'System.Xml.XmlDocument' does not contain a definition for 'XXXX' in BizTalk 2006

I was recently involved in a project where the filename was named depending on one of the distinguished fields in the XML. The code I  used was similar to:

OutboundMessage(FILE.ReceivedFileName) = InboundDespatch.InnerSchema1.Property1.ToString() + ".txt";

This caused a compilation error stating:

'System.Xml.XmlDocument' does not contain a definition for 'Property1'

It turns out that the generated code does not like the .ToString() method. To get around this, you can use the convert methods to convert it:

OutboundMessage(FILE.ReceivedFileName) = System.Convert.ToString(InboundDespatch.InnerSchema1.Property1) + ".txt"

And the problem vanishes!