Expression must evaluate to a node-set. April 27, 2016
Sitecore
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This isn't good.

This isn’t good.

There I was, minding my own business when I was greeted by this lovely YSOD.  I was creating a new site within my custom SiteProvider for a client and when I loaded up Sitecore, everything was down.  From the stack trace, it was clear that it was cache related.  Some query to the cache configuration was breaking everything.  To test this, I created a XML-based site configuration:

<site name="12-monkeys"  virtualFolder="/12-monkeys" physicalFolder="/12-monkeys" rootPath="/sitecore/content" startItem="/home2" database="web" htmlCacheSize="50MB"  registryCacheSize="0" viewStateCacheSize="0" xslCacheSize="25MB"  filteredItemsCacheSize="10MB" />

Everything was fine.  A big more digging had me remove the cache sizes in the site definition.

<site name="12-monkeys"  virtualFolder="/12-monkeys" physicalFolder="/12-monkeys" rootPath="/sitecore/content" startItem="/home2" database="web" />

Boom. The error is back.  It appears that when it tries to default a cache size, it explodes.  I renamed “12-monkeys” to “twelve-monkeys” and the error went away.  Interesting.

Time to open up dotPeek.  Down inside Sitecore.Web.SiteInfo, there’s a method to create all the caches which is called as part of the constructor.  When this is called, there’s a method invoked that will get the cache size from the site.

private long GetCacheSize(string attribute, Sitecore.Collections.StringDictionary properties, long defaultSize)
{
  Assert.ArgumentNotNull((object) attribute, "attribute");
  Assert.ArgumentNotNull((object) properties, "properties");
  string str1 = StringUtil.GetString(new string[1]
  {
    properties[attribute]
  }).ToUpperInvariant();
  if (str1.Length > 0)
    return StringUtil.ParseSizeString(str1);
  string str2 = Factory.CreateObject("cacheSizes/sites/" + this.name + "/" + attribute.Substring(0, attribute.Length - "CacheSize".Length), false) as string;
  if (!string.IsNullOrEmpty(str2))
    return StringUtil.ParseSizeString(str2);
  return defaultSize;
}

Noting line 11, you can see it tries to build an XPath to the cacheSizes node:

<cacheSizes>
  <sites>
    <website>
      <html>50MB</html>
      <registry>0</registry>
      <viewState>0</viewState>
      <xsl>25MB</xsl>
    </website>
  </sites>
</cacheSizes>

Creating a website in here with a name that starts with a number is against the W3 standards for XML and would break the whole entire world.

I’ll be submitting this as a bug to the Sitecore Support Portal, but in the interim, I suggest you always specific your cache sizes if you’re going to have the possibility of a site name that starts with a number.

Update 5.16:  Sitecore’s official response was to always include the cache settings on the site definition node.  That’s about right, I guess.