How to check dead clients before callback in wshttpduplexbinding

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WCF

i have created a wcf service using wshttpduplex binding, i wanted to check (if the client is connected or not) before calling him back.

i checked timeout exception to detect a dead client but the exception is thrown after 2,3 successful tries when the client is down.

when i disconnect client, the service callback should throw exception on first try after the client is down.

does anyone know the reason?