If you've ever worked with XSLT, you'd know that it's pretty limited when it comes to its programming capabilities. Try writing a for loop in XSLT and you'd know what I mean. XSLT is not designed to be a programming language so you should never put too much programming logic in your XSLT. That code can be a pain to write and maintain and so it should be avoided at all costs. Keep your xslt simple and put any complex logic that your xslt transformation requires in a class. Here is how you can create a helper class and call that from your xslt. For example, this is my helper class:  public class XsltHelper     {         public string GetStringHash(string originalString)         {             return originalString.GetHashCode().ToString();         }     }   And this is my xslt file(notice the namespace declaration that references the helper class): <?xml version="1.0" encoding="UTF-8" ?> <xsl:stylesheet  xmlns:xsl="http://www.w3.org/1999/XSL/Transform" version="1.0" xmlns:ext="http://MyNamespace">     <xsl:output method="text" indent="yes" omit-xml-declaration="yes"/>     <xsl:template  match="/">The hash code of "<xsl:value-of select="stringList/string1" />" is "<xsl:value-of select="ext:GetStringHash(stringList/string1)" />".     </xsl:template> </xsl:stylesheet>   Here is how you can include the helper class as part of the transformation: string xml = "<stringList><string1>test</string1></stringList>";             XmlDocument xmlDocument = new XmlDocument();             xmlDocument.LoadXml(xml);               XslCompiledTransform xslCompiledTransform = new XslCompiledTransform();             xslCompiledTransform.Load("XSLTFile1.xslt");               XsltArgumentList xsltArgs = new XsltArgumentList();                        xsltArgs.AddExtensionObject("http://MyNamespace", Activator.CreateInstance(typeof(XsltHelper)));               using (FileStream fileStream = new FileStream("TransformResults.txt", FileMode.OpenOrCreate, FileAccess.ReadWrite, FileShare.ReadWrite))             {                 // transform the xml and output to the output file ...                 xslCompiledTransform.Transform(xmlDocument, xsltArgs, fileStream);                            }