How can we notify clients for any changes in WCF service? or in other terms, if we change anything in WCF service how should it reflect to clients?

by implementing the IServiceCallback contract in the client application so that the Callback from the WCF service can be received in the client.

the client application behaves as service and service behaves as a client.

 

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You can use "INotifyPropertyChanged" interface is used to notify clients, typically binding clients, that a property value has changed.
Consider a Person object with a property called FirstName.
To provide generic property-change notification, the Person type implements the INotifyPropertyChanged interface and raises a PropertyChanged event when FirstName is changed.