Guidance needed with Task and async await

Question

After going through a lot of articles and videos i am still having problem with the asynchronous programming.I am working on a project where in service layer I have created all the methods as async. All of the return Task <T> or Task (I made sure not to return void). Now to the issue. My Api calls async methods which internally calls other async methods which may even call other async methods. So I await every time I encounter an async method. The downside with this approach, I think, is that cause I am awaiting for result every time I encounter async it will take a lot of time. For ex:

public async Task<xyz> DoMainTask(int ActionId, int ItemId, int UserId)
{
   await DoTask1(ActionId,ItemId,UserId);  3 sec
   await DoTask2(ActionId,ItemId,UserId);  3 sec
   await DoTask3(ActionId,ItemId,UserId);  3 sec
}

So I don't want to wait for 9 sec because all tasks here are independent of each other. I want to do something like:

public async Task<xyz> DoMainTask(int ActionId, int ItemId, int UserId)
{
   List<Task> lst = new List<Task>();
   t1= DoTask1(ActionId,ItemId,UserId);  
   lst.Add(t1);

   t2 = DoTask2(ActionId,ItemId,UserId);
   lst.Add(t2);

   t3 = DoTask3(ActionId,ItemId,UserId);
   lst.Add(t3);

   await Task.WhenAll(lst);

   // do some work
   return xyz;
}

Which will probably take around 5-6 sec. How do I do this? Whenever I try to use 2nd approach it get error:

A second operation started on this context before a previous asynchronous operation completed

DoTask1 is defined as:

  public async Task DoTask1 (int ActionId, int ItemId, int UserId)
    {
        try
        {
            DailyActivityPoint dailyActivityPoint = new DailyActivityPoint()
            {
                ActionId = ActionId,
                CreatedDate = DateTime.Now,
                ItemId = ItemId,
                UserId = UserId,
                PointsAccumulated = await GetPointsAwardedForAction(ActionId)
            };

            _entities.DailyActivityPoints.Add(dailyActivityPoint);
            await _entities.SaveChangesAsync();
        }
        catch (Exception ex)
        {

        }
    }

inside DoTask1 I am also calling an async method.

How can it be done and what's the best practice?

Answer 1

I believe you are encountering a thread-safety concern as described here. If so, you'll need to be sure that for each 'awaitable' that reaches into EF it is using its own DbContext instance.

So, be sure you aren't using a DbContext singleton; instantiate a new one as you go if you can or get tricky with a container like he does in the link (containers are your friends)

Answer 2

Maybe you should use this guideline in writing your asynchronous methods

Guideline:

1. Write the method the normal way you would write a conventional method, then convert it to an async method.

2. Use the async keyword in the method declaration.

   public async Task<int> ExampleMethodAsync()  
   {  
       // . . . .  
   } 
   

3. Use the await keyword in the code calling an asynchronous process/method.

     int resultValue = await ExampleMethodAsync();
   

Note: await can only be used in an async method modified by the async keyword.

4. Use one of the correct return types in the async method in order to get results

   return types for an async method need to be one of the following:
   

   • Task if your method has a return statement in which the operand has type TResult.
   • Task if your method has no return statement or has a return statement with no operand.
   • void if you're writing an async event handler.

5. Adding the “Async” suffix at the end of the calling method name. This is not required but is considered a convention for writing async methods in C#.

   public async Task<int> ExampleCallingMethodAsync()  
   {  
       // . . . .  
   }
   

6. Include at least one await expression in the async method code. The suspension of an async method at an await expression doesn't constitute an exit from the method, and finally blocks don’t run.

   public async Task<int> ExampleMethodAsync()  
   {  
    //some code
    string pageContents = await client.GetStringAsync(uri);   
    //some more code
    return pageContents.Length;
   }
   

   Note:

   1. You have to adjust what your async method is returning. The object being returned has to match the type Task<T> of your async method
   2. If an async method doesn’t use an await operator to mark a suspension point, the method executes as a synchronous method does, despite the async modifier. The compiler issues a warning for such methods.

---

Below is a typical method converted to an asynchronous method

Example

    private void WebPage(string someURI) 
 { 
    WebClient webClient = new WebClient(); 
    string pageContent = webClient.DownloadString(someURI);
    Console.WriteLine(pageContent);
 }

changed to:

   private async void WebPageAsync(string someURI)
 { 
  WebClient webClient = new WebClient();
  string pageContent = await webClient.DownloadStringTaskAsync(someURI); 
  Console.WriteLine(pageContent); 
 }

I hope this was helpful?