using System;

using System.Xml;

using System.Xml.Xsl;

using System.IO;

public class Program

{

    public static void Main()

    {

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

<xsl:stylesheet version=""1.0""

xmlns:xsl=""http://www.w3.org/1999/XSL/Transform"">

​

<xsl:template match=""/"">

  <root>

    <xsl:for-each select=""root/book"">

        <book>

        <author>

        <xsl:for-each select=""author"">

          <xsl:value-of select=""current()""/>

          <xsl:if test=""last() > position()"">

            <xsl:text>, </xsl:text>

          </xsl:if>

        </xsl:for-each>

        </author>

        <title>

          <xsl:for-each select=""title"">

          <xsl:value-of select=""current()""/>

          <xsl:if test=""last() > position()"">

            <xsl:text>, </xsl:text>

          </xsl:if>

        </xsl:for-each>

        </title>

      </book>

    </xsl:for-each>

  </root>

</xsl:template>

</xsl:stylesheet>";

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

        <root>

  <book>

    <title>book1Title</title>

    <author>book1Author1</author>

  </book>

  <book>

    <title>book2Title</title>

    <author>book2Author1</author>

    <author>book2Author2</author>

  </book>

  <book>

    <title>book9Title1</title>

    <title>book9Title2</title>

    <author>book9Author</author>

  </book>

</root>";

        string output = String.Empty;

        using (StringReader srt = new StringReader(xsl)) // xslInput is a string that contains xsl

        using (StringReader sri = new StringReader(xml)) // xmlInput is a string that contains xml

        {

            using (XmlReader xrt = XmlReader.Create(srt))

            using (XmlReader xri = XmlReader.Create(sri))

            {

                var xslt = new XslCompiledTransform();

                xslt.Load(xrt);

                using (StringWriter sw = new StringWriter())

                using (XmlWriter xwo = XmlWriter.Create(sw, xslt.OutputSettings)) // use OutputSettings of xsl, so it can be output as HTML

                {

                    xslt.Transform(xri, xwo);

                    output = sw.ToString();

                }

            }

        }

        Console.WriteLine(output);

    }

}