Q: There is a Windows Workflow Foundation (WF) workflow service which contains only one pair of Receive and SendReply. Data will be sent through the Receive activity and how do I get the workflow service to return the GUID of the workflow instance in SendReply activity.

A: Write a custom activity to retrieve it (See below). Bind that to a variable and bind that variable to the SendReply.Content

    public sealed class GetWorkflowInstanceId : CodeActivity<Guid>
    {
        protected override Guid Execute(CodeActivityContext context)
        {
            return context.WorkflowInstanceId;
        }
    }