Does Task.WhenAll run Tasks in background thread parallel

Question

Are the following 2 code fragments do the same?

//--------------------------------------------------
1.
//--------------------------------------------------

var producer = Task.Run(async () =>
{
    await bar.ReadDataAsync();
});

var consumer = Task.Run(async () =>
{
    await bar.WriteDataAsync();
});


await Task.WhenAll(consumer, producer);

//--------------------------------------------------
2.
//--------------------------------------------------

await Task.WhenAll(bar.ReadDataAsync(), bar.WriteDataAsync());

Answer 1

Task.WhenAll does not run the tasks. No tasks are started by this method.

What Task.WhenAll does do is return a new Task that only completes when all the original tasks have completed.

From msdn

Task.WhenAll Method

.NET Framework 4.6 and 4.5

Creates a task that will complete when all of the supplied tasks have completed.

https://msdn.microsoft.com/en-us/library/system.threading.tasks.task.whenall%28v=vs.110%29.aspx

In your example, you have called Task.Run . This is a request that you would like to run a unit of work asynchronously. However, the thread affinity is not guaranteed. Both units of work may run synchronously - that is up to the default TaskScheduler to decide.

Answer 2

There's no magic in Task.WhenAll - it's a method like any other.

As such, first all its arguments are evaluated, and only after that the method itself is executed. So yeah, you get parallel execution.