What could cause async lambda to return unwanted data about the operation in Ok IActionResult?

user_12953341 0
Viewed 74

I tried to use await in my lambda function to have an operation done asynchronously, to simplify the question I left out unnecessary operations (where clauses etc) and I named the async method Services.CustomerAmountService.QueryDbAndReturnResult

The function takes a double (sum of amount) and does numerous tasks, but most importantly an async call (firstAsync, which makes the function awaitable)

[HttpGet]
public async Task<IActionResult> GetCustomersAndAmount()
{
    try
    {
        return Ok((await _orderRepository.CustomMadeQueryThatReturnsIQueryable()
            .Select(x => new
            {
                CustomerId = x.CustomerId,
                CustomerName = x.Customer.Name
                Amount = x.Amount
            })
            .ToListAsync())
            .GroupBy(x => x.CustomerId )
            // this "async x" causes the unwanted result in my Ok() IActionResult
            .Select( async x => new
            {
                CustomerName = x.First().CustomerName,
                SumAmount = await Services.CustomerAmountService.QueryDbAndReturnResult(x.Sum(x => x.Amount)) 
            })
        ));
    }
    catch (Exception ex)
    {
        return BadRequest(ex.Message);
    }
}

The result looks like this:

{"1":[{"result":{"customerName":"VeryNiceCustomer","amount":1.0},"id":1269,"exception":null,"status":5,"isCanceled":false,"isCompleted":true,"isCompletedSuccessfully":true,"creationOptions":0,"asyncState":null,"isFaulted":false}]}

But the desired result and the result without "async x" and an async function (so just returning '1' and doing no further operations at all) looks like this:

{"1":[{"customerName":"VeryNiceCustomer","amount":1}]}

Im wondering what I can change to always get the second result, and not worry about returning unnecessary data about the operation ? It seems to be returing a Result, but I'm only interested in the Result of the Result (what the function returns, not the async Result)

asp.net-core asynchronous linq c#
2 Answers

async lambda is transformed into Func<T, Task<T>> so your return is actually of type IEnumerable<Task<GeneratedAnonymousType>>. You can use Task.WhenAll to transform the results:

var resultTasks = (await _orderRepository.CustomMadeQueryThatReturnsIQueryable()
    .Select(x => new
    {
        CustomerId = x.CustomerId,
        CustomerName = x.Customer.Name
        Amount = x.Amount
    })
    .ToListAsync())
    .GroupBy(x => x.CustomerId )
    .Select( async x => new
    {
        CustomerName = x.First().CustomerName,
        SumAmount = await Services.CustomerAmountService.QueryDbAndReturnResult(x.Sum(x => x.Amount)) 
    }));

return Ok(await Task.WhenAll(resultTasks));

But in general I would recommend to revise tha whole approach - usually having parallel processing in action is a smell. Writing a method wich will fetch all needed data in one query should be better.

user_25012790

You cannot use async in LINQ methods. You have to enumerate results by foreach and make await calls. Schematically:

foreach (var x in items)
   SumAmount = await Services.CustomerAmountService.QueryDbAndReturnResult(x.Sum(x => x.Amount))

But, I think, your query just not optimal:

var query = _orderRepository.CustomMadeQueryThatReturnsIQueryable()
   .GroupBy(x => new
    {
        CustomerId = x.CustomerId,
        CustomerName = x.Customer.Name
    })
   .Select(g => new 
    {
         CustomerName = g.Key.CustomerName,
         SumAmount = g.Sum(x => x.Amount)
    });

return return Ok(await query.ToListAsync());
user_106463160
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