arcgis 10.1 - Performing Tool Validation in Python Toolbox?

I'm trying to make sure there is no '-' in the output filename, and if there is, to set an error (circle with the red 'X') so that the tool can't be run, and have a tooltip that explains the issue. I've tried a bunch of things, but from what I've read, it seems this should work:

if parameters[7].value:
            if parameters[7].value.value.count("-") > 0:
                parameters[7].setErrorMessage("Output cannot have a '-'." + \
                    "Please change the dash and rename the output after the tool has run")
            else:
                parameters[7].clearMessage()

        return

I'm not super clear on what is returned from the list of parameters. The paramater type is "DEShapefile". There is no error message being thrown when I use "--" as the filename.

Can someone see why it's not working for me?

Answer

I spoofed up a Python script of 1 parameter and wired it into toolbox and this is how I got it to check a FeatureClass input:

import arcpy

class ToolValidator(object):
  """Class for validating a tool's parameter values and controlling

  the behavior of the tool's dialog."""

  def __init__(self):
    """Setup arcpy and the list of tool parameters."""
    self.params = arcpy.GetParameterInfo()

  def initializeParameters(self):
    """Refine the properties of a tool's parameters.  This method is
    called when the tool is opened."""
    return


  def updateParameters(self):
    """Modify the values and properties of parameters before internal
    validation is performed.  This method is called whenever a parameter
    has been changed."""

    return

  def updateMessages(self):
    """Modify the messages created by internal validation for each tool

    parameter.  This method is called after internal validation."""
    self.params[0].clearMessage()
    if self.params[0].value is None:
        self.params[0].clearMessage()
    else:
        fc = str(self.params[0].value.value)
        if fc.count("-") > 0:
            self.params[0].setErrorMessage("Output cannot have a dash")
        else:
            self.params[0].clearMessage()

    return