I had a problem clearing the SearchText in an MVVM Silverlight application. I could clear clear the SelectedItem and Text but the SearchText was left behind. It is read only and cannot be changed by binding.
Example: AutoCompleteBox with a list of Countries. When the user wants to enter Australia they enter 'au' at this point the list appers with Austria and Australia. The user can then select Australia and move on. At the end of editing they click on a 'Save' button. At this point it is likely that you would want to clear the data forn for entering new data.
Even if you have bindings to the SelectedItem and the Text properties and you set them to 'null' and string.Empty respectively the SearchText property remains and the AutoCompleteBox will not clear but will contain 'au'.
I posted about this all over the internet but could get no answer on the control itself and so I came at it from a different angle which may help someone who ends up frustrated like me.
I am using a Silverlight Navigation template application which uses a NavigationFrame in which to load Silverlight pages. I noticed that if I navigated to another page and returned to my data form the SearchText was cleared. Any values that were bound to properties remained valid, just the SearchText had cleared on all AutoCompleteBoxes. I therefore used the PageConductor method of injecting the NavigationFrame into the ViewModel where I could call the refresh method. I got this method from John Papa's example from the Silverlight Firestarter event , I simply added a Refresh method to the IPageConductor interface so I am now able to call 'PageConductor.Refresh()' which is like reloading the page. I hope this helps someone out there.