Programming WCF Services

Callbacks

Callback contracts, just like service contracts, can propagate the service transaction to the callback client. You apply the TransactionFlow attribute, as with a service contract, for example:

interface IMyContractCallback
{
   [OperationContract][TransactionFlow(TransactionFlowOption.Allowed)]
   void OnCallback( );
}
[ServiceContract(CallbackContract = typeof(IMyContractCallback))]
interface IMyContract
{...}

The callback method implementation can use the OperationBehavior attribute just like a service operation and specify requiring a transaction scope and auto-completion:

class MyClient : IMyContractCallback
{
   [OperationBehavior(TransactionScopeRequired = true)]
   public void OnCallback( )
   {
      Transaction transaction = Transaction.Current;
      Debug.Assert(transaction!= null);
   }
}

Callback Transaction Modes

The callback client can have four modes of configuration: Service, Service/Callback, Callback, and None, analogous to the service transaction modes, except the service now plays the client role and the callback plays the service role in the previous service-side modes. For example, to configure the callback for Service transaction mode (that is, always using the service transaction), follow these steps:

1. Use a transaction-aware duplex binding with transaction flow enabled.

2. Set transaction flow to mandatory on the callback operation.

3. Configure the callback operation to require a transaction scope.

Example 7-25 shows a callback client configured for Service transaction.

Example 7-25. Configuring ...