Get Results from Task.WhenAll for multiple tasks

Question

Hi Folks,

I am trying to get the results back from running multiple tasks. Each task point to a function and returns either true or false.

Here is my code:

var t1 = new Task(() => { CheckNC(); }) ;

var t2 = new Task(() => { CheckNL(); });

var t3 = new Task(() => { CheckNP(); });

var t4 = new Task(() => { CheckNK(); });

t1.Start();

t2.Start();

t3.Start();

t4.Start();

await Task.WhenAll(t1, t2, t3, t4);

There is no t1.Result property to look up ??

Am I doing this correct or is there a better way?

Visual Studio 2019 on WinForms

Thanks,

J

Accepted Answer

Updated by Jiale:

This is the code with added Progress Controls. It works fine.

var t1 = Task.Run(() => CheckNC(progress, statusNC));

var t2 = Task.Run(() => CheckNL(progress, statusNL));

var t3 = Task.Run(() => CheckAD(progress, statusAD, token));

var t4 = Task.Run(() => CheckAC(progress, statusAC, token));

// Wait till all tasks are complete

await Task.WhenAll(t1, t2, t3, t4);

your Tasks all return void, so there is no result, just that they completed. only Task<> has a result.

Task<> example:

var t1 = new Task(() => 10);
var t2 = new Task(() => "hi");
t1.Start();
t2.Start();
await Task.WhenAll(t1, t2);
Console.WriteLine(t1.Result);
Console.WriteLine(t2.Result);
}

Task<> example with common result type:

var t1 = new Task(() => 10);
var t2 = new Task(() => 20);
t1.Start();
t2.Start();
var list = await Task.WhenAll(t1, t2);
foreach (var i in list)
	Console.WriteLine(i);

or simpler syntax:

var list = await Task.WhenAll(
   Task.Run(() => 10),
   Task.Run(() => 20)
);
foreach (var i in list)
	Console.WriteLine(i);

Answer

You should not be creating tasks directly. That is the wrong approach. Do something like this.

var t1 = Task.Run(CheckNC);
var t2 = Task.Run(CheckNL);
var t3 = Task.Run(CheckNP);
var t4 = Task.Run(CheckNK);

await Task.WhenAll(t1, t2, t3, t4);

//Get results
var result = t1.Result;

Task.Run accepts either a function with no parameters or a function that returns a result. If a function returns a result then the returned task is typed ask Task so the result is available from Result.

Note that if you want to use WhenAll you can but it is only necessary if you want to wait for all tasks to complete before you process any results. Result and Wait on each task will automatically block, although they will also throw if the underlying task fails.

//Alternatives to WhenAll
await Task.WhenAll(t1, t2, t3, t4);

await t1;
await t2;
...

//Or - block on result
var result1 = t1.Result;
var result2 = t2.Result;
...

//Or - just wait
t1.Wait();
t2.Wait();
...

Answer

Thanks guys for your help.

This is my new code with added Progress Controls. It works fine.

var t1 = Task.Run(() => CheckNC(progress, statusNC));

var t2 = Task.Run(() => CheckNL(progress, statusNL));

var t3 = Task.Run(() => CheckAD(progress, statusAD, token));

var t4 = Task.Run(() => CheckAC(progress, statusAC, token));

// Wait till all tasks are complete

await Task.WhenAll(t1, t2, t3, t4);

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Get Results from Task.WhenAll for multiple tasks

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