Async task waiting for another task

Question

I have code like :

private async Task<bool> DoAsyncThing()
{
  await doOtherThings();
} 

private async Task<bool> DoAsyncThing2()
{
  await doOtherThings2();
} 

private async Task<bool> SaveAll()
{
   return await _context.SaveChangesAsync() > 0;
}

public async Task<bool> FirstBatchProcess()
{
    var tasks = new List<Task<bool>>();
    ...
    users.Foreach(user => {
        task.Add(this.DoAsyncThing());
    });
    await Task.WhenAll(tasks);
    return await this.SaveAll();
}

public async Task<bool> SecondBatchProcess()
{
    // get all data from batch 1 and then do calculation
    var tasks = new List<Task<bool>>();
    ...
    users.Foreach(user => {
        task.Add(this.DoAsyncThing2());
    });
    await Task.WhenAll(tasks);
    return await this.SaveAll();
}


public async Task<bool> ProcessAll()
{
    await this.FirstBatchProcess();
    await this.SecondBatchProcess();
}

In the ProcessAll I want firstBatchProcess to be done first before doing SecondBatchProcess. Because I have some data from FirstBatchPRocess to be used later in SecondBatchProcess. If I run this code, both will be executed async and caused error because the SecondBatchProcess required the data generated from FirstBatchProcess.

note: both BatchProcesses contains multiple async loop so I use Task.WhenAll() How to wait FirstBatchProcess finished then executed SecondBatchProcess ?

Answer 1

Update

so when I call Task.Wait() it will waiting this task to be done then it will continue another process ?

Since you edited your question, and if i understand you correctly (i'm reading between the lines)

await this.FirstBatchProcess();  // will wait for this to finish
await this.SecondBatchProcess(); // will wait for this to finish

The answer is yes all tasks started in FirstBatchProcess will complete before it executes SecondBatchProcess

Original

Task.WhenAll Method

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

I think you maybe getting confused with the await operator

await (C# Reference)

The await operator is applied to a task in an asynchronous method to insert a suspension point in the execution of the method until the awaited task completes . The task represents ongoing work.

It actually waits!

Your Full Demo Here

private static async Task DoAsyncThing()
{
    Console.WriteLine("waiting");
    await Task.Delay(1000);
    Console.WriteLine("waited");
}

private static async Task SaveAll()
{
    Console.WriteLine("Saving");
    await Task.Delay(1000);
}

public static async Task ProcessAll()
{
    var tasks = new List<Task>();
    for (int i = 0; i < 10; i++)
    {
        tasks.Add(DoAsyncThing());
    }

    await Task.WhenAll(tasks);
    await SaveAll();
    Console.WriteLine("Saved");
}

public static void Main()
{
    ProcessAll().Wait();
    Console.WriteLine("sdf");
}

Output

waiting
waiting
waiting
waiting
waiting
waiting
waiting
waiting
waiting
waiting
waited
waited
waited
waited
waited
waited
waited
waited
waited
waited
Saving
Saved
sdf

All tasks were completed.

Answer 2

How about using Task.Factory.StartNew();

Task.Factory.StartNew(() =>
{
     return DoAsyncThing();
}).ContinueWith(x =>
{
     if (x.Result)
        SaveAll();
});

if DoAsyncThing() do something with UI, you should use TaskScheduler.FromCurrentSynchronizationContext() with StartNew()

I hope it helps.

Thank you.