Regex Replace inputString       w   -            The regular expression pattern    w  -  matches any character that is not a word character  a period  an   symbol  or a hyphen  A word character is any letter  decimal digit  or punctuation connector such as an underscore  Any character that matches this pattern is replaced by String Empty  which is the string defined by the replacement pattern  To allow additional characters in user input  add those characters to the character class in the regular expression pattern  For example  the regular expression pattern    w  -    also allows a percentage symbol and a backslash in an input string   Regex Replace inputString                      Refer this too    Removing Invalid Characters from XML Name Tag - RegEx C   Here is a function to remove the characters from a specified XML string   using System  using System IO  using System Text  using System Text RegularExpressions   namespace XMLUtils       class Standards                    lt summary gt              Strips non-printable ascii characters              Refer to http   www w3 org TR xml11  charsets for XML 1 1             Refer to http   www w3 org TR 2006 REC-xml-20060816  charsets for XML 1 0              lt  summary gt               lt param name  content  gt contents lt  param gt               lt param name  XMLVersion  gt XML Specification to use  Can be 1 0 or 1 1 lt  param gt          private void StripIllegalXMLChars string tmpContents  string XMLVersion                            string pattern   String Empty              switch  XMLVersion                                case  1 0                       pattern      x  10   2-F  FFF EF  FDD 0-9A-F  7F 8 0-46-9A-F 9 0-9A-F                         break                  case  1 1                       pattern      x  10   2-F  FFF EF  FDD 0-9A-F   19  0-9A-F  7F 8 0-46-9A-F  0  1-8BCEF                         break                  default                      throw new Exception  Error  Invalid XML Version                                Regex regex   new Regex pattern  RegexOptions IgnoreCase               if  regex IsMatch tmpContents                                 tmpContents   regex Replace tmpContents  String Empty                             tmpContents   string Empty

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If you are writing xml  just use the classes provided by the framework to create the xml  You won t have to bother with escaping or anything   Console Write new XElement  Data     lt   gt   amp        Will output   lt Data gt  amp lt   amp gt   amp amp  lt  Data gt    If you need to read an XML file that is malformed  do not use regular expression  Instead  use the Html Agility Pack

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As the way to remove invalid XML characters I suggest you to use XmlConvert IsXmlChar method  It was added since  NET Framework 4 and is presented in Silverlight too  Here is the small sample   void Main         string content     v f 0       Console WriteLine IsValidXmlString content       False      content   RemoveInvalidXmlChars content       Console WriteLine IsValidXmlString content       True    static string RemoveInvalidXmlChars string text        var validXmlChars   text Where ch   gt  XmlConvert IsXmlChar ch   ToArray        return new string validXmlChars      static bool IsValidXmlString string text        try           XmlConvert VerifyXmlChars text           return true        catch           return false            And as the way to escape invalid XML characters I suggest you to use XmlConvert EncodeName method   Here is the small sample   void Main         const string content     v f 0       Console WriteLine IsValidXmlString content       False      string encoded   XmlConvert EncodeName content       Console WriteLine IsValidXmlString encoded       True      string decoded   XmlConvert DecodeName encoded       Console WriteLine content    decoded      True    static bool IsValidXmlString string text        try           XmlConvert VerifyXmlChars text           return true        catch           return false            Update  It should be mentioned that the encoding operation produces a string with a length which is greater or equal than a length of a source string  It might be important when you store a encoded string in a database in a string column with length limitation and validate source string length in your app to fit data column limitation

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Use SecurityElement Escape  using System  using System Security   class Sample     static void Main         string text    Escape characters    lt   gt   amp              string xmlText   SecurityElement Escape text     output    Escape characters    amp lt   amp gt   amp amp   amp quot   amp apos      Console WriteLine xmlText

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Here is an optimized version of the above method RemoveInvalidXmlChars which doesn t create a new array on every call  thus stressing the GC unnecessarily   public static string RemoveInvalidXmlChars string text        if  text    null          return text      if  text Length    0          return text          a bit complicated  but avoids memory usage if not necessary     StringBuilder result   null      for  int i   0  i  lt  text Length  i                  var ch   text i           if  XmlConvert IsXmlChar ch                         result  Append ch                     else if  result    null                        result   new StringBuilder                result Append text Substring 0  i                         if  result    null          return text     no invalid xml chars detected - return original text     else         return result ToString

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The RemoveInvalidXmlChars method provided by Irishman does not support surrogate characters  To test it  use the following example   static void Main         const string content     v U00010330        string newContent   RemoveInvalidXmlChars content        Console WriteLine newContent       This returns an empty string but it shouldn t  It should return   U00010330  because the character U 10330 is a valid XML character   To support surrogate characters  I suggest using the following method   public static string RemoveInvalidXmlChars string text        if  string IsNullOrEmpty text           return text       int length   text Length      StringBuilder stringBuilder   new StringBuilder length        for  int i   0  i  lt  length    i                if  XmlConvert IsXmlChar text i                          stringBuilder Append text i                      else if  i   1  lt  length  amp  amp  XmlConvert IsXmlSurrogatePair text i   1   text i                          stringBuilder Append text i                stringBuilder Append text i   1                  i                       return stringBuilder ToString

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string XMLWriteStringWithoutIllegalCharacters string UnfilteredString        if  UnfilteredString    null          return string Empty       return XmlConvert EncodeName UnfilteredString      string XMLReadStringWithoutIllegalCharacters string FilteredString        if  UnfilteredString    null          return string Empty       return XmlConvert DecodeName UnfilteredString       This simple method replace the invalid characters with the same value but accepted in the XML context     To write string use XMLWriteStringWithoutIllegalCharacters string UnfilteredString   To read string use XMLReadStringWithoutIllegalCharacters string FilteredString

[c#] Escape invalid XML characters in C#

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