How to check for valid xml in string input before calling .LoadXml()

Just catch the exception. The small overhead from catching an exception drowns compared to parsing the XML.

If you want the function (for stylistic reasons, not for performance), implement it yourself:

public class MyXmlDocument: XmlDocument
{
  bool TryParseXml(string xml){
    try{
      ParseXml(xml);
      return true;
    }catch(XmlException e){
      return false;
    }
 }

Using a XmlValidatingReader will prevent the exceptions, if you provide your own ValidationEventHandler.

I was unable to get XmlValidatingReader & ValidationEventHandler to work. The XmlException is still thrown for incorrectly formed xml. I verified this by viewing the methods with reflector.

I indeed need to validate 100s of short XHTML fragments per second.

public static bool IsValidXhtml(this string text)
{
   bool errored = false;
   var reader = new XmlValidatingReader(text, XmlNodeType.Element, new XmlParserContext(null, new XmlNamespaceManager(new NameTable()), null, XmlSpace.None));
   reader.ValidationEventHandler += ((sender, e) => { errored = e.Severity == System.Xml.Schema.XmlSeverityType.Error; });

   while (reader.Read()) { ; }
   reader.Close();
   return !errored;
}

XmlParserContext did not work either.

Anyone succeed with a regex?

If catching is too much for you, then you might want to validate the XML beforehand, using an XML Schema, to make sure that the XML is ok, But that will probably be worse than catching.

AS already been said, I'd rather catch the exception, but using XmlParserContext, you could try to parse "manually" and intercept any anomaly; however, unless you're parsing 100 xml fragments per second, why not catching the exception?