Getting a series of unique random numbers

If you've tried looping through subnodes to display, for example, random images - you will have noticed the propensity of most solutions to display some identical nodes.This is because most "random" solutions use a time-based seed for their random-number generation. This becomes a problem when you're requesting multiple random numbers within a very short time frame (milliseconds). The seed will be the same, and therefore the random numbers don't differ.

Umbraco's XSLT extension library has a way around this, but you need a bit of extra inline code in your XSLT to be able to take advantage of it.

The function in question is umbraco.library:GetRandom(). This isn't really useful by itself, as it only returns an object, not a number.

To take advantage of the GetRandom() method in the library, and to return a random number within a certain range, you can insert the following inline function into your XSLT file:

  <msxsl:script language="c#" implements-prefix="randomTools">
    <msxsl:assembly href="../bin/umbraco.dll"/>
    <![CDATA[
        /// <summary>
        /// Gets a random integer that falls between the specified limits
        /// </summary>
        /// <param name="lowerLimit">An integer that defines the lower-boundary of the range</param>
        /// <param name="upperLimit">An integer that defines the upper-boundary of the range</param>
        /// <returns>A random integer within the specified range</returns>
        public static int GetRandom(int lowerLimit,int upperLimit) {
            Random r = umbraco.library.GetRandom();
            int returnedNumber = 0;
            lock (r)
            {
                returnedNumber = r.Next(lowerLimit, upperLimit);
            }
            return returnedNumber;
        }
    ]]>
  </msxsl:script>

Don't forget to add the prefix and namespace information to the <xsl:stylesheet> element. Shown below are the parts that you will need to add to your existing <xsl:stylesheet> declaration.

<xsl:stylesheet
    xmlns:randomTools="http://www.umbraco.org/randomTools"
    exclude-result-prefixes="randomTools">

You can then generate a random number by calling the inline function. The following example will return a random number between 1 and 100:

<xsl:value-of select="randomTools:GetRandom(1,100)"/>

The following example shows how to list 8 randomly-selected nodes that fall below the "current page":

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE xsl:stylesheet [
  <!ENTITY nbsp "&#x00A0;">
]>
<xsl:stylesheet
    version="1.0"
    xmlns:xsl="http://www.w3.org/1999/XSL/Transform"
    xmlns:msxml="urn:schemas-microsoft-com:xslt"
    xmlns:msxsl="urn:schemas-microsoft-com:xslt"
    xmlns:umbraco.library="urn:umbraco.library"
    xmlns:randomTools="http://www.umbraco.org/randomTools"
    exclude-result-prefixes="msxml umbraco.library msxsl randomTools">

  <xsl:output method="xml" omit-xml-declaration="yes" />

  <xsl:param name="currentPage"/>

  <xsl:variable name="maxItems" select="number(8)" />

  <xsl:template match="/">
      <xsl:for-each select="$currentPage//node">
        <xsl:sort select="randomTools:GetRandom(0,count($currentPage//node))" order="ascending" />
        <xsl:if test="position() &lt;= $maxItems">
          <li>
            <a>
              <xsl:attribute name="href">
                <xsl:value-of select="umbraco.library:NiceUrl(@id)"/>
              </xsl:attribute>
              <xsl:value-of select="@nodeName"/>
            </a>
          </li>
        </xsl:if>
      </xsl:for-each>
  </xsl:template>

  <msxsl:script language="c#" implements-prefix="randomTools">
    <msxsl:assembly href="../bin/umbraco.dll"/>
    <![CDATA[
        /// <summary>
        /// Gets a random integer that falls between the specified limits
        /// </summary>
        /// <param name="lowerLimit">An integer that defines the lower-boundary of the range</param>
        /// <param name="upperLimit">An integer that defines the upper-boundary of the range</param>
        /// <returns>A random integer within the specified range</returns>
        public static int GetRandom(int lowerLimit,int upperLimit) {
            Random r = umbraco.library.GetRandom();
            int returnedNumber = 0;
            lock (r)
            {
                returnedNumber = r.Next(lowerLimit, upperLimit);
            }
            return returnedNumber;
        }
    ]]>
  </msxsl:script>
</xsl:stylesheet>

The code on this page, and the GetRandom() method in umbraco.library, is based on this blog post by Eli Robillard.