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4GuysFromRolla.com : ASP FAQS : XML


In ASP.NET, how can I create an XmlDocument instance from an XML string? Similarly, given an XML string, how can I display the data in a data Web control?

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Answer: Loading XML Strings Into XML Objects
The XmlDocument class - and related classes, such as XmlTextReader, XPathDocument, and so on - have a variety of constructor overloads or provide means for XML data to be loaded in a number of ways. These ways, while not common to all XML-related classes in the .NET Framework, but found in most, are:

* Specifying a string that is a URL or file path of an XML file
* Passing in a stream instance that contains the XML data
* Passing in a TextReader instance that contains the XML data

What many developers might think is missing from the list is, being able to just pass in a string of XML. That is, one might want to create an XmlDocument instance like do:

XmlDocument xmlDoc = new XmlDocument();

This approach can be useful if you are needing to load an XmlDocument (or other XML-related object) based on the XML returned by a Web service or some component. However, none of the XML-related classes in the .NET Framework accept a raw XML string for initialization.

(NOTE: The XmlDocument class has the LoadXml() method that will load an XmlDocument from XML string data; however, if you want to do any validation, this method is not an option. Also, the other XML-related classes - XmlTextReader, XPathDocument, and so on - do not expose such a method.)

One approach would be to take the XML string, dump it to a temporary file, and then speicfy the file's path in the Load() method or the constructor. However, this approach is unnecessarily inefficient and leaves a mess of temporary files.

The ideal approach is to load the data into a TextReader. We can then pass the TextReader instance into the appropriate XML class instance. If you look at the TextReader class's technical documentation, you'll find that two classes are derived from the TextReader class: StreamReader and StringReader. Checking out the StringReader's technical documentation you'll see that a StringReader instance can be created by passing in the string into its constructor.

Since these XML-related classes will accept a TextReader, we can simply create a StringReader populated with the XML data in string form, and then pass in the StringReader. The following code shows how to load string XML data into an XmlDocument instance:

StringReader sr = new StringReader("<book><title>...");

XmlDocument xmlDoc = new XmlDocument();

Displaying an XML String in a Data Web Control
Displaying an XML string in a data Web control, such as the DataGrid, DataList, or Repeater, is easy to do once you have loaded a DataSet with the XML data. The DataSet class contains a ReadXml() method, which like XML-related classes we looked at earlier, accepts:

* Specifying a string that is a URL or file path of an XML file
* Passing in a stream instance that contains the XML data
* Passing in a TextReader instance that contains the XML data

Again, we are faced with the same problem as before if what we have is not a URL or file with the XML data, but a string of XML data. Again, the solution is to use a StringReader to load the XML string data, and then pass in this StringReader into the DataSet's ReadXml() method. Finally, the DataSet can then be bound to the data Web control.

The following source code demonstrates how this would be done:

StringReader sr = new StringReader("<book><title>...");
DataSet ds = new DataSet();


myDataGrid.DataSource = ds;

For a more thorough discussion on this, be sure to read Anthony Hart's DataGrids, DataSets, and XML Strings.

Happy Programming!

FAQ posted by Scott Mitchell at 10/16/2003 12:34:42 PM to the XML category. This FAQ has been viewed 73,586 times.

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