Question

I have a method as follows:

      public decimal GetExchangeRate(string fromCurrency, string toCurrency)
      {
            GoogleCurrencyService googleCurrencyService = new GoogleCurrencyService();
            return googleCurrencyService.GetRateForCurrency(fromCurrency, toCurrency);

      } 

and another class as follows

public class GoogleCurrencyService
{
    public decimal GetRateForCurrency(string fromCurrency, string toCurrency)
    {

        try
        {
            WebClient client = new WebClient();
            client.DownloadStringCompleted += new DownloadStringCompletedEventHandler(StringDownloadCompleted);
            client.DownloadStringAsync(new Uri(_requestUri + fromCurrency + "=?" + toCurrency));

        }
        catch (Exception)
        {
            ExchangeRate = 0;
        }

        return ExchangeRate;
    }

    private void StringDownloadCompleted(object sender, DownloadStringCompletedEventArgs e)
    {
        _response = e.Result;
        ExchangeRate = ParseResponseAndGetExchangeRate();
    }

}//class GoogleCurrencyService

the variable ExchangeRate always come out as zero, so I believe the function call “GetRateForCurrency” returns before the async callback gets called. How do I make sure that does not happen as I need the variable ExchangeRate to be set before being returned. Thanks. Also, I have noticed that the callback never gets called as I have a breakpoint in it and the exception as well which does not get called. So I donot know where the problem is.Any help appreciated.

Answer 1

You can use an event wait handle to block the current thread and wait for the async call…

public class GoogleCurrencyService
{
    private const string RequestUri = "http://www.google.com/ig/calculator?hl=en&q=1{0}%3D%3F{1}";

    public decimal ExchangeRate { get; private set; }

    public decimal GetRateForCurrency(string fromCurrency, string toCurrency)
    {
        ExchangeRate = 0;
        // use a signaler to block this thread and wait for the async call.
        var signaler = new ManualResetEvent(false);
        try
        {
            var client = new WebClient();
            client.DownloadStringCompleted += StringDownloadCompleted;
            // pass the signaler as user token
            client.DownloadStringAsync(new Uri(String.Format(RequestUri, fromCurrency, toCurrency)), signaler);

            // wait for signal, it will be set by StringDownloadCompleted
            signaler.WaitOne();
        }
        finally
        {
            signaler.Dispose();
        }

        return ExchangeRate;
    }

    private void StringDownloadCompleted(object sender, DownloadStringCompletedEventArgs e)
    {
        try
        {
            ExchangeRate = ParseResponseAndGetExchangeRate(e.Result);
        }
        finally
        {
            // set signal
            ((ManualResetEvent)e.UserState).Set();
        }
    }

    private decimal ParseResponseAndGetExchangeRate(string result)
    {
        return 123;
    }
}

EDIT: The same class using an async pattern

public class GoogleCurrencyService
{
    private const string RequestUri = "http://www.google.com/ig/calculator?hl=en&q=1{0}%3D%3F{1}";

    public void GetRateForCurrency(string fromCurrency, string toCurrency, Action<decimal> callback)
    {
        var client = new WebClient();
        client.DownloadStringCompleted += StringDownloadCompleted;
        // pass the callback as user token
        client.DownloadStringAsync(new Uri(String.Format(RequestUri, fromCurrency, toCurrency)), callback);
    }

    private void StringDownloadCompleted(object sender, DownloadStringCompletedEventArgs e)
    {
        // parse response to get the rate value
        var rate = ParseResponseAndGetExchangeRate(e.Result);

        // if a callback was specified, call it passing the rate.
        var callback = (Action<decimal>)e.UserState;
        if (callback != null)
            callback(rate);
    }

    private decimal ParseResponseAndGetExchangeRate(string result)
    {
        return 123;
    }
}

Consuming the async class:

// this is your UI form/control/whatever
public class MyUI
{
    public void OnButtonToGetRateClick()
    {
        var from = "USD"; // or read from textbox...
        var to = "EUR";

        // call the rate service
        var service = new GoogleCurrencyService();
        service.GetRateForCurrency(from, to, (rate) =>
            {
                // do stuff here to update UI.
                // like update ui.
            });
    }
}

Maybe you’ll have to dispatch the UI changes to ui thread. I not have WP framework here to confirm that this is the case, but I think it is.