OnPropertyChanged Event Handler

Question

I am writing an application for Windows Phone. I have a variable result which stores the response sent by a server after a request from the Windows Phone app.

I have a class called request which generates the request and receives the response. I want to check when the result variable changes to that I can do some further processing on my application.

Here is the request class:

namespace PhoneApp1
{
    public class Request: INotifyPropertyChanged
    {
        public string data;
        public string result = "test";

        public Request()
        {

        }

        public string Result
        {
            get
            {
                return result;
            }
            set
            {
                if (this.result == value)
                {
                    return;
                }
                this.result = value;
                OnPropertyChanged("Result");
            }
        }

        public event PropertyChangedEventHandler PropertyChanged;
        public void OnPropertyChanged(string propertyName)
        {
            PropertyChangedEventHandler handler = PropertyChanged;

            if (handler != null)
            {
                handler(this, new PropertyChangedEventArgs(propertyName));
            }
        }

        public void doRequest(string parameters, string URL)
        {
            data = parameters;

            HttpWebRequest request = (HttpWebRequest)HttpWebRequest.Create(URL);
            request.Method = "POST";
            request.ContentType = "application/x-www-form-urlencoded";
            request.BeginGetRequestStream(new AsyncCallback(GetRequestStreamCallback), request);
        }

        public void GetRequestStreamCallback(IAsyncResult callbackResult)
        {
            HttpWebRequest myRequest = (HttpWebRequest)callbackResult.AsyncState;
            Stream postStream = myRequest.EndGetRequestStream(callbackResult);
            byte[] byteArray = Encoding.UTF8.GetBytes(data);

            postStream.Write(byteArray, 0, byteArray.Length);
            postStream.Close();

            myRequest.BeginGetResponse(new AsyncCallback(GetResponsetStreamCallback), myRequest);
        }

        public void GetResponsetStreamCallback(IAsyncResult callbackResult)
        {
            HttpWebRequest request = (HttpWebRequest)callbackResult.AsyncState;
            HttpWebResponse response = (HttpWebResponse)request.EndGetResponse(callbackResult);

            StreamReader httpWebStreamReader = new StreamReader(response.GetResponseStream());
            result = httpWebStreamReader.ReadToEnd();
        }
    }
}

As you can see, I implemented an OnPropertyChanged event handler on the result variable.

Here is the code from where I am creating an instance of the class to do the request:

                    **//Log-in Button Event Handler**
                    Request req = new Request();
                    req.PropertyChanged += new PropertyChangedEventHandler(req_PropertyChanged);
                    req.doRequest("function=LogIn&username=" + username + "&password=" + password, "http://localhost:4000/Handler.ashx");
                }
            }
        }**//End of Log-in Button Event Handler**

        public void req_PropertyChanged(object sender, PropertyChangedEventArgs e)
        {
            string result = ((Request)sender).Result;

            if (result.Equals("True") || result.Equals("true"))
            {
                PhoneApplicationService.Current.State["Username"] = username;
                NavigationService.Navigate(new Uri("/Authenticated.xaml", UriKind.Relative));
            }

            else
            {
                MessageBox.Show("The log-in details are invalid!");
            }
        }

For some reason or another, nothing happens when I click the log-in button. No message box, no redirection, not even an exception. I can't figure out what I am doing wrong. Please help me. I have wasted these past two hours trying to solve this problem.

Why is the req_PropertyChanged code not executing, even though I know that the result variable is changing?

Answer 1

PropertyChanged is not magic.
It will only fire if you explicitly raise the event.

You do raise the event in your Result setter, but you aren't calling that.
When you write result = httpWebStreamReader.ReadToEnd();, you set the field directly, without running the property setter.
Thus, the event never fires.