Task vs Thread differences

Question

I'm new to parallel programming. There are two classes available in .NET: Task and Thread.

So, my questions are:

• What is difference between those classes?
• When is it better to use Thread and when Task?

Answer 1

Thread is a lower-level concept: if you're directly starting a thread, you know it will be a separate thread, rather than executing on the thread pool etc.

Task is more than just an abstraction of "where to run some code" though - it's really just "the promise of a result in the future". So as some different examples:

• Task.Delay doesn't need any actual CPU time; it's just like setting a timer to go off in the future
• A task returned by WebClient.DownloadStringTaskAsync won't take much CPU time locally; it's representing a result which is likely to spend most of its time in network latency or remote work (at the web server)
• A task returned by Task.Run() really is saying "I want you to execute this code separately"; the exact thread on which that code executes depends on a number of factors.

Note that the Task<T> abstraction is pivotal to the async support in C# 5.

In general, I'd recommend that you use the higher level abstraction wherever you can: in modern C# code you should rarely need to explicitly start your own thread.