C# Exception handling in classes

Question

Hello,

C# 2008

I have developed the class below. I have to get the balance from the web server. Once that is done it will call back into my main app with the result.

However, sometime the web server fails for some unknown reason. Could be high volume of traffic or something else. However, I haven't implemented any exception handling in my class. As the app that uses this handles the exception.

However, the client has confirmed that when the web server does fail it displays a unhandled exception dialog box. Then they have to click continue to keep using my application.

So below I am not sure if I should implement the exception handling in my class. However, I am confused as to why the exception was not caught in my app that as below.

Many thanks for any suggestions, or if you see anything else wrong,

private void OnGetBalanceCompleted(object sender, SIPPhoneLibraryEventArgs e)
    {
        try
        {
            //If the balance starts with 'null' there has been an error trying to get the balance.
            if (e.Balance.StartsWith("null"))
            {
                statusDisplay1.CurrentBalance = CATWinSIP_MsgStrings.BalanceError;
            }
            else
            {
                // Display the current balance and round to 2 decimal places.
                statusDisplay1.CurrentBalance = Math.Round(Convert.ToDecimal(e.Balance), 2).ToString();

                //If the balance is zero display in the status message
                if (decimal.Parse(e.Balance) == 0)
                {
                    this.statusDisplay1.CallStatus = "Zero Balance";
                }
            }
            //Remove the event as no longer needed
            siplibrary.GetBalanceCompletedEvent -= new EventHandler<SIPPhoneLibraryEventArgs>(OnGetBalanceCompleted);
        }
        catch (WebException ex)
        {
            MessageBox.Show(ex.Message);
        }
        catch (Exception ex)
        {
            MessageBox.Show(ex.Message);
        }
    }




//Control library for all importing functions
public class Balance : IDisposable
{
    //Constructor
    WebClient wc;
    public Balance()
    {
        using (wc = new WebClient())
        {
            //Create event handler for the progress changed and download completed events
            wc.DownloadProgressChanged += new DownloadProgressChangedEventHandler(wc_DownloadProgressChanged);
            wc.DownloadStringCompleted += new DownloadStringCompletedEventHandler(wc_DownloadStringCompleted);
        }
    }

    ~Balance()
    {
        this.Dispose(false);
    }

    //Event handler and the method that handlers the event
    public EventHandler<SIPPhoneLibraryEventArgs> GetBalanceCompletedEvent;

    //The method that raises the event
    public void OnGetBalanceCompleted(SIPPhoneLibraryEventArgs e)
    {
        if (GetBalanceCompletedEvent != null)
        {
            GetBalanceCompletedEvent(this, e);
        }
    }

    //Get the current balance for the user that is logged in.
    //If the balance returned from the server is NULL display error to the user.
    //Null could occur if the DB has been stopped or the server is down.       
    public void GetBalance(string sipUsername)
    {
        //Remove the underscore ( _ ) from the username, as this is not needed to get the balance.
        sipUsername = sipUsername.Remove(0, 1);

        string strURL = string.Format("http://xxx.xxx.xx.xx:xx/voipbilling/servlet/advcomm.voipbilling.GetBalance?CustomerID={0}", sipUsername);

        //Download only when the webclient is not busy.
        if (!wc.IsBusy)
        { 
            // Sleep for 1/2 second to give the server time to update the balance.
            System.Threading.Thread.Sleep(500);
            // Download the current balance.
            wc.DownloadStringAsync(new Uri(strURL));
        }
        else
        {
            System.Windows.Forms.MessageBox.Show("Busy please try again");
        }
    }

    //return and display the balance after the download has fully completed
    void wc_DownloadStringCompleted(object sender, DownloadStringCompletedEventArgs e)
    {
        //Pass the result to the event handler
        this.OnGetBalanceCompleted(new SIPPhoneLibraryEventArgs(e.Result));
    }

    //Progress state of balance.
    void wc_DownloadProgressChanged(object sender, DownloadProgressChangedEventArgs e)
    {
        //Write the details to the screen.
        Console.WriteLine(e.TotalBytesToReceive);
        Console.WriteLine(e.BytesReceived);
        Console.WriteLine(e.ProgressPercentage);
    }


    //Dispose of the balance object
    public void Dispose()
    {
        Dispose(true);

        GC.SuppressFinalize(this);
    }

    //Remove the event handlers
    private bool isDisposed = false;
    private void Dispose(bool disposing)
    {
        if (!this.isDisposed)
        {
            if (disposing)
            {
                wc.DownloadProgressChanged -= new DownloadProgressChangedEventHandler(wc_DownloadProgressChanged);
                wc.DownloadStringCompleted -= new DownloadStringCompletedEventHandler(wc_DownloadStringCompleted);

                wc.Dispose();
            }               
            isDisposed = true;
        }
    }
}

Answer 1

1. There is more information in the exception than just its Message property. You are throwing all of that information away by only displaying the Message property. Use ex.ToString() instead.
2. Is the code you posted part of the user interface? If not, then it has no business knowing anything about the user interface. In particular, it should not be using MessageBox.Show.
3. I'd remove all the UI stuff, and instead raise an event. The caller would listen to the event and do any UI work.

Answer 2

It seems that you are catching the exception on the OnGetBalanceCompleted event only, instead on the process of fetching the balance.

When there is any error on the fetching, the OnGetBalanceCompleted is not even called, that's why your exception handler is not called.