C# – Parse multiple XML files with ASP.NET (C#) and return those with particular element

asp.netc++xml

Greetings.

I'm looking for a way to parse a number of XML files in a particular directory with ASP.NET (C#). I'd like to be able to return content from particular elements, but before that, need to find those that have a certain value between an element.

Example XML file 1:

<file>
    <title>Title 1</title>
    <someContent>Content</someContent>
    <filter>filter</filter>
</file>

Example XML file 2:

<file>
    <title>Title 2</title>
    <someContent>Content</someContent>
    <filter>filter, different filter</filter>
</file>

Example case 1:

Give me all XML that has a filter of 'filter'.

Example case 2:

Give me all XML that has a title of 'Title 1'.

Looking, it seems this should be possible with LINQ, but I've only seen examples on how to do this when there is one XML file, not when there are multiples, such as in this case.

I would prefer that this be done on the server-side, so that I can cache on that end.

Functionality from any version of the .NET Framework can be used.

Thanks!

~James

Best Solution

If you are using .Net 3.5, this is extremely easy with LINQ:

//get the files
XElement xe1 = XElement.Load(string_file_path_1);
XElement xe2 = XElement.Load(string_file_path_2);

//Give me all XML that has a filter of 'filter'.
var filter_elements1 = from p in xe1.Descendants("filter") select p;
var filter_elements2 = from p in xe2.Descendants("filter") select p;
var filter_elements = filter_elements1.Union(filter_elements2);

//Give me all XML that has a title of 'Title 1'.
var title1 = from p in xe1.Descendants("title") where p.Value.Equals("Title 1") select p;
var title2 = from p in xe2.Descendants("title") where p.Value.Equals("Title 1") select p;
var titles = title1.Union(title2);

This can all be written shorthand and get you your results in just 4 lines total:

XElement xe1 = XElement.Load(string_file_path_1);
XElement xe2 = XElement.Load(string_file_path_2);
var _filter_elements = (from p1 in xe1.Descendants("filter") select p1).Union(from p2 in xe2.Descendants("filter") select p2);
var _titles = (from p1 in xe1.Descendants("title") where p1.Value.Equals("Title 1") select p1).Union(from p2 in xe2.Descendants("title") where p2.Value.Equals("Title 1") select p2);

These will all be IEnumerable lists, so they are super easy to work with:

foreach (var v in filter_elements)
    Response.Write("value of filter element" + v.Value + "<br />");

LINQ rules!

Use a TextWriter

Do not use Response.OutputStream

Do use Response.Output

Both are streams, but Output is a TextWriter. When an XmlDocument saves itself to a TextWriter, it will use the encoding specified by that TextWriter. The XmlDocument will automatically change the xml declaration node to match the encoding used by the TextWriter. e.g. in this case the XML declaration node:

<?xml version="1.0" encoding="ISO-8859-1"?>

would become

<?xml version="1.0" encoding="UTF-8"?>

This is because the TextWriter has been set to UTF-8. (More on this in a moment). As the TextWriter is fed character data, it will encode it with the byte sequences appropriate for its set encoding.

Incorrect:

doc.Save(Response.OutputStream);

In this example the document is incorrectly saved to the OutputStream, which performs no encoding change, and may not match the response's content-encoding or the XML declaration node's specified encoding.

Correct

doc.Save(Response.Output);

The XML document is correctly saved to a TextWriter object, ensuring the encoding is properly handled.

Set Encoding

The encoding given to the client in the header:

Response.ContentEncoding = ...

must match the XML document's encoding:

<?xml version="1.0" encoding="..."?>

must match the actual encoding present in the byte sequences sent to the client. To make all three of these things agree, set the single line:

Response.ContentEncoding = System.Text.Encoding.UTF8;

When the encoding is set on the Response object, it sets the same encoding on the TextWriter. The encoding set of the TextWriter causes the XmlDocument to change the xml declaration:

<?xml version="1.0" encoding="UTF-8"?>

when the document is Saved:

doc.Save(someTextWriter);

Save to the response Output

You do not want to save the document to a binary stream, or write a string:

Incorrect:

doc.Save(Response.OutputStream);

Here the XML is incorrectly saved to a binary stream. The final byte encoding sequence won't match the XML declaration, or the web-server response's content-encoding.

Incorrect:

Response.Write(doc.ToString());
Response.Write(doc.InnerXml);

Here the XML is incorrectly converted to a string, which does not have an encoding. The XML declaration node is not updated to reflect the encoding of the response, and the response is not properly encoded to match the response's encoding. Also, storing the XML in an intermediate string wastes memory.

You don't want to save the XML to a string, or stuff the XML into a string and response.Write a string, because that:

- doesn't follow the encoding specified
- doesn't set the XML declaration node to match
- wastes memory

Do use doc.Save(Response.Output);

Do not use doc.Save(Response.OutputStream);

Do not use Response.Write(doc.ToString());

Do not use 'Response.Write(doc.InnerXml);`

Set the content-type

The Response's ContentType must be set to "text/xml". If not, the client will not know you are sending it XML.

Final Answer

Response.Clear(); //Optional: if we've sent anything before
Response.ContentType = "text/xml"; //Must be 'text/xml'
Response.ContentEncoding = System.Text.Encoding.UTF8; //We'd like UTF-8
doc.Save(Response.Output); //Save to the text-writer
    //using the encoding of the text-writer
    //(which comes from response.contentEncoding)
Response.End(); //Optional: will end processing

Complete Example

Rob Kennedy had the good point that I failed to include the start-to-finish example.

GetPatronInformation.ashx:

<%@ WebHandler Language="C#" Class="Handler" %>

using System;
using System.Web;
using System.Xml;
using System.IO;
using System.Data.Common;

//Why a "Handler" and not a full ASP.NET form?
//Because many people online critisized my original solution
//that involved the aspx (and cutting out all the HTML in the front file),
//noting the overhead of a full viewstate build-up/tear-down and processing,
//when it's not a web-form at all. (It's a pure processing.)

public class Handler : IHttpHandler
{
   public void ProcessRequest(HttpContext context)
   {
      //GetXmlToShow will look for parameters from the context
      XmlDocument doc = GetXmlToShow(context);

      //Don't forget to set a valid xml type.
      //If you leave the default "text/html", the browser will refuse to display it correctly
      context.Response.ContentType = "text/xml";

      //We'd like UTF-8.
      context.Response.ContentEncoding = System.Text.Encoding.UTF8;
      //context.Response.ContentEncoding = System.Text.Encoding.UnicodeEncoding; //But no reason you couldn't use UTF-16:
      //context.Response.ContentEncoding = System.Text.Encoding.UTF32; //Or UTF-32
      //context.Response.ContentEncoding = new System.Text.Encoding(500); //Or EBCDIC (500 is the code page for IBM EBCDIC International)
      //context.Response.ContentEncoding = System.Text.Encoding.ASCII; //Or ASCII
      //context.Response.ContentEncoding = new System.Text.Encoding(28591); //Or ISO8859-1
      //context.Response.ContentEncoding = new System.Text.Encoding(1252); //Or Windows-1252 (a version of ISO8859-1, but with 18 useful characters where they were empty spaces)

      //Tell the client don't cache it (it's too volatile)
      //Commenting out NoCache allows the browser to cache the results (so they can view the XML source)
      //But leaves the possiblity that the browser might not request a fresh copy
      //context.Response.Cache.SetCacheability(HttpCacheability.NoCache);

      //And now we tell the browser that it expires immediately, and the cached copy you have should be refreshed
      context.Response.Expires = -1;

      context.Response.Cache.SetAllowResponseInBrowserHistory(true); //"works around an Internet&nbsp;Explorer bug"

      doc.Save(context.Response.Output); //doc saves itself to the textwriter, using the encoding of the text-writer (which comes from response.contentEncoding)

      #region Notes
      /*
       * 1. Use Response.Output, and NOT Response.OutputStream.
       *  Both are streams, but Output is a TextWriter.
       *  When an XmlDocument saves itself to a TextWriter, it will use the encoding
       *  specified by the TextWriter. The XmlDocument will automatically change any
       *  XML declaration node, i.e.:
       *     <?xml version="1.0" encoding="ISO-8859-1"?>
       *  to match the encoding used by the Response.Output's encoding setting
       * 2. The Response.Output TextWriter's encoding settings comes from the
       *  Response.ContentEncoding value.
       * 3. Use doc.Save, not Response.Write(doc.ToString()) or Response.Write(doc.InnerXml)
       * 3. You DON'T want to save the XML to a string, or stuff the XML into a string
       *  and response.Write that, because that
       *   - doesn't follow the encoding specified
       *   - wastes memory
       *
       * To sum up: by Saving to a TextWriter: the XML Declaration node, the XML contents,
       * and the HTML Response content-encoding will all match.
       */
      #endregion Notes
   }

   private XmlDocument GetXmlToShow(HttpContext context)
   {
      //Use context.Request to get the account number they want to return
      //GET /GetPatronInformation.ashx?accountNumber=619

      //Or since this is sample code, pull XML out of your rear:
      XmlDocument doc = new XmlDocument();
      doc.LoadXml("<Patron><Name>Rob Kennedy</Name></Patron>");

      return doc;
   }

   public bool IsReusable { get { return false; } }
}

C# – How to read and parse an XML file in C#

XmlDocument to read an XML from string or from file.

XmlDocument doc = new XmlDocument();
doc.Load("c:\\temp.xml");

doc.LoadXml("<xml>something</xml>");

then find a node below it ie like this

XmlNode node = doc.DocumentElement.SelectSingleNode("/book/title");

foreach(XmlNode node in doc.DocumentElement.ChildNodes){
   string text = node.InnerText; //or loop through its children as well
}

then read the text inside that node like this

string text = node.InnerText;

or read an attribute

string attr = node.Attributes["theattributename"]?.InnerText

Always check for null on Attributes["something"] since it will be null if the attribute does not exist.