Multiple Async Methods, best to use WhenAll with different return types

Question

I have an Async controller action method which calls 4 async methods on my Backend to return List from each. The List of objects is different for each method. ie List List etc.

I have this working as so:

BizProvider bp = new BizProvider();
List<biz.Customer> custReturn = await bp.GetCustomerAsync();
List<biz.Account> acctReturn = await bp.GetAccountAsync();
...plus 2 more
List<object> returnArr = new List<object>();
returnArr.Add(custReturn);
returnArr.Add(acctReturn);  ...plus 2 more
return JsonConvert.SerializeObject(returnArr);

Should I use Task.WhenAll because of multiple Tasks?

The example at the microsoft https://msdn.microsoft.com/en-us/library/hh194874%28v=vs.110%29.aspx

All the Tasks return List<int>

So I used and abstract class and had all my list object types inherit from this. and I changed the return type of my business object provider to return the List of abstract type so now I can do:

var tasks1 = new List<Task<List<Biz.AbstractClass>>>();
tasks1.Add(bp.GetCustomerAsAbstractAsync());
tasks1.Add(bp.GetAccountAsAbstractAsync());
...plus 2 more

I then call var continuation = Task.WhenAll(tasks1);

this executes then the next line it gets to the .Results but then stops executing???

foreach (var result in continuation.Result)
                                       ^ stops here

further on I call

returnArrays.Add(result1);
return JsonConvert.SerializeObject(returnArrays);

but these never get hit...I don't know why. Perhaps I don't need a WhenAll, but then I am still curious what is going wrong. perhaps I need an await on the specific functions, or invoke the Action somehow, as in the Microsoft link the functions are called inline.

Task.Run(async () => { x=x,etc...)

Edit 20150306 => adding more implementation details

CustMan cm = new CustMan(); 
List<object> returnArr = new List<object>();
var aTask = cm.GetCustomersAsync(); 
var bTask = cm.GetAccountsAsync(); 
await Task.WhenAll(aTask, bTask);  
returnArr.Add(aTask.Result);//same for bTask <-- breakpoint never hits 
return JsonConvert.SerializeObject(returnArr); 
//also .js ajax return method never comes back.

//in CustMan()
public async Task<List<biz.Customer>> GetCustomersAsync() {
    List<biz.Customer> custList = await (from contact in ObjectContextDb.GetData<da.ContactInfo>()//<--generic returns IQueryable
     join customerContact in ObjectContextDb.GetData<da.CustomerContact>() on contact.Id equals customerContact.ContactInfoID
     join customerOrg in ObjectContextDb.GetData<da.CustomerOrganisation>() on customerContact.OrgID equals customerOrg.Id
     orderby contact.LastName
     select new biz.Customer {
       CustomerContactInfo = new biz.Contact() {
         ID = contact.Id,
         WorkPhone = contact.WorkPhone
       },
       CustomerOrg = new biz.CustomerOrganisation {
         ID = facultyOrg.Id,
         Name = facultyOrg.OrgName,
         ClientID = (customerContact.ClientID.HasValue ? customerContact.ClientID.Value : 0)
       }
     }).ToListAsync<biz.Customer>();
    return custList;// <-- Breakpoint hits here, the List has items
}

public async Task<List<biz.Account>> GetAccountsAsync()
{
 var roles = (from acctType in ObjectContextDb.GetData<da.AccountInType>()
  join r in ObjectContextDb.GetData<da.AccountType>() on acctType.AccountTypeID equals r.ID
  select new
  {
   AccountId = acctType.AccountID,
   Type = r.TypeName
  });   //.ToList();

  List<biz.Account> allContacts = await (from account in ObjectContextDb.GetData<da.Account>()
       orderby account.Name
       select new biz.Account()
       {
         Number = account.Id,
         Name = account.Name,
         Roles = (from r in roles where account.Id == r.AccountId select r.Type).ToList()
       }).ToListAsync<biz.Account>();

  return allContacts;//  <-- Breakpoint hits here, the List has items
}

The method which calls the Manager Class methods without WhenAll works!

CustMan cm = new CustMan(); 
List<object> returnArrays = new List<object>();
List<biz.Customer> custReturn = await cm.GetCustomersAsync();
List<biz.Account> acctReturn = await cm.GetAccountsAsync();

returnArrays.Add(custReturn);
returnArrays.Add(acctReturn);

return JsonConvert.SerializeObject(returnArrays);

Answer 1

You can use Task.WhenAll without changing anything. Just store the tasks in a variable and add their results afterwards:

var aTask = GetAAsync();
var bTask = GetBAsync();
...
await Task.WhenAll(aTask, bTask);
returnArr.Add(aTask.Result);
returnArr.Add(bTask.Result);  

In you case however, the async operations use Entity Framework which doesn't support multiple operations concurrently so you can't do that.

Call and await your operations one at a time:

returnArr.Add(await GetAAsync());
returnArr.Add(await GetBAsync());  
...

Answer 2

It sounds like you're doing this:

var continuation = Task.WhenAll(tasks1);
foreach (var result in continuation.Result) { }

Which is probably deadlocking. WhenAll returns a Task too, so you'll need to await that:

var results = await Task.WhenAll(tasks1);
foreach (var result in results) { }

Otherwise, building a List of tasks and passing them into WhenAll is a perfectly reasonable way to await multiple tasks concurrently.