What do you mean by "the current task ID of the process"? Do you want the ID of a specific task that the User service created, or the ID of the process?
The id of the specific task created by the User Service. I want to use that task id to complete the task outside of workspace environment through .Net code.
If you are not using any parallel tasks, execute this script against the LC database. This should give you the current task id
Select max (id) from tb_task where process_instance_id = <id>
Generally, the TaskManager service is used to query tasks. I've not used the wsdl for that service before so I can't help you much there.
In the process, you can get the taskID if you use a custom render service. To externalize the taskID you'd have to do something like have the render service save the ID to a file, or call a web service and pass it to the service, or use JMS service, or email service.....