前言：我知道代码摘录很长，但我不想遗漏其他人可能会发现问题原因的细节。代码有些冗长的性质和许多异常陷阱的原因是由于寻找我在下面描述的NullReferenceException。您可以通过跳转到在相互调用的异步方法中找到的await关键字来快速浏览代码到显着部分。
更新： InvalidOperationException 正在发生，因为我正在更改某些按钮的 IsEnabled 状态。我在主线程上，所以我不确定为什么会这样。有谁知道为什么？
我有一个用 C# 编写的 Windows Phone 7 应用程序，当在特定代码上下文中调用GetResponseAsync()时，它会收到System.InvalidOperationException 。该应用程序使用 PetFinder API 创建猫品种猜测游戏，旨在帮助动物收容所中的猫被收养。这是完整的确切异常消息：
Message: An unhandled exception of type 'System.InvalidOperationException' occurred in System.Windows.ni.dll
在发生异常之前，有几次成功调用GetResponseAsync()。我已经按照下面调用它们的顺序包含了异常中涉及的方法的代码。有人能告诉我为什么我会得到这个异常以及如何解决它吗？
异常完全在调用它的当前代码上下文之外发生，因此下面的代码和包含GetResponseAsync()的库之间的一些交互为问题创造了条件。调用GetResponseAsync()之前的线程代码上下文是主线程。
背景说明：
这一切都始于我追查在doLoadRandomPet()中调用getRandomPetExt()期间发生的 NullReferenceException 。根据我对 SO 所做的阅读，我的猜测是从getRandomPetExt()返回了一个NULL 任务。但是，如果您查看该代码，您会发现我正在尽我所能捕获虚假异常并避免返回 NULL 任务。我目前仍然认为 NULL 任务正在发生，因为其他一些代码正在我的代码之外生成虚假异常。也许Microsoft.Bcl.Async中的某些内容？这是一些奇怪的同步上下文问题还是隐藏的跨线程访问问题？
奇怪的是，在我进行特定更改之前，我根本没有收到InvalidOperationException，只有间歇性的NullReferenceException，每 20 到 30 次调用下面显示的方法链中的 1 次。另一方面，每次使用新的代码结构都会发生InvalidOperationException 。我所做的更改对我来说是一个小改动，旨在帮助我进行调试工作。我唯一做的就是创建一个方法包装器，将 loadRandomPet() 的内容移到doLoadRandomPet()中。我这样做是为了禁用一些触发方法调用的按钮，这些按钮可能会干扰获取随机宠物的操作。我将电话转至doLoadRandomPet()在 try/finally 块中，以确保在操作退出时重新启用按钮。为什么这会导致代码执行发生如此重大的变化？
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();
}
}