I read somewhere that TRY CATCH is not recommended in Web API methods.
I'm making the following call into a method and if all goes well, I want to return an Employee object along with Status 200 but if something goes wrong e.g. database call fails, etc. I want to return status 500. What's the right way to handle that code?
[HttpPost]
public async Task<IHttpActionResult> PostNewEmployeeAsync(Employee emp)
{
var newEmployee = await RegisterEmployee(emp);
return Ok(emp);
// What if I had a database error in RegisterEmployee method. How do I detect the error and send InternalServerError()
}
private async Task<Employee> RegisterEmployee(Employee emp)
{
// Call DB to register new employee, then return Employee object
}
Your code should return the error code that matches the case that you have, for example if your code couldn't find the required resource in the database return NotFound,
but if you code raises an exception, avoid wrapping your code by try/catch block and instead the exception should bubble up to the level that you can handle it globally, to do this you have many options like :
1- Implement an ExceptionFilter where you can handle all the unhandled exceptions raised in your controllers (this doesn't include any exception happens before the controllers in the pipeline).
See this for more details about ExceptionFilterAttribute.
2- If you are using Web API 2, you can implement the interface IExceptionHandler where you can handle all the exception happens anywhere in the pipeline and there you can return the errors you want.
See this for more details about Global Exception Handling in Web API 2.
Hope that helps.
You don't want to avoid try/catch entirely, you just need to be really careful about it. Wrap your code in a try block, and catch the exception you're expecting. Inside the catch, return the error response.
Related
The problem: I'm crashing when I want to render my incoming data which was retrieved asynchronously.
The app starts and displays some dialog boxes using XAML. Once the user fills in their data and clicks the login button, the XAML class has in instance of a worker class that does the HTTP stuff for me (asynchronously using IXMLHTTPRequest2). When the app has successfully logged in to the web server, my .then() block fires and I make a callback to my main xaml class to do some rendering of the assets.
I am always getting crashes in the delegate though (the main XAML class), which leads me to believe that I cannot use this approach (pure virtual class and callbacks) to update my UI. I think I am inadvertently trying to do something illegal from an incorrect thread which is a byproduct of the async calls.
Is there a better or different way that I should be notifying the main XAML class that it is time for it to update it's UI? I am coming from an iOS world where I could use NotificationCenter.
Now, I saw that Microsoft has it's own Delegate type of thing here: http://msdn.microsoft.com/en-us/library/windows/apps/hh755798.aspx
Do you think that if I used this approach instead of my own callbacks that it would no longer crash?
Let me know if you need more clarification or what not.
Here is the jist of the code:
public interface class ISmileServiceEvents
{
public: // required methods
virtual void UpdateUI(bool isValid) abstract;
};
// In main XAML.cpp which inherits from an ISmileServiceEvents
void buttonClick(...){
_myUser->LoginAndGetAssets(txtEmail->Text, txtPass->Password);
}
void UpdateUI(String^ data) // implements ISmileServiceEvents
{
// This is where I would render my assets if I could.
// Cannot legally do much here. Always crashes.
// Follow the rest of the code to get here.
}
// In MyUser.cpp
void LoginAndGetAssets(String^ email, String^ password){
Uri^ uri = ref new URI(MY_SERVER + "login.json");
String^ inJSON = "some json input data here"; // serialized email and password with other data
// make the HTTP request to login, then notify XAML that it has data to render.
_myService->HTTPPostAsync(uri, json).then([](String^ outputJson){
String^ assets = MyParser::Parse(outputJSON);
// The Login has returned and we have our json output data
if(_delegate)
{
_delegate->UpdateUI(assets);
}
});
}
// In MyService.cpp
task<String^> MyService::HTTPPostAsync(Uri^ uri, String^ json)
{
return _httpRequest.PostAsync(uri,
json->Data(),
_cancellationTokenSource.get_token()).then([this](task<std::wstring> response)
{
try
{
if(_httpRequest.GetStatusCode() != 200) SM_LOG_WARNING("Status code=", _httpRequest.GetStatusCode());
String^ j = ref new String(response.get().c_str());
return j;
}
catch (Exception^ ex) .......;
return ref new String(L"");
}, task_continuation_context::use_current());
}
Edit: BTW, the error I get when I go to update the UI is:
"An invalid parameter was passed to a function that considers invalid parameters fatal."
In this case I am just trying to execute in my callback is
txtBox->Text = data;
It appears you are updating the UI thread from the wrong context. You can use task_continuation_context::use_arbitrary() to allow you to update the UI. See the "Controlling the Execution Thread" example in this document (the discussion of marshaling is at the bottom).
So, it turns out that when you have a continuation, if you don't specify a context after the lambda function, that it defaults to use_arbitrary(). This is in contradiction to what I learned in an MS video.
However by adding use_currrent() to all of the .then blocks that have anything to do with the GUI, my error goes away and everything is able to render properly.
My GUI calls a service which generates some tasks and then calls to an HTTP class that does asynchronous stuff too. Way back in the HTTP classes I use use_arbitrary() so that it can run on secondary threads. This works fine. Just be sure to use use_current() on anything that has to do with the GUI.
Now that you have my answer, if you look at the original code you will see that it already contains use_current(). This is true, but I left out a wrapping function for simplicity of the example. That is where I needed to add use_current().
I am currently developing a Windows Phone 7 App that calls a WCF web service which I also control. The service offers an operation that returns the current user's account information when given a user's login name and password:
[ServiceContract]
public interface IWindowsPhoneService
{
[OperationContract]
[FaultContract(typeof(AuthenticationFault))]
WsAccountInfo GetAccountInfo(string iamLogin, string password);
}
Of course, there is always the possibility of an authentication failure and I want to convey that information to the WP7 app. I could simply return null in that case, but I would like to convey the reason why the authentication failed (i.e. login unknown, wrong password, account blocked, ...).
This is my implementation of the above operation (for testing purposes, all it does is throwing an exception):
public WsAccountInfo GetAccountInfo(string iamLogin, string password)
{
AuthenticationFault fault = new AuthenticationFault();
throw new FaultException<AuthenticationFault>(fault);
}
Now, if I call this operation in my WP7 app, like this:
Global.Proxy.GetAccountInfoCompleted += new EventHandler<RemoteService.GetAccountInfoCompletedEventArgs>(Proxy_GetAccountInfoCompleted);
Global.Proxy.GetAccountInfoAsync(txbLogin.Text, txbPassword.Password);
void Proxy_GetAccountInfoCompleted(object sender, RemoteService.GetAccountInfoCompletedEventArgs e)
{
if (e.Error != null)
{
MessageBox.Show(e.Error.Message);
return;
}
}
The debugger breaks in Reference.cs, saying that FaultException'1 was unhandled, here:
public PhoneApp.RemoteService.WsAccountInfo EndGetAccountInfo(System.IAsyncResult result) {
object[] _args = new object[0];
PhoneApp.RemoteService.WsAccountInfo _result = ((PhoneApp.RemoteService.WsAccountInfo)(base.EndInvoke("GetAccountInfo", _args, result)));
return _result;
}
BEGIN UPDATE 1
When pressing F5, the exception bubbles to:
public PhoneApp.RemoteService.WsAccountInfo Result {
get {
base.RaiseExceptionIfNecessary(); // <-- here
return ((PhoneApp.RemoteService.WsAccountInfo)(this.results[0]));
}
}
and then to:
private void Application_UnhandledException(object sender, ApplicationUnhandledExceptionEventArgs e)
{
if (System.Diagnostics.Debugger.IsAttached)
{
// An unhandled exception has occurred; break into the debugger
System.Diagnostics.Debugger.Break();
}
}
After that, the app terminates (with or without the debugger).
END UPDATE 1
Now, I would love to catch the exception in my code, but I am never given the chance, since my Completed handler is never reached.
Based on similar questions on this site, I have already tried the following:
Re-add the service reference --> no change
Re-create a really simple WCF service from scratch --> same problem
Start the app without the debugger to keep the app from breaking into the debugger --> well, it doesn't break, but the exception is not caught either, the app simply exits
Tell VS 2010 not to break on FaultExceptions (Debug > Options) --> does not have any effect
wrap every line in my app in try { ... } catch (FaultException) {} or even catch (Exception) --> never called.
BEGIN UPDATE 2
What I actually would like to achieve is one of the following:
ideally, reach GetAccountInfoCompleted(...) and be able to retrieve the exception via the GetAccountInfoCompletedEventArgs.Error property, or
be able to catch the exception via a try/catch clause
END UPDATE 2
I would be grateful for any advice that would help me resolve this issue.
The framework seems to read your WsAccountInfo.Result property.
This rethrows the exception on client side.
But you should be the first to read this property.
I don't know your AuthenticationFault class, does it have a DataContractAttribute and is it a known type like the example in
http://msdn.microsoft.com/en-us/library/system.servicemodel.faultcontractattribute.aspx ?
I believe I had the same problem. I resolved it by extending the proxy class and calling the private Begin.../End... methods within the Client object rather than using the public auto-generated methods on the Client object.
For more details, please see:
http://cbailiss.wordpress.com/2014/02/09/wcf-on-windows-phone-unable-to-catch-faultexception/
With the following service method example:-
[PrincipalPermission(SecurityAction.Demand, Role="BUILTIN\\Administrator")]
public string GetTest()
{
try
{
return "Hello";
}
catch (Exception ex)
{
throw ex;
}
}
How do I get an error from the method when the caller is not in the correct Role. In design time the error breaks on the method line (i.e. public string GetTest) and does not reach the catch. At run time it is reported in my silverlight application as an unhandled error (I have try.. catch blocks there too).
There doesn't seem to be a place to catch the error as it never gets into the try blocks!!
The check for the role is made (by the WCF runtime) before the method is actually called - not inside the method!
You need to handle this exception on the caller's side when you make this call.
If you need to check certain conditions inside your service code, don't decorate the method with an attribute, but instead use the role provider in code to check for a given condition.
If you want global error handler for your WCF service you can implement IErrorHandler and add it in custom behavior. Operation can't catch exceptions thrown outside of its try block.
I'm trying to find an elegant way to retry an operation when a WCF channel is in faulted state. I've tried using the Policy Injection AB to reconnect and retry the operation when a faulted state exception occurs on first call, but the PolicyInjection.Wrap method doesn't seem to like wrapping the TransparentProxy objects (proxy returned from ChannelFactory.CreateChannel).
Is there any other mechanism I could try or how could I try get the PIAB solution working correctly - any links, examples, etc. would be greatly appreciated.
Here is the code I was using that was failing:
var channelFactory = new ChannelFactory(endpointConfigurationName);
var proxy = channelFactory.CreateChannel(...);
proxy = PolicyInjection.Wrap<IService>(proxy);
Thank you.
I would rather use callback functions, something like this:
private SomeServiceClient proxy;
//This method invokes a service method and recreates the proxy if it's in a faulted state
private void TryInvoke(Action<SomeServiceClient> action)
{
try
{
action(this.proxy);
}
catch (FaultException fe)
{
if (proxy.State == CommunicationState.Faulted)
{
this.proxy.Abort();
this.proxy = new SomeServiceClient();
//Probably, there is a better way than recursion
TryInvoke(action);
}
}
}
//Any real method
private void Connect(Action<UserModel> callback)
{
TryInvoke(sc => callback(sc.Connect()));
}
And in your code you should call
ServiceProxy.Instance.Connect(user => MessageBox.Show(user.Name));
instead of
var user = ServiceProxy.Instance.Connect();
MessageBox.Show(user.Name);
Although my code uses proxy-class approach, you can write a similar code with Channels.
Thank you so much for your reply. What I ended up doing was creating a decorator type class that implemented the interface of my service, which then just wrapped the transparent proxy generated by the ChannelFactory. I was then able to use the Policy Injection Application Block to create a layer on top of this that would inject code into each operation call that would try the operation, and if a CommunicationObjectFaultedException occurred, would abort the channel, recreate it and retry the operation. It's working great now - although it works great, the only downside though is the wrapper class mentioned having to implement every service operation, but this was the only way I could use the PIAB as this made sense for me for in case I did find a way in future, it was easy enough to change just using interfaces.
In classic ASP.NET I’d persist data extracted from a web service in base class property as follows:
private string m_stringData;
public string _stringData
{ get {
if (m_stringData==null)
{
//fetch data from my web service
m_stringData = ws.FetchData()
}
return m_stringData;
}
}
This way I could simply make reference to _stringData and know that I’d always get the data I was after (maybe sometimes I’d use Session state as a store instead of a private member variable).
In Silverlight with a WCF I might choose to use Isolated Storage as my persistance mechanism, but the service call can't be done like this, because a WCF service has to be called asynchronously.
How can I both invoke the service call and retrieve the response in one method?
Thanks,
Mark
In your method, invoke the service call asynchronously and register a callback that sets a flag. After you have invoked the method, enter a busy/wait loop checking the flag periodically until the flag is set indicating that the data has been returned. The callback should set the backing field for your method and you should be able to return it as soon as you detect the flag has been set indicating success. You'll also need to be concerned about failure. If it's possible to get multiple calls to your method from different threads, you'll also need to use some locking to make your code thread-safe.
EDIT
Actually, the busy/wait loop is probably not the way to go if the web service supports BeginGetData/EndGetData semantics. I had a look at some of my code where I do something similar and I use WaitOne to simply wait on the async result and then retrieve it. If your web service doesn't support this then throw a Thread.Sleep -- say for 50-100ms -- in your wait loop to give time for other processes to execute.
Example from my code:
IAsyncResult asyncResult = null;
try
{
asyncResult = _webService.BeginGetData( searchCriteria, null, null );
if (asyncResult.AsyncWaitHandle.WaitOne( _timeOut, false ))
{
result = _webService.EndGetData( asyncResult );
}
}
catch (WebException e)
{
...log the error, clean up...
}
Thanks for your help tvanfosson. I followed your code and have also found a pseudo similar solution that meets my needs exactly using a lambda expression:
private string m_stringData;
public string _stringData{
get
{
//if we don't have a list of departments, fetch from WCF
if (m_stringData == null)
{
StringServiceClient client = new StringServiceClient();
client.GetStringCompleted +=
(sender, e) =>
{
m_stringData = e.Result;
};
client.GetStringAsync();
}
return m_stringData;
}
}
EDIT
Oops... actually this doesn't work either :-(
I ended up making the calls Asynchronously and altering my programming logic to use MVVM pattern and more binding.