Develop Reference

Catching Exception message from Boolean method

I have seen similar questions, but not exactly this:
I would like to know the right way of determining whether a method is executed correctly or not, returning a boolean, and if the method is not executed know the reason, even if an exception is thrown.
I do it in this way, but I think that return inside the catch is a bad practice, so which is the right way?:
if(!myObject.DoSomething('A', out result))
{
MessageBox.Show(myObject.ErrorMessage);
[...]
}else{
MessageBox.Show(result);
[...]
}
class myObject()
{
public string ErrorMessage;
bool DoSomething(char inputValue, out string result)
{
try
{
if(inputValue == 'A')
{
ErrorMessage = "Bad input value: " + inputValue;
return false;
}
[...]
return true;
}catch(Exception ex){
ErrorMessage = ex.Message;
return false;
}
}
I don't like trhow the exception inside the catch because I lose the control of the application (and I can't get the description), and the exception always finish in the form. And if I show the exception in the form, I don't need try catch in the rest of the classes.
I mean that try {} catch(Exception ex) { throw ex;} is the same as not putting try catch.
thanks a lot

My suggestion would be to create your own Exception type (possibly global), and pass it in as a reference.
Thereafter you can still get back your boolean indicating success or failure (and having only one return outside of the try..catch).
public class CustomException
{
private string _message;
private string _title;
public CustomException()
{
_title = "";
_message = "";
}
public CustomException(string title, string message)
{
_title = title;
_message = message;
}
}
Then call DoSomething passing in an instance of CustomException (ce in this case).
CustomException ce = new CustomException();
Be advised this is the best process to solve the problem of having to return a boolean indicating success or failure and know the message, for example; dumping it to a log file or logging to database (particularly for Service Calls - WCF)
However this is not a solution for bad logic in handling business process.

Return false inside a catch isn't by itself bad practice. It's useful when you handle a piece of code's exceptions and it must not fail.
For example, I'm working on a printer piloting DLL at the time, and this DLL must read a XML file containing multiple records to print. The method must not fail because one record fails to print, but it still can return exception if the XML file is not correctly formated.
public void Print(string xmlFile)
{
if (String.IsNullOrWhiteSpace(xmlFile))
throw new ArgumentNullException("No xml file has been passed to the Print method.");
// This line will most likely throw an exception if the XMl file is not well formated
XDocument dom = XDocument.Load(xmlFile);
foreach (XElement n in dom.XPathSelectElements("//RECORDS/RECORD"))
{
try
{
// send commands to the printer, if the printer fails to print, throw a PrinterRecordException
}
catch (PrinterRecordException e)
{
// log print failure, but keep on printing the rest
continue;
}
catch (Exception e)
{
// dunno what happened, but still have to print the rest
continue;
}
}
}
In this example, my function could return false instead of throwing exceptions to the main program, if this program doesn't care. In my case it does :p In my opinion, that's how you should think your method.

Exception handling methods and best practices are a some-what subjective matter. I cannot attest to the method I'm about to present because I have only just started to use it in my own project.
What I suggest is having a static ExceptionHandler class with which you can register any exception to be handled by Generic Parameter and its corresponding handler. This will decouple your business logic from your UI in case you wanted to display some kind of message box when a particular exception occurs.
Here's an example:
/// the real implementation uses lambda's and/or implementations of IExceptionHandler<TException>
ExceptionHandler.Register<InvalidPasswordException>(() => /*some handler logic*/);
// ... else where in the code ...
catch (InvalidPasswordException ex)
{
// do resource clean-up and raise exception for listeners such as the UI or logging infrastructure.
ExceptionHandler.Raise(ex);
}
So far this looks promising, especially when compared with my previous approaches. But only time will tell.
Update
The ExceptionHandler class itself need not be static, for example you might want to have different instances of ExceptionHandlers at different layers of your application if you are using a layered architecture.

Getting InvalidOperationException during call to GetResponseAsync()?

PREFACE: I know the code excerpt is lengthy but I did't want to leave out a detail that someone else might spot as the cause of the issue. The reason for the somewhat verbose nature of the code and the many Exception traps is due to the hunt for the NullReferenceException that I describe below. You can wade through the code quickly to the salient parts by jumping to the await keywords found amongst the async methods that call each other.
UPDATE: The InvalidOperationException is occurring because I am altering the IsEnabled status of certain buttons. I am on the Main thread so I am not sure why this is happening. Does anyone know why?
I have a Windows Phone 7 application written C# that is getting a System.InvalidOperationException when GetResponseAsync() is called in a particular code context. The application uses the PetFinder API to create a Cat Breed Guessing game with the intention of helping cats in animal shelters get adopted. Here is the exact Exception message in its entirety:
Message: An unhandled exception of type 'System.InvalidOperationException' occurred in System.Windows.ni.dll
Before the Exception occurs, there are several successful calls to GetResponseAsync(). I have included the code for the methods involved in the Exception in the order that they are called below. Can someone tell me why I am getting this Exception and how to fix it?
The Exception is occurring completely out of the current code context that calls it, so it's some interaction between the code below and the library that contains GetResponseAsync() that is creating the conditions for the problem. The thread code context just before the call to GetResponseAsync() is the Main thread.
BACKGROUND NOTE:
This all began while I was chasing down a NullReferenceException that was occuring during the call to getRandomPetExt() within doLoadRandomPet(). From the reading I did on SO, my guess is that a NULL Task was being returned from getRandomPetExt(). But if you look at that code you'll see I'm doing everything in my power to trap a spurious Exception and to avoid returning a NULL Task. My current belief still at this time is that the NULL Task is occurring because some other code is generating a spurious Exception outside of my code. Perhaps something in Microsoft.Bcl.Async? Is this some strange synchronization context problem or hidden cross-thread access issue?
The strange part is that before I made a particular change I did not get the InvalidOperationException at all, only the intermittent NullReferenceException every 1 out of 20 to 30 calls to the method chain shown below. The InvalidOperationException on the other hand happens every time with the new code structure. The change I made was to me a minor one meant to help my debugging efforts. The only thing I did was create a method wrapper that moved the guts of loadRandomPet() into doLoadRandomPet(). I did this so I could disable some buttons that triggered method calls that might interfere with the operation to get a random pet. I wrapped the call to doLoadRandomPet() in a try/finally block, to make sure the buttons were re-enabled when the operation exited. Why would this cause such a major change in code execution?
async private void loadRandomPet(int maxRetries = 3)
{
// Do not allow the Guess My Breed or Adopt Me buttons to be
// clicked while we are getting the next pet.
btnAdoptMe.IsEnabled = false;
btnGuessMyBreed.IsEnabled = false;
try
{
await doLoadRandomPet(maxRetries);
}
finally
{
// >>>>> THIS CODE IS NEVER REACHED.
// Make sure the buttons are re-enabled.
btnAdoptMe.IsEnabled = true;
btnGuessMyBreed.IsEnabled = true;
}
}
// -------------------- CALLED NEXT
/// <summary>
/// (awaitable) Loads a random pet with a limit on the number of retries in case of failure.
/// </summary>
/// <param name="maxRetries">The number of retries permitted.</param>
async private Task doLoadRandomPet(int maxRetries = 3)
{
// Show the busy indicator.
radbusyMain.Visibility = Visibility.Visible;
try
{
// Get a random pet.
List<KeyValuePair<string, string>> listUrlArgs = new List<KeyValuePair<string, string>>();
// Only cats.
listUrlArgs.addKVP("animal", PetFinderUtils.EnumAnimalType.cat.ToString());
if (!String.IsNullOrWhiteSpace(MainMenu.ZipCode))
{
listUrlArgs.addKVP(PetFinderUtils.URL_FIELD_LOCATION, MainMenu.ZipCode);
}
if (maxRetries < 0)
throw new ArgumentOutOfRangeException("The maximum retries value is negative.");
Debug.WriteLine("------------------ START: LOADING Random Pet ----------------");
// Loop until a random pet is found.
int numRetries = 0;
// Select the breed, otherwise we will get a ton of "Domestic Short Hair" responses,
// which are not good for the game. Breeds that are returning empty search
// results this session are filtered too.
string strBreedName = MainMenu.GetRandomBreedName();
listUrlArgs.addKVP("breed", strBreedName);
while (numRetries <= maxRetries)
{
try
{
// Save the last successful retrieval.
if (this._pbi != null)
_pbiLast = this._pbi;
this._pbi = await getRandomPetExt(listUrlArgs);
}
catch (EmptySearchResultsException esr)
{
// getRandomPetExt() could not find a suitable cat given the current parameters.
// Swallow the Exception without notifying the user. Just allow the code
// further down to re-use the last cat retrieved in the hopes the next
// quiz won't have the problem.
Debug.WriteLine(">>>>>>>>>> doLoadRandomPet() - getRandomPet() failed to find a cat.");
}
catch (PetFinderApiException pfExc)
{
if (pfExc.ResponseCode == PetFinderUtils.EnumResponseCodes.PFAPI_ERR_LIMIT)
// Swallow the Exception, but let the user know to stop playing for the awhile
// since we have exceeded our rate limit.
CatQuizAux.EasyToast("The PetFinder server is busy.\nPlease try playing the game\nlater.");
else
// Swallow the Exception, but let the user know to stop playing for the awhile
// since we have exceeded our rate limit.
CatQuizAux.EasyToast("The PetFinder may be down.\nPlease try playing the game\nlater.");
// Just exit.
return;
} // try
catch (Exception exc)
{
// This is really bad practice but we're out of time. Just swallow the Exception
// to avoid crashing the program.
Debug.WriteLine(">>>>>>>>>> doLoadRandomPet() - getRandomPet() Other Exception occurrred. Exception Message: " + exc.Message);
}
// If the returned pet is NULL then no pets using the current search criteria
// could be found.
if (this._pbi != null)
{
// Got a random pet, stop looping. Save it to the backup cat field too.
break;
}
else
{
// Are we using a location?
if (listUrlArgs.hasKey(PetFinderUtils.URL_FIELD_LOCATION))
// Retry without the location to open the search to the entire PetFinder API
// inventory.
listUrlArgs.deleteKVP(PetFinderUtils.URL_FIELD_LOCATION);
else
{
// Use a differet breed. Add the current breed to the list of breeds returning
// empty search results so we don't bother with that breed again this session.
MainMenu.ListEmptyBreeds.Add(strBreedName);
// Remove the current breed.
listUrlArgs.deleteKVP("breed");
// Choose a new breed.
strBreedName = MainMenu.GetRandomBreedName();
listUrlArgs.addKVP("breed", strBreedName);
} // else - if (listUrlArgs.hasKey(PetFinderUtils.URL_FIELD_LOCATION))
} // if (this._pbi == null)
// Sleep a bit.
await TaskEx.Delay(1000);
numRetries++;
} // while (numRetries <= maxRetries)
// If we still have a null _pbi reference, use the back-up one.
if (this._pbi == null)
this._pbi = this._pbiLast;
if (this._pbi == null)
throw new ArgumentNullException("(ViewPetRecord::doLoadRandomPet) Failed completely to find a new cat for the quiz. Please try again later.");
// Add the pet to the already quizzed list.
MainMenu.AddCatQuizzed(this._pbi.Id.T.ToString());
// Show the cat's details.
lblPetName.Text = this._pbi.Name.T;
imgPet.Source = new BitmapImage(new Uri(this._pbi.Media.Photos.Photo[0].T, UriKind.Absolute));
// Dump the cat's breed list to the Debug window for inspection.
dumpBreedsForPet(this._pbi);
}
finally
{
// Make sure the busy indicator is hidden.
radbusyMain.Visibility = Visibility.Collapsed;
}
} // async private void doLoadRandomPet(int maxRetries = 3)
// -------------------- CALLED NEXT
/// <summary>
/// Gets a Random Pet. Retries up to maxRetries times to find a pet not in the already <br />
/// quizzed list before giving up and returning the last one found. Also skips pets without <br />
/// photos.
/// </summary>
/// <param name="listUrlArgs">A list of URL arguments to pass add to the API call.</param>
/// <param name="maxRetries">The number of retries to make.</param>
/// <returns>The basic info for the retrieved pet or NULL if a pet could not be found <br />
/// using the current URL arguments (search criteria).</returns>
async private Task<PetBasicInfo> getRandomPetExt(List<KeyValuePair<string, string>> listUrlArgs, int maxRetries = 3)
{
PetBasicInfo newPbi = null;
try
{
newPbi = await doGetRandomPetExt(listUrlArgs, maxRetries);
}
catch (Exception exc)
{
Debug.WriteLine(">>>>>> (ViewPetRecord::getRandomPetExt) EXCEPTION: " + exc.Message);
throw;
} // try/catch
return newPbi;
} // async private void getRandomPetExt()
// -------------------- CALLED NEXT
// This was done just to help debug the NullReferenceException error we are currently fighting.
// see getRandomPetExt() below.
async private Task<PetBasicInfo> doGetRandomPetExt(List<KeyValuePair<string, string>> listUrlArgs, int maxRetries = 3)
{
if (maxRetries < 0)
throw new ArgumentOutOfRangeException("The maximum retries value is negative.");
Debug.WriteLine("------------------ START: Getting Random Pet ----------------");
// Loop until a random pet is found that has not already been used in the quiz or until
// we hit the maxRetries limit.
int numRetries = 0;
PetBasicInfo pbi = null;
while (numRetries <= maxRetries)
{
try
{
pbi = await MainMenu.PetFinderAPI.GetRandomPet_basic(listUrlArgs);
}
catch (PetFinderApiException pfExcept)
{
// pfExcept.ResponseCode = PetFinderUtils.EnumResponseCodes.PFAPI_ERR_LIMIT;
switch (pfExcept.ResponseCode)
{
case PetFinderUtils.EnumResponseCodes.PFAPI_ERR_NOENT:
Debug.WriteLine("The PetFinder API returned an empty result set with the current URL arguments.");
// No results found. Swallow the Exception and return
// NULL to let the caller know this.
return null;
case PetFinderUtils.EnumResponseCodes.PFAPI_ERR_LIMIT:
Debug.WriteLine("The PetFinder API returned a rate limit error.");
// Throw the Exception. Let the caller handler it.
throw;
default:
// Rethrow the Exception so we know about it from the crash reports.
// Other Exception. Stop retrying and show the user the error message.
Debug.WriteLine("Exception during getRandomPetExt()\n" + pfExcept.ErrorMessage);
throw;
} // switch()
}
// Does the pet have a photo?
if (pbi.Media.Photos.Photo.Length > 0)
{
// Yes. Has the pet already been used in a quiz?
if (!MainMenu.IsCatQuizzed(pbi.Id.T.ToString()))
// No. Success.
return pbi;
} // if (pbi.Media.Photos.Photo.Length > 0)
// Retry required.
Debug.WriteLine(String.Format("Retrying, retry count: {0}", numRetries));
// No photo or already used in a quiz. Wait a little before retrying.
await TaskEx.Delay(1000);
// Count retires.
numRetries++;
} // while (numRetries <= maxRetries)
// Unable to find a cat not already quizzed. Just return the last retrieved.
Debug.WriteLine("Retry count exceeded. Returning last retreived pet.");
// Returning NULL results in a await throwing a non-specific NullReferenceException.
// Better to throw our own Exception.
throw new EmptySearchResultsException("(ViewPetRecord::getRandomPetExt) Unable to retrieve a new random cat from the PetFinder API server.");
// return pbi;
} // async private PetBasicInfo doGetRandomPetExt()
// ------------------ CALLED NEXT
/// <summary>
/// Returns the basic information for a randomly chosen pet of the given animal type.
/// </summary>
/// <param name="enAnimalType">The desired animal type to restrict the search to.</param>
/// <returns></returns>
async public Task<JSON.JsonPetRecordTypes.PetBasicInfo> GetRandomPet_basic(List<KeyValuePair<string, string>> urlArgumentPairs = null)
{
Debug.WriteLine("(GetRandomPet_basic) Top of call.");
// If the URL Argument Pairs parameter is null, then create one.
if (urlArgumentPairs == null)
urlArgumentPairs = new List<KeyValuePair<string, string>>();
// Add the "output" parameter that tells PetFinder we want the Basic information for the pet,
// not the ID or full record.
urlArgumentPairs.addKVP("output", "basic");
// Add a unique URL argument to defeat URL caching that may be taking
// place in the Windows Phone library or at the PetFinder API server.
// This defeats the problem so that a new random pet is returned
// each call, instead of the same one.
long n = DateTime.Now.Ticks;
urlArgumentPairs.addKVP("nocache", n.ToString());
// Build the API call.
string strApiCall =
buildPetFinderApiUrl(METHOD_RANDOM_PET,
urlArgumentPairs);
Debug.WriteLine("(GetRandomPet_basic) URL for call: \n" + strApiCall);
// Make the call.
string strJsonReturn = await Misc.URLToStringAsyncEZ(strApiCall);
bool bIsJsonReturnValid = false;
try
{
JSON.JsonPetRecordTypes.PetRecordBasicInfo jsonPetBasic = JsonConvert.DeserializeObject<JSON.JsonPetRecordTypes.PetRecordBasicInfo>(strJsonReturn);
// Deserialization succeeded.
bIsJsonReturnValid = true;
// Success code?
// For some strange reason T is cast to an "int" here where in GetBreedList it's equivalent is cast to a string.
int iResponseCode = jsonPetBasic.Petfinder.Header.Status.Code.T;
if (iResponseCode != 100)
throw new PetFinderApiException("PetFinder::GetRandomPet_basic", iResponseCode);
// throw new Exception("(PetFinder::GetRandomPet_basic) The response document contains a failure response code: " + iResponseCode.ToString() + ":" + jsonPetBasic.Petfinder.Header.Status.Message);
// Return the pet record basic info.
return jsonPetBasic.Petfinder.Pet;
}
finally
{
if (!bIsJsonReturnValid)
// Setting debug trap to inspect JSON return.
Debug.WriteLine("JSON Deserialization failure.");
Debug.WriteLine("(GetRandomPet_basic) BOTTOM of call.");
} // try/finally
}
// -------------------- CALLED NEXT, never returns
/// <summary>
/// (awaitable) Simpler version of above call. Same warnings about getting byte stream <br />
/// objects apply here as they do to URLtoStringAsync()
/// </summary>
/// <param name="stUrl"></param>
/// <param name="reqMethod"></param>
/// <returns></returns>
async public static Task<string> URLToStringAsyncEZ(string strUrl, HttpRequestMethod reqMethod = HttpRequestMethod.HTTP_get)
{
strUrl = strUrl.Trim();
if (String.IsNullOrWhiteSpace(strUrl))
throw new ArgumentException("(Misc::URLToStringAsyncEZ) The URL is empty.");
HttpWebRequest request = (HttpWebRequest)WebRequest.Create(strUrl);
// Get the string value for the request method.
request.Method = reqMethod.GetDescription();
// >>>>> THIS CALL to GetResponseAsync() TRIGGERS THE EXCEPTION (see stack trace below)
// Async wait for the respone.
HttpWebResponse response = (HttpWebResponse)await request.GetResponseAsync();
// Use a stream reader to return the string.
using (var sr = new StreamReader(response.GetResponseStream()))
{
return sr.ReadToEnd();
}
}
// -------------------- STACK TRACE JUST BEFORE URLToStringAsync(), the call the triggers the exception.
> Common_WP7.DLL!Common_WP7.Misc.URLToStringAsyncEZ(string strUrl, Common_WP7.Misc.HttpRequestMethod reqMethod) Line 1079 C#
CatQuiz.DLL!CatQuiz.PetFinderUtils.GetRandomPet_basic(System.Collections.Generic.List<System.Collections.Generic.KeyValuePair<string,string>> urlArgumentPairs) Line 441 C#
CatQuiz.DLL!CatQuiz.ViewPetRecord.doGetRandomPetExt(System.Collections.Generic.List<System.Collections.Generic.KeyValuePair<string,string>> listUrlArgs, int maxRetries) Line 55 C#
CatQuiz.DLL!CatQuiz.ViewPetRecord.getRandomPetExt(System.Collections.Generic.List<System.Collections.Generic.KeyValuePair<string,string>> listUrlArgs, int maxRetries) Line 123 C#
CatQuiz.DLL!CatQuiz.ViewPetRecord.doLoadRandomPet(int maxRetries) Line 243 C#
CatQuiz.DLL!CatQuiz.ViewPetRecord.loadRandomPet(int maxRetries) Line 343 C#
CatQuiz.DLL!CatQuiz.ViewPetRecord.PageViewPetRecord_Loaded(object sender, System.Windows.RoutedEventArgs e) Line 355 C#
System.Windows.ni.dll!MS.Internal.CoreInvokeHandler.InvokeEventHandler(int typeIndex, System.Delegate handlerDelegate, object sender, object args) Unknown
System.Windows.ni.dll!MS.Internal.JoltHelper.FireEvent(System.IntPtr unmanagedObj, System.IntPtr unmanagedObjArgs, int argsTypeIndex, int actualArgsTypeIndex, string eventName) Unknown
======================= EXCEPTION
// -------------------- STACK TRACE when EXCEPTION occurs
> CatQuiz.DLL!CatQuiz.App.Application_UnhandledException(object sender, System.Windows.ApplicationUnhandledExceptionEventArgs e) Line 101 C#
System.Windows.ni.dll!MS.Internal.Error.CallApplicationUEHandler(System.Exception e) Unknown
System.Windows.ni.dll!MS.Internal.Error.IsNonRecoverableUserException(System.Exception ex, out uint xresultValue) Unknown
System.Windows.ni.dll!MS.Internal.JoltHelper.FireEvent(System.IntPtr unmanagedObj, System.IntPtr unmanagedObjArgs, int argsTypeIndex, int actualArgsTypeIndex, string eventName) Unknown
// -------------------- CODE CONTEXT when EXCEPTION occurs
// Code to execute on Unhandled Exceptions
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();
}
}

how to ignore an exception

I am writing a kinect app in C# and I have this code
try //start of kinect code
{
_nui = new Runtime();
_nui.Initialize(RuntimeOptions.UseSkeletalTracking | RuntimeOptions.UseDepthAndPlayerIndex | RuntimeOptions.UseColor);
// hook up our events for video
_nui.DepthFrameReady += _nui_DepthFrameReady;
_nui.VideoFrameReady += _nui_VideoFrameReady;
// hook up our events for skeleton
_nui.SkeletonFrameReady += new EventHandler<SkeletonFrameReadyEventArgs>(_nui_SkeletonFrameReady);
// open the video stream at the proper resolution
_nui.DepthStream.Open(ImageStreamType.Depth, 2, ImageResolution.Resolution320x240, ImageType.DepthAndPlayerIndex);
_nui.VideoStream.Open(ImageStreamType.Video, 2, ImageResolution.Resolution640x480, ImageType.Color);
// parameters used to smooth the skeleton data
_nui.SkeletonEngine.TransformSmooth = true;
TransformSmoothParameters parameters = new TransformSmoothParameters();
parameters.Smoothing = 0.8f;
parameters.Correction = 0.2f;
parameters.Prediction = 0.2f;
parameters.JitterRadius = 0.07f;
parameters.MaxDeviationRadius = 0.4f;
_nui.SkeletonEngine.SmoothParameters = parameters;
//set camera angle
_nui.NuiCamera.ElevationAngle = 17;
}
catch (System.Runtime.InteropServices.COMException)
{
MessageBox.Show("Could not initialize Kinect device.\nExiting application.");
_nui = null;
}
I am looking for a way for my app to not crash (the exception to be ignored) when kinect is not connected. I created another question here but the solutions could not be applied to my occasion as I am forced to use outdated sdk and nobody can solve that quesiton so I am trying to use a different approach. How can I ignore this exception? (I can reverse the changes made to _nui myself afterwards)

Currently you're catching all of ComExceptions. If you want to catch other exceptions, you need to provide specific types for each exception.
You can add your exception types after your catch block like this:
catch (System.Runtime.InteropServices.COMException)
{
MessageBox.Show("Could not initialize Kinect device.\nExiting application.");
_nui = null;
} catch (Exception ex) //this will catch generic exceptions.
{
}
If you want your code to execute after catch No matter what. you can also try to use finally
like this
try
{
//logic
}
finally
{
//logic. This will be executed and then the exception will be catched
}

IF you want to ignore all exceptions:
try
{
// your code...
}
catch (Exception E)
{
// whatever you need to do...
};
The above is a catch-all (although some exception can't be caught like Stackoverflow).
REMARK
You should NOT use the above... you should find out what sort of exception is thrown and catch that!

WP7 IsolatedStorage Exception: {"Operation not permitted on IsolatedStorageFileStream."}

In my Silverlight/XNA app/game I save and load a custom class Level.cs to Isolated Storage. I serialize and deserialize the level using SharpSerializer. The code below is combined from several tutorials on saving and serializing. The code works most of the time, but if I save and load a level repeatedly, usually 2 or 3 times, I will get an exception thrown in the Load() method. I have been unable to track down the cause of this exception. Right now I am handling it by restarting the level from the default XML file when this occurs.
My Questions:
1.) Do you know what is causing my exception, and how can I fix it?
2.) Are there any additional details in the Exception that I am catching that might help me track down what is causing this?
3.) Is there a better method for structuring this code? Most examples I have found use "using" statements. Is there an advantage to that method that might help me here?
Additional Details:
The strange part is that if I quit the app without saving and then run the app again, it will successfully load the Isolated Storage file that it just failed to load. This leads me to believe that the save file is not corrupted, but that I am likely not disposing of some resources properly each time I save/load and as those resources build up during an instance of running the app, it eventually causes this issue.
When the exception is thrown, it is always thrown by this line in the Load() method:
LoadStream = LoadStorage.OpenFile(loadName, FileMode.Open);
The exception description is {"Operation not permitted on IsolatedStorageFileStream."}. Are there any other relavant details in the exception that I should look for? The inner exception could not be read.
My method for saving:
public void Save()
{
IsolatedStorageFile SaveStorage = null;
SaveStorage = IsolatedStorageFile.GetUserStoreForApplication();
IsolatedStorageFileStream SaveStream = null;
string saveLName = "levelSave_" + currentLevel.info.number + ".XML";
if (SaveStorage.FileExists(saveLName))
{
SaveStorage.DeleteFile(saveLName);
}
try
{
SaveStream = SaveStorage.CreateFile(saveLName);
var serializer = new SharpSerializer();
serializer.Serialize(currentLevel, SaveStream);
saveState = SaveState.Successful;
}
catch (Exception ex)
{
saveState = SaveState.Failed;
}
finally
{
if (SaveStream != null)
{
SaveStream.Close();
SaveStream.Dispose();
} if (SaveStorage != null)
{
SaveStorage.Dispose();
}
}
}
My method for loading:
public Level LoadLevel(int levelNumber)
{
IsolatedStorageFile LoadStorage;
LoadStorage = IsolatedStorageFile.GetUserStoreForApplication();
Level tmpLevel;
string loadName = "levelSave_" + levelNumber + ".XML";
if (LoadStorage.FileExists(loadName))
{
IsolatedStorageFileStream LoadStream = null;
try
{
LoadStream = LoadStorage.OpenFile(loadName, FileMode.Open);
var serializer = new SharpSerializer();
tmpLevel = (Level)serializer.Deserialize(LoadStream);
}
catch (Exception ex)
{
tmpLevel = LoadLevelXML(levelNumber);
// Level failed to load from save,
// restart unsaved level from beginning
}
finally
{
if (LoadStream != null)
{
LoadStream.Close();
LoadStream.Dispose();
}
if (LoadStorage != null)
{
LoadStorage.Dispose();
}
}
}
else
{
tmpLevel = LoadLevelXML(levelNumber);
// Level save file not found,
// restart unsaved level from beginning
}
return tmpLevel;
}
I believe this code is probably unnecessary in the Save() method. I just added it to make sure overwriting a file was not the problem.
if (SaveStorage.FileExists(saveLName))
{
SaveStorage.DeleteFile(saveLName);
}
I also believe that this code in both methods is unnecessary, but again, I added it to ensure that the "storage" floating around was not the issue.
if (LoadStorage != null)
{
LoadStorage.Dispose();
}
The exception was thrown before I added either of those segments of code.

I have a feeling that blocking IO operations may throw an IllegalOperationException if performed on the UI thread.
Have you tried performing your serialisation on a background thread (for example, using BackgroundWorker)?

Now, just for kicks, give this a spin:
lock (SaveStorage)
{
if (SaveStorage.FileExists(saveLName))
{
SaveStorage.DeleteFile(saveLName);
}
}

You should be using Mutex concept..refer these..
http://msdn.microsoft.com/en-us/library/ff402541(v=vs.92).aspx
http://forums.create.msdn.com/forums/p/94965/568077.aspx

Exception handling in C# - How?

Using MODI (Microsoft Office Document Imaging) OCR, sometimes the image doesn't contain any text. Therefore doc.OCR throws an exception.
public static string recognize(string filepath, MODI.MiLANGUAGES language = MODI.MiLANGUAGES.miLANG_RUSSIAN, bool straightenimage = true)
{
if (!File.Exists(filepath)) return "error 1: File does not exist";
MODI.Document doc = new MODI.Document();
doc.Create(filepath);
try
{
doc.OCR(language, false, false);
}
catch
{
//
}
MODI.Image image = (MODI.Image)doc.Images[0];
string result="";
foreach (MODI.Word worditems in image.Layout.Words)
{
result += worditems.Text + ' ';
if (worditems.Text[worditems.Text.Length - 1] == '?') break;
}
doc.Close(false);
System.Runtime.InteropServices.Marshal.ReleaseComObject(doc);
System.Runtime.InteropServices.Marshal.FinalReleaseComObject(doc);
System.Runtime.InteropServices.Marshal.ReleaseComObject(image);
System.Runtime.InteropServices.Marshal.FinalReleaseComObject(image);
image = null;
doc = null;
GC.Collect();
GC.WaitForPendingFinalizers();
return result;
}
This code terminates the application, not what I need :(
How do I just make it fade away like nothing happened?

You are 95% of the way there with the code you posted:
try
{
doc.OCR(language, false, false);
}
catch
{
// Here you would check the exception details
// and decide if this is an exception you need
// and want to handle or if it is an "acceptable"
// error - at which point you could popup a message
// box, write a log or doing something else
}
That said it would be prudent to catch the exception type that occurs when the document is empty and then have a different exception handler for any other errors that may occur
try
{
doc.OCR(language, false, false);
}
catch (DocumentEmptyException dex)
{
}
catch
{
}
DocumentEmptyException is, I assume, not the exception type thrown - if you look at the docs for the OCR method (or via debug) you will be able to work out which exception type to catch
EDIT (After seeing your edit)
Are you sure the exception is being thrown from the doc.OCR(...) method? In your edit you added additional code after the catch, could it be coming from there instead?
For example, the line after the catch:
MODI.Image image = (MODI.Image)doc.Images[0];
If your document is empty and therefore the exception is thrown and ignored (as the catch block has nothing in it), does this line continue to work?

You are doing nothing in the catch block, just swallowing the exception which is very bad. The code continues to execute and you try to use the doc variable but because the .OCR call has failed it is more than possible that another exception is thrown later. For example doc.Images[0] will probably crash if the OCR has failed. So either terminate the execution of the method by returning something or put the entire method in a try/catch block.