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This topic applies to Windows Workflow Foundation 4 (WF4).
Workflow services may depend on messages being sent in a specific order. A workflow service contains one or more Receive activities and each Receive activity is expecting a specific message. Without particular transport delivery guarantees, messages sent by clients may be delayed and therefore delivered in an order the workflow service may not expect. Implementing a workflow service that does not require messages be sent in a specific order is normally done using a Parallel activity. For a more complicated application protocol, the workflow would become very complex very quickly. The out-of-order message processing feature in Windows Communication Foundation (WCF) allows you to create such a workflow without all of the complexity of nested Parallel activities. Out-of-order message processing is only supported on channels that support ReceiveContext such as the WCF MSMQ bindings.
Enabling Out-Of-Order Message Processing
Out-of-order message processing is enabled by setting the AllowBufferedReceive property to true on the WorkflowService. The following example shows how to set the AllowBufferedReceive property in code.
// Code: Opt-in to Buffered Receive processing...
WorkflowService service = new WorkflowService
{
Name="MyService",
Body = workflow,
AllowBufferedReceive = true
};
You can also apply the AllowBufferedReceive attribute to a workflow service in XAML as shown in the following example.
// Xaml: Opt-in to Buffered Receive processing...
<WorkflowService AllowBufferedReceive="True">
<!—the actual children activities -->
</Sequence>