callback function cannot accept object as a parameter

I found if I defined callback method like

void  call(object o)

I will received the following error

Unhandled Exception: System.ServiceModel.CommunicationObjectFaultedException: The communication object, System.Se开发者_StackOverflow社区rviceModel.Channels.ServiceChannel, cannot be used for communication because it is in the Faulted state.

if using

void call(string s)

then it is good. Anyone knows the reason.

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Yes you can't use object type. Operations (including callbacks) always have to know exact type of parameter otherwise they are not able to perform serialization and deserialization.