How to supress WPF tool tips?

Is there a way to supress WPF tool tips, other than by setting their content to null?

I am extending the Calendar control to provide date highlighting, using Charles Petzold's MSDN article as a guide. As per the article, I use a value converter to return a string if a date on in the Calendar was found in a date list. The string (number of appointments for the day, for example) is displayed as a tool tip when the mouse is over the date. If the date wasn't found in the list, the开发者_StackOverflow社区 value converter returns null.

Overall, it's pretty cool, but I'd like to provide an option to turn the tool tips off and just show highlighting. I am already using a null value when a date is not found, so that a tooltip is not displayed for that date. The null value also supresses the highlighting for that date. So, I can't simply return a blanket null if tool tips are turned off, because it would turn off the highlighting, as well.

I tried creating the tool tip in a separate tag and giving it a name, so that I could use a data trigger to turn it on and off. No go-- a tool tip is considered a property, and while it will compile as a separate object, it throws a runtime exception.

I also tried returning a coded value, 'NOSHOW' if the date was not found in the date list, so that I could use null to supress tool tips. Unfortunately, that gives every unhighlighted date a tool tip that says NOSHOW.

So, at this point I am scratching my head. Any suggestions as to how I can get this done? Thanks for your help!

ToolTipService.IsEnabled=False

<TextBox ToolTipService.IsEnabled="False" />