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How to run multiple methods in parallel in ASP.NET

I have 8 methods in an ASP.NET Console App, like Fun1(), Fun2(), Fun3() ... and so on. In my console application I have called all these methods sequentially. But now the requirement is I have do that using parallel programming concepts. I have read about task and threading concepts in Java but completely new to .NET parallel Programming.
Here is the flow of methods I needed in my console app,
As you can see the diagram, Task1 and Task2 should run in parallel, and Task3 will only occur after completion of the previous two.
The functions inside each task, for example Fun3 and Fun4 for the Task1, should run sequentially, the one after the other.
Can anyone please help me out?

One way to solve this is, by using WhenAll.
To take an example, I have created X number of methods with the name FuncX() like this:
async static Task<int> FuncX()
{
await Task.Delay(500);
var result = await Task.FromResult(1);
return result;
}
In this case, we have Func1, Func3, Func4, Func5, and Func6.
So we call methods and pass them to a list of Task.
var task1 = new List<Task<int>>();
task1.Add(Func3());
task1.Add(Func4());
var task2 = new List<Task<int>>();
task2.Add(Func1());
task2.Add(Func5());
task2.Add(Func6());
You have 2 options to get the result:
// option1
var eachFunctionIsDoneWithAwait1 = await Task.WhenAll(task1);
var eachFunctionIsDoneWithAwait2 = await Task.WhenAll(task2);
var sum1 = eachFunctionIsDoneWithAwait1.Sum() + eachFunctionIsDoneWithAwait2.Sum();
Console.WriteLine(sum1);
// option2
var task3 = new List<List<Task<int>>>();
task3.Add(task1);
task3.Add(task2);
var sum2 = 0;
task3.ForEach(async x =>
{
var r = await Task.WhenAll(x);
sum2 += r.Sum();
});
Console.WriteLine(sum2);
This is just example for inspiration, you can change it and do it the way you want.

Here is how you could create the tasks according to the diagram, using the Task.Run method:
Task task1 = Task.Run(() =>
{
Fun3();
Fun4();
});
Task task2 = Task.Run(() =>
{
Fun1();
Fun5();
Fun6();
});
Task task3 = Task.Run(async () =>
{
await Task.WhenAll(task1, task2);
Fun7();
Fun8();
});
The Task.Run invokes the delegate on the ThreadPool, not on a dedicated thread. If you have some reason to create a dedicated thread for each task, you could use the advanced Task.Factory.StartNew method with the TaskCreationOptions.LongRunning argument, as shown here.
It should be noted that the above implementation has not an optimal behavior in case of exceptions. In case the Fun3() fails immediately, the optimal behavior would be to stop the execution of the task2 as soon as possible. Instead this implementation will execute all three functions Fun1, Fun5 and Fun6 before propagating the error. You could fix this minor flaw by creating a CancellationTokenSource and invoking the Token.ThrowIfCancellationRequested after each function, but it's going to be messy.
Another issue is that in case both task1 and task2 fail, only the exception of the task1 is going to be propagated through the task3. Solving this issue is not trivial.
Update: Here is one way to solve the issue of partial exception propagation:
Task task3 = Task.WhenAll(task1, task2).ContinueWith(t =>
{
if (t.IsFaulted)
{
TaskCompletionSource tcs = new();
tcs.SetException(t.Exception.InnerExceptions);
return tcs.Task;
}
if (t.IsCanceled)
{
TaskCompletionSource tcs = new();
tcs.SetCanceled(new TaskCanceledException(t).CancellationToken);
return tcs.Task;
}
Debug.Assert(t.IsCompletedSuccessfully);
Fun7();
Fun8();
return Task.CompletedTask;
}, default, TaskContinuationOptions.DenyChildAttach, TaskScheduler.Default)
.Unwrap();
In case both task1 and task2 fail, the task3 will propagate the exceptions of both tasks.

how to wait for task.run

I am writing a windows store app and needs some help on Task.Run method. I am calling service to retrieve data from a service; because I want to cancel the task when internet is disconnected I am passing CancellationToken
await Task.Run(async () => await Download1(), cts.Token);
There is another download method which should run after this above task is finished so I am using await. Both the tasks write to same files so I want to make sure that they do not run in parallel.
await Task.Run(async () => await Download2(), cts.Token);
The issue is that above task 2 starts without task 1 is finished and so both task run in parallel. What I am doing wrong? Please advise.
Download1 looks like this:-
public async Task Download1()
{
await Windows.ApplicationModel.Core.CoreApplication.MainView.CoreWindow.Dispatcher.RunAsync(CoreDispatcherPriority.Normal,
async () =>
{
Status = "Downloading!";
var ListSetupTasks = new List<SetupView>();
foreach (var setup in AllSetupTasks.Setup)
{
ListSetupTasks.Add(new SetupViewModel(setup));
}
IEnumerable<Task> downloadTasksQuery = from setup in ListSetupTasks where setup.TaskType == TaskType.Web select _masterlistrepository.GetTask(setup, false, datetime, branch);
Task[] downloadTasks = downloadTasksQuery.ToArray();
await Task.WhenAll(downloadTasks);
IEnumerable<Task> FinalTasksQuery = from setup in ListSetupTasks where setup.TaskType == TaskType.Web select _masterlistrepository.GetTask(setup, false);
foreach (var task in FinalTasksQuery)
{
await task;
}
});
}
The CancellationToken is used like this:-
async void NetworkChanged(object sender, NetworkConnectionStatusChangedEventArgs e)
{
if (e.Value == false && LoadingData == true)
{
await Windows.ApplicationModel.Core.CoreApplication.MainView.CoreWindow.Dispatcher.RunAsync(CoreDispatcherPriority.Normal,
async () =>
{
await _alertmessageservice.ShowAsync("", "Network error. Please retry!");
cts.Cancel();
});
}
}

You use the CoreDispatcher.RunAsync which accepts a DispatchedHandler. DispatchedHandler has this signature:
public delegate void DispatchedHandler()
So when you pass in an async lambda it will end up being async void. That makes it impossible to await as there's no task being returned. This means that everything after an await in your lambdas will run on a different thread concurrently.
You shouldn't be passing an async delegate to that method.
As a side note, passing a CancellationToken doesn't enable to automatically stop the operation. It can only stop the operation before it starts or if you're observing the token somewhere inside the operation.
So, to actually use the CancellationToken Download should accept use it which makes the Task.Run redundant:
await Download1Async(cts.Token);

Can you just call them sequentially?
await Task.Run(async () =>
{
await Download1();
await Download2();
});
You might not even need Task.Run, unless you are calling from a UI thread. If you're already on a background thread, then try just:
await Download1();
await Download2();
Of course, your cancellation token should probably be passed into the download function either way.

react different to multiple async calls

Imagine the following scenario :
public async Task DoMultipleWork() {
var uploadTask = UploadAsync(file);
var processingTask = Task.Run( () => DoCpuWork() );
await Task.WhenAll(uploadTask, processingTask);
Console.WriteLine("upload is done");
Console.WirteLine("processing is done");
}
How can I change that code so that it doesn't matter which one ends first, it execute some particular (sync or async) code.
So I fire the both task and when taskA or taskB ends, I just run some code (sync or async) independently of the other.
I think maybe ContinueWith but I'm not sure because it needs an another async method which is not really needed.
EDIT
As suggested by comments on answer, I want to clear that I want to execute different code depending on the task that completes, and get both Console.WriteLine executed as soon as the original task completes.

You want to use Task.WhenAny which returns the first task that completes. You can then tell which task completed by comparing to the original tasks. Before returning you should wait for the other one to complete explicitly (or wait for both with Task.WhenAll):
public async Task DoMultipleWork()
{
var uploadTask = UploadAsync(file);
var processingTask = Task.Run( () => DoCpuWork() );
var completedTask = await Task.WhenAny(uploadTask, processingTask);
Console.WriteLine("upload or processing is done");
if (completedTask == uploadTask)
{
// Upload completed
}
else
{
// Processing completed
}
await Task.WhenAll(uploadTask, processingTask) // Make sure both complete
Console.WriteLine("upload and processing are done");
}

As I describe on my blog, ContinueWith is dangerous unless you explicitly pass a scheduler. You should use await instead of ContinueWith in ~99% of cases (more detail in another blog post).
In your case:
private async Task UploadAsync(string filepath)
{
var result = await fileManager.UploadAsync(filepath);
Console.WriteLine($"Result from uploading file {result}");
}
private async Task ProcessAsync(string filepath, IProgress<T> progress)
{
await Task.Run(() => wordProcessor.Process(filepath, progress));
Console.WriteLine("processing completed");
}
...
await Task.WhenAll(UploadAsync(filepath), ProcessAsync(filepath, processingProgress));

public async Task DoMultipleWork() {
var uploadTask = UploadAsync(file);
var processingTask = Task.Run( () => DoCpuWork() );
uploadTask.ContinueWith((Task t) => Console.WriteLine("YOUR_MESSAGE"), TaskContinuationOptions.OnlyOnRanToCompletion);
processingTask.ContinueWith((Task t) => Console.WriteLine("YOUR_MESSAGE"), TaskContinuationOptions.OnlyOnRanToCompletion);
await Task.WhenAll(new []{uploadTask, processingTask});
}

Serializing async task and async continuation

I have long running processing that I want to perform in a background task. At the end of the task, I want to signal that it has completed. So essentially I have two async tasks that I want to run in the background, one after the other.
I am doing this with continuations, but the continuation is starting prior to the initial task completing. The expected behavior is that the continuation run only after the initial task has completed.
Here is some sample code that demonstrates the problem:
// Setup task and continuation
var task = new Task(async () =>
{
DebugLog("TASK Starting");
await Task.Delay(1000); // actual work here
DebugLog("TASK Finishing");
});
task.ContinueWith(async (x) =>
{
DebugLog("CONTINUATION Starting");
await Task.Delay(100); // actual work here
DebugLog("CONTINUATION Ending");
});
task.Start();
The DebugLog function:
static void DebugLog(string s, params object[] args)
{
string tmp = string.Format(s, args);
System.Diagnostics.Debug.WriteLine("{0}: {1}", DateTime.Now.ToString("HH:mm:ss.ffff"), tmp);
}
Expected Output:
TASK Starting
TASK Finishing
CONTINUATION Starting
CONTINUATION Ending
Actual Output:
TASK Starting
CONTINUATION Starting
CONTINUATION Ending
TASK Finishing
Again, my question is why is the continuation starting prior to the completion of the initial task? How do I get the continuation to run only after the completion of the first task?
Workaround #1
I can make the above code work as expected if I make the intial task synchronous - that is if I Wait on the Task.Delay like so:
var task = new Task(() =>
{
DebugLog("TASK Starting");
Task.Delay(1000).Wait(); // Wait instead of await
DebugLog("TASK Finishing");
});
For many reasons, it is bad to use Wait like this. One reason is that it blocks the thread, and this is something I want to avoid.
Workaround #2
If I take the task creation and move it into it's own function, that seems to work as well:
// START task and setup continuation
var task = Test1();
task.ContinueWith(async (x) =>
{
DebugLog("CONTINUATION Starting");
await Task.Delay(100); // actual work here
DebugLog("CONTINUATION Ending");
});
static public async Task Test1()
{
DebugLog("TASK Starting");
await Task.Delay(1000); // actual work here
DebugLog("TASK Finishing");
}
Credit for the above approach goes to this somewhat related (but not duplicate) question: Use an async callback with Task.ContinueWith
Workaround #2 is better than Workaround #1 and is likely the approach I'll take if there is no explanation for why my initial code above does not work.

Your original code is not working because the async lambda is being translated into an async void method underneath, which has no built-in way to notify any other code that it has completed. So, what you're seeing is the difference between async void and async Task. This is one of the reasons that you should avoid async void.
That said, if it's possible to do with your code, use Task.Run instead of the Task constructor and Start; and use await rather than ContinueWith:
var task = Task.Run(async () =>
{
DebugLog("TASK Starting");
await Task.Delay(1000); // actual work here
DebugLog("TASK Finishing");
});
await task;
DebugLog("CONTINUATION Starting");
await Task.Delay(100); // actual work here
DebugLog("CONTINUATION Ending");
Task.Run and await work more naturally with async code.

You shouldn't be using the Task constructor directly to start tasks, especially when starting async tasks. If you want to offload work to be executed in the background use Task.Run instead:
var task = Task.Run(async () =>
{
DebugLog("TASK Starting");
await Task.Delay(1000); // actual work here
DebugLog("TASK Finishing");
});
About the continuation, it would be better to just append it's logic to the end of the lambda expression. But if you're adamant on using ContinueWith you need to use Unwrap to get the actual async task and store the it so you could handle exceptions:
task = task.ContinueWith(async (x) =>
{
DebugLog("CONTINUATION Starting");
await Task.Delay(100); // actual work here
DebugLog("CONTINUATION Ending");
}).Unwrap();
try
{
await task;
}
catch
{
// handle exceptions
}

Change your code to
// Setup task and continuation
var t1 = new Task(() =>
{
Console.WriteLine("TASK Starting");
Task.Delay(1000).Wait(); // actual work here
Console.WriteLine("TASK Finishing");
});
var t2 = t1.ContinueWith((x) =>
{
Console.WriteLine("C1");
Task.Delay(100).Wait(); // actual work here
Console.WriteLine("C2");
});
t1.Start();
// Exception will be swallow without the line below
await Task.WhenAll(t1, t2);

Call long running method and continue with other tasks

public class PerformMainTask()
{
Task1();
Task2();
PerformLongTask();
Task3();
Task4();
}
What I would like to achieve here is to PerformLongTask() onto another thread, and to continue with Task3 & Task4 even when PerformLongTask() is still running.
How should my PerformLongTask() be like in a C# 5.0 way?
Do I need to use async/await?

The simplest solution I know of is:
Task1();
Task2();
var task3 = Task.Run(() => PerformLongTask());
Task4();
Task5();
task3.Wait(); //if task3 has not started yet it will be inlined here
Simple and efficient. If you need to propagate errors you should probably use Parallel.Invoke:
Parallel.Invoke(
() => { PerformLongTask(); },
() => { Task4(); Task5(); }
);

Assuming you are using C# 5 and all your Task() methods truly return a Task (or anything awaitable), your code should look like that:
public async Task PerformMainTask()
{
await Task1();
await Task2();
// Start long task
var longTask = PerformLongTask();
await Task3();
await Task4();
//wait for long task to finish
await longTask;
}
However, if your long task does not run parallelly on its own, you can force it to do so with Task.Run:
public async Task PerformMainTask()
{
await Task1();
await Task2();
// Start long task
var longTask = Task.Run(PerformLongTask);
await Task3();
await Task4();
//wait for long task to finish
await longTask;
}
If none are your tasks are really tasks, just strip all the await except the last one, and you will be good to go.

public class PerformMainTask()
{
Task1();
Task2();
Task.WaitAll(Task.Factory.StartNew(() => PerformLongTask()), Task.Factory.StartNew(() => { Task3(); Task4(); }));
}