The communication object, System.ServiceModel.Channels.ServiceChannel, cannot be used for communication

Question

The communication object, System.ServiceModel.Channels.ServiceChannel, cannot be used for communication because it is in the Faulted state.

Answer 1

To prevent the Server from fall in Fault state, you have to ensure that no unhandled exception is raised. If WCF sees an unexpected Exception, no more calls are accepted - safety first.
Two possibilties to avoid this behaviour:

1. Use a FaultException (this one is not unexpected for WCF, so the WCF knows that the server has still a valid state)
   instead of

   throw new Exception("Error xy in my function")  
   

   use always

   throw new FaultException("Error xy in my function")  
   

   perhaps you can try..catch the whole block and throw a FaultException in all cases of an Exception

   try   
   {  
       ... some code here   
   }
   catch (Exception ex)
   {  
       throw new FaultException(ex.Message)   
   }
   

2. Tell WCF to handle all Exceptions using an Errorhandler. This can be done in several ways, I chose a simple one using an Attribute:
   All we have to do more, is to use the attribute [SvcErrorHandlerBehaviour] on the wanted Service Implementation

   using System;
   using System.Collections.ObjectModel;
   using System.ServiceModel;
   using System.ServiceModel.Channels;
   using System.ServiceModel.Description;
   using System.ServiceModel.Dispatcher;
   
   namespace MainService.Services
   {
       /// 
   
       /// Provides FaultExceptions for all Methods Calls of a Service that fails with an Exception
       /// 
       public class SvcErrorHandlerBehaviourAttribute : Attribute, IServiceBehavior
       {
           public void Validate(ServiceDescription serviceDescription, ServiceHostBase serviceHostBase)
           { } //implementation not needed
   
           public void AddBindingParameters(ServiceDescription serviceDescription, ServiceHostBase serviceHostBase, Collection endpoints,
                                            BindingParameterCollection bindingParameters)
           { } //implementation not needed
   
           public void ApplyDispatchBehavior(ServiceDescription serviceDescription, ServiceHostBase serviceHostBase)
           {
               foreach (ChannelDispatcherBase chanDispBase in serviceHostBase.ChannelDispatchers)
               {
                   ChannelDispatcher channelDispatcher = chanDispBase as ChannelDispatcher;
                   if (channelDispatcher == null)
                       continue;
                   channelDispatcher.ErrorHandlers.Add(new SvcErrorHandler());
               }
           }
       }
   
       public class SvcErrorHandler: IErrorHandler
       {
           public bool HandleError(Exception error)
           {
               //You can log th message if you want.
               return true;
           }
   
           public void ProvideFault(Exception error, MessageVersion version, ref Message msg)
           {
               if (error is FaultException)
                   return;
   
               FaultException faultException = new FaultException(error.Message);
               MessageFault messageFault = faultException.CreateMessageFault();
               msg = Message.CreateMessage(version, messageFault, faultException.Action);
           }
       }
   }
   

This is an easy example, you can dive deeper into IErrorhandler by not using the naked FaultException, but a FaultException<> with a type that provides additional info see IErrorHandler for an detailed example.