WorkflowServiceHost Class

protected override Guid OnGetInstanceId(object[] inputs, OperationContext operationContext)
{
    //Create was called by client
    if (operationContext.IncomingMessageHeaders.Action.EndsWith("Create"))
    {
        return Guid.Empty;
    }
    //CreateWithInstanceId was called by client
    else if (operationContext.IncomingMessageHeaders.Action.EndsWith("CreateWithInstanceId"))
    {
        return (Guid)inputs[1];
    }
    else
    {
        throw new InvalidOperationException("Invalid Action: " + operationContext.IncomingMessageHeaders.Action);
    }
}