AutoCompleteExtender - authentication failure (forms authentication)

I'm using the AutoCompleteExtender from the AJAX control toolkit on my aspx page - I have it wired up to a WCF service that is returning a string array and everything works happily.

If I change my service definition to include a demand for the caller to be authenticated, like so:

<OperationContract(), PrincipalPermission(SecurityAction.Demand, Authenticated:=True)> _
Public Function GetLookupValues(ByVal prefixText As String, ByVal count As Integer, ByVal contextKey As String) As String()

Then the autocomplete extender stops working, and I get an authentication error in the service. The service is set up to use ASPNetCompatibility mode, and I was hoping that the extender w开发者_Go百科ould pass the authentication credentials for my logged in user - does anyone know how to make this work?

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The sample over here mentions a ContextKey property on the AutoCompleteExtender. Does the behaviour change if you set this property on the extender?

Also this StackOverflow post might have some answers.

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Just in case anybody finds this and is curious, I wound up switching back to an 'old' style (asmx) web service and that worked just fine.