PRB: Cannot unload assemblies that you create and load by using script in XSLT (316775)

CAUSE

The classes of the System.Xml namespace support the use of embedded scripting by using the script element in XSL Transformation (XSLT) applications. Therefore, the <msxsl:script> element allows you to choose the programming language (such as Microsoft Visual C# .NET or Microsoft Visual Basic .NET) to perform certain tasks.

Declared functions are contained within script blocks. When you use embedded script with an XSL file, an assembly that contains Microsoft Intermediate Language (MSIL) is created and loaded into memory. Because of a design limitation in this version of the Microsoft .NET Framework, you cannot unload that assembly from memory. This may lead to a memory leak if assemblies are created and loaded repeatedly or in a loop.

RESOLUTION

To resolve this problem, do not repeatedly load the XSLT with the script. Develop your application in such a way that you load the XSLT once and reuse it as many times as needed. This practice also improves performance.

For example, the following code leaks memory:

For(int i=0;i<1000;i++)
{
      xslt.Load(stylesheet);
      //Do other stuff
      xslt.Transform(doc, null, writer);
}

Change the code as follows to load XSLT only once and reuse it in a loop:

xslt.Load(stylesheet);
For(int i=0;i<1000;i++)
{

      //Do other stuff
      xslt.Transform(doc, null, writer);
}

This code only loads the assembly once and does not leak memory.

MORE INFORMATION

Steps to reproduce the behavior

Create a Visual C# .NET Console Application project.

Create an XSLT application, and then add the following code:

using System;
using System.IO;
using System.Xml;
using System.Xml.XPath;
using System.Xml.Xsl;

public class Sample
{
	private const String filename = "..\\..\\XmlFile1.xml";
	private const String stylesheet = "..\\..\\XSLTFile1.xslt";

	public static void Main() 
	{

                for(int i=0;i<1000;i++)
                {

		XslTransform xslt = new XslTransform();
                xslt.Load(stylesheet);

		//Load the XML data file.
		XPathDocument doc = new XPathDocument(filename);

		//Create an XmlTextWriter to write to the console.         
		XmlTextWriter writer = new XmlTextWriter(Console.Out);
		writer.Formatting = Formatting.Indented;

		//Transform the file.
		xslt.Transform(doc, null, writer);
		writer.Close();
                }
		System.Console.Read();
        
	} 
}

Create an XSLT file that is named XSLTFile1.xslt, and then add the following code:

 <xsl:stylesheet version="1.0" xmlns:xsl="http://www.w3.org/1999/XSL/Transform"
    xmlns:msxsl="urn:schemas-microsoft-com:xslt"
    xmlns:user="urn:my-scripts">

  <msxsl:script language="C#" implements-prefix="user">

<![CDATA[

     public double circumference(double radius){
       double pi = 3.14;
       double circ = pi*radius*2;
       return circ;
     }      
            ]] >     
<!-- Remove the space between ]] and > in the preceding line. -->
<!-- The space is inserted because of a publishing constraint. -->

   </msxsl:script>

  <xsl:template match="data">  
  <circles>

  <xsl:for-each select="circle">
    <circle>
    <xsl:copy-of select="node()"/>
       <circumference>
          <xsl:value-of select="user:circumference(radius)"/>        
       </circumference>
    </circle>
  </xsl:for-each>
  </circles>
  </xsl:template>
</xsl:stylesheet>

Create an XML file named XMLFile1.xml, and then add the following code:

<?xml version='1.0'?>
<data>
  <circle>
    <radius>12</radius>
  </circle>
  <circle>
    <radius>37.5</radius>
  </circle>
</data>

Save the XML and XSL files in the application folder.
Step through the code. Notice that the assemblies are created and loaded in the Output window of Visual Studio .NET.
Open Performance Monitor. Notice that the private bytes display the increase in memory.