Adding dropdown-value to ContextKey of Autocomplete control

Hi,

I'm using the contextKey parameter of the Autocomplete control to pass additional parameters to the webservice.if The TargetControlID="txtA" and contextkey="dropdownB" then it can return the right value--"value-B".

But I found I change then contextkey="dropwondC" and the TargetControlID is the same="txtA" ,then return value is still "value-B",it can't return the right value--"value-C".

If I postback the webpage,it can return the right value--"value-C".How can I get the right return value for any contextkey parameter? to clear the Autocomplete control buffer?

I am so appreciated for any help.