cannot return value from async method in view

Question

I'm trying to return value from async html helper but it is giving following string instead of desired values.

"System.Threading.Tasks.Task+WhenAllPromise`1[System.Decimal]"

Method:

public async static Task<decimal> CalculateCurrency(this HtmlHelper helper, decimal amount, string from, string country)
    {
        if (await getValue(country))
        {
            string fromCurrency = string.IsNullOrEmpty(from) ? "USD" : from;
            string toCurrency = country;
            WebClient client = new WebClient();
            string url = string.Format("http://finance.yahoo.com/d/quotes.csv?e=.csv&f=sl1d1t1&s={0}{1}=X", fromCurrency.ToUpperInvariant(), toCurrency.ToUpperInvariant());
            Stream response = await client.OpenReadTaskAsync(url);
            StreamReader reader = new StreamReader(response);
            string yahooResponse = await reader.ReadLineAsync();
            response.Close();
            if (!string.IsNullOrWhiteSpace(yahooResponse))
            {
                string[] values = Regex.Split(yahooResponse, ",");
                if (values.Length > 0)
                {
                    decimal rate = System.Convert.ToDecimal(values[1]);
                    string res = string.Format("{0:0.00}", rate * amount);
                    return decimal.Parse(res);
                 }
            }
            return decimal.Zero;
        }
        return decimal.Zero;
    }

Call HTML Helper:

@Html.CalculateCurrency(22, "USD", "EUR")

@DavidG how i can solve this issue? i want that method to run asynchronous — DevWithSigns, Jul 12 '14 at 02:01
Do the asynchronous work inside the method but return synchronously. — DavidG, Jul 12 '14 at 02:04
@DavidG i want to improve application performance thats why i used async method, is there any other way to do this? or how i can do this with this method? — DevWithSigns, Jul 12 '14 at 02:10
@Usama Async code isn't intended to improve performance, it's intended to improve *responsiveness*. For what you're doing here, the best approach would probably be to have an async controller method, which you call asynchronously from your front end via AJAX. — Daniel Mann, Jul 12 '14 at 02:14
@DanielMann can u please guide me with code how i can do this? — DevWithSigns, Jul 12 '14 at 02:17
@Usama I'm sorry, but I'm not going to write your code for you. I've given you a nudge in the right direction -- start researching! If you hit any problems, feel free to post another question! — Daniel Mann, Jul 12 '14 at 02:21

score 4 · Accepted Answer · edited May 23 '17 at 10:28

Views don't support asynchronous methods. So as result you get result of default .ToString() function for type of result which indeed returns just type name.

Options:

move code to controller and call in from asynchronous top level (non-child) action with await. Pass data to view via model or ViewBag
convert to real synchronous code if you must call it from the view or child action
if not possible try .Result, but watch out for deadlocks. See await vs Task.Wait - Deadlock? for details/links.

Note: moving async code to child action will not help.

cannot return value from async method in view

1 Answers1

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