Is Task.WhenAll required in the sample code?

问题

In the following code task1 and task2 are independent of each other and can run in parallel. What is the difference between following two implementations?

var task1 = GetList1Async();
var task2 = GetList2Async();

await Task.WhenAll(task1, task2);

var result1 = await task1; 
var result2 = await task2; 

and

var task1 = GetList1Async();
var task2 = GetList2Async();

var result1 = await task1; 
var result2 = await task2; 

Why should I choose one over the other?

Edit: I would like to add that return type of GetList1Async() and GetList2Async() methods are different.

回答1:

Your first example will wait for both tasks to complete and then retrieve the results of both.

Your second example will wait for the tasks to complete one at a time.

You should use whichever one is clearer for your code. If both tasks have the same result type, you can retrieve the results from WhenAll as such:

var results = await Task.WhenAll(task1, task2);

回答2:

The first construct is more readable. You clearly state that you intend to wait for all tasks to complete, before obtaining the results. I find it reasonable enough to use that instead the second.

Also less writing, if you add a 3rd or 4th task... I.e. :

await Task.WhenAll(task1, task2, task3, task4);

compared to:

var result1 = await task1; 
var result2 = await task2; 
var result3 = await task3; 
var result4 = await task4; 

来源：https://stackoverflow.com/questions/18747213/is-task-whenall-required-in-the-sample-code