I get a null exception if I try to pass a null parameter to a delegate during an invoke. Here's what the code looks like:
public void RequestPhoto()
{
WCF.Service.BeginGetUserPhoto(Contact.UserID,
new AsyncCallback(RequestPhotoCB), null);
}
public void RequestPhotoCB(IAsyncResult result)
{
var photo = WCF.Service.EndGetUserPhoto(result);
UpdatePhoto(photo);
}
public delegate void UpdatePhotoDelegate(Binary photo);
public void UpdatePhoto(Binary photo)
{
if (InvokeRequired)
{
var d = new UpdatePhotoDelegate(UpdatePhoto);
Invoke(d, new object[] { photo });
}
else
{
var ms = new MemoryStream(photo.ToArray());
var bmp = new Bitmap(ms);
pbPhoto.BackgroundImage = bmp;
}
}
The problem is with the line:
Invoke(d, new object[] { photo });
If the variable "photo" is null. What is the correct way to pass a null parameter during an invoke?
Thanks!
Just for the benefit of others, you can pass null arguments to delegates (if the type allows it? Clarification needed here). In your case, IAsyncResult will allow it.
As for the debugging, the exception occurs on Invoke because you are debugging on a given Thread A, the exception occurs on Thread B. You can breakpoint multiple threads. Breakpoint the Thread B code and you will see the exception closer to or on the source.
Notice though that your debugger will jump around if multiple threads are running code at the same time. Debugging in multiple threads is always at least a little tricky, but satisfying when you solve the problems.
You could also further improve your answer code to check the null before it checks the InvokeRequired, as this is thread-independent to your logic (your code checks it just prior to use, after Invoking). This will save pushing the Invoke onto the message pump (assuming WinForms).
OK I figured it out. The problem was NOT with passing the null parameter to the delegate like I thought. The problem was with the delegate executing it was causing a null exception on the line:
var ms = new MemoryStream(photo.ToArray());
I didn't realize the problem was there because it was crashing on the Invoke line.
So I changed the UpdatePhoto method as follows:
public void UpdatePhoto(Binary photo)
{
if (InvokeRequired)
{
var d = new UpdatePhotoDelegate(UpdatePhoto);
Invoke(d, new object[] { photo});
}
else
{
if (photo != null)
{
var ms = new MemoryStream(photo.ToArray());
var bmp = new Bitmap(ms);
pbPhoto.BackgroundImage = bmp;
}
}
}
And all is well!
Related
I try to execute this code async and parallel by using the ParallelForEachAsync method from this project: Dasync/AsyncEnumerable. Unfortunately the method never returns.
The SampleProduct is a simple DTO which has a boolean property and two string properties.
The GetOsmData method tries to get data via http-request, but often throws an exception.
My first attempt was without .ConfigureAwait(false), but has the same result...
If I try this method with a list of products (products.Count = 8) result.Count always stops by 7.
private async Task<ConcurrentBag<SampleProduct>> CheckOsmDataAsync(
List<SampleProduct> products)
{
var result = new ConcurrentBag<SampleProduct>();
await products.ParallelForEachAsync(
async product =>
{
OsmData osmData;
try
{
osmData = await GetOsmData(_osmUrl.Replace("articlenumber",
product.MaterialNumber.ToString())).ConfigureAwait(false);
}
catch (Exception e)
{
osmData = null;
}
if (osmData != null && osmData.PrintingData.Count > 0)
{
product.OsmPrintImageAvailable = true;
}
else
{
product.OsmPrintImageAvailable = false;
}
result.Add(product);
},
// 0 => Chooses a default value based on the processor count
maxDegreeOfParallelism: 0
);
return result;
}
With the help of my colleague I could solve the problem... The problem was not inside the method itself, but only how it was called. I called it form the Main/UI-Thread synchronously. That seems to have caused a deadlock. Making the calling method async and awaiting the CheckOsmDataAsync() solved the problem.
Nevertheless thanks for your responds!
Specifically I am doing C# development using Xamarin.Forms, however working on the native Android side writing a GPS wrapper class which will be usable in the Xamarin.Forms side via dependency injection. The calls should be the same between C# and Java in regards to Android for the most part.
Essentially, I have this method in my Geolocator object (which implements ILocationListener) on the Android side:
public async Task<Tuple<bool, string, GPSData>> GetGPSData() {
gpsData = null;
var success = false;
var error = string.Empty;
if (!manager.IsProviderEnabled(LocationManager.GpsProvider)) {
//request permission or location services enabling
//set error
} else {
manager.RequestSingleUpdate(LocationManager.GpsProvider, this, null);
success = true;
}
return new Tuple<bool, string, GPSData>(success, error, gpsData);
}
and
public void OnLocationChanged(Location location) {
gpsData = new GPSData(location.Latitude, location.Longitude);
}
I want to be able to call GetGPSData and have it return the tuple, which for now the only important thing about the Tuple is gpsData is filled in. I know it can take seconds to find a fix, so I want this method to be async and be awaitable in the Xamarin.Forms side once I actually need the value.
My issue is I can't figure out a way to have manager.RequestSingleUpdate work synchronously, or any work around. You call that method, and then eventually OnLocationChanged fires off. I tried throwing in a disgusting, barbaric
while (gpsData == null);
after the call to force it not to continue until OnLocationChanged was fired, however when I put that line in, OnLocationChanged never gets called. I'm assuming that is because OnLocationChanged is invoked on the same thread instead of being a background thread.
Is there any way for me to take this scenario and have GetGPSData not return until OnLocationChanged has fired off?
Thanks
EDIT: To add, this method will not be regularly called. It's spontaneous and rare, so I don't want to use RequestLocationUpdates, get regular updates and return the most recent one because that would require always having GPS on, while would rain batteries unnecessarily.
You can do what you want with a TaskCompletionSource. I had the same issue and this is how I solved it:
TaskCompletionSource<Tuple<bool, string, GPSData> tcs;
// No need for the method to be async, as nothing is await-ed inside it.
public Task<Tuple<bool, string, GPSData>> GetGPSData() {
tcs = new TaskCompletionSource<Tuple<bool, string, GPSData>>();
gpsData = null;
var success = false;
var error = string.Empty;
if (!manager.IsProviderEnabled(LocationManager.GpsProvider)) {
//request permission or location services enabling
//set error
tcs.TrySetException(new Exception("some error")); // This will throw on the await-ing caller of this method.
} else {
manager.RequestSingleUpdate(LocationManager.GpsProvider, this, null);
success = true;
}
//return new Tuple<bool, string, GPSData>(success, error, gpsData); <-- change this to:
return this.tcs.Task;
}
And:
public void OnLocationChanged(Location location) {
gpsData = new GPSData(location.Latitude, location.Longitude);
// Here you set the result of TaskCompletionSource. Your other method completes the task and returns the result to its caller.
tcs.TrySetResult(new Tuple<bool, string, GPSData>(false, "someString", gpsData));
}
I have a class with a set of functions that differ in the number of parameters and the parameter types. I've been trying to figure out a way to invoke a call to a desired function inside an allocated thread.
What's a simple way of doing this? I've looked into System.Action, but that requires the parameters to be known. I've also gone over TaskFactory and TPL, but from the examples I've seen, I can't put together the solution in my head.
What I want to eventually do is queue up jobs that will be executed by a limited number of threads. The jobs performed are simple HTTP requests.
I feel like this has been done before and has a simple solution, but it has eluded me for weeks. I'm hoping for an elegant way of doing it instead of a lot of complex code.
I'm also trying to implement MEF plugins to make matters worse.
public bool AddThreads(int numOfThreads)
{
try
{
// Exit if no plugin type is set
if (PluginType == "") return false;
int totalThreads = numOfThreads + threads.Count;
for (int i = threads.Count; i < totalThreads; i++)
{
// Create an instance of the MEF plugin
var task = PluginHandler.CreateInstance(PluginType);
threads.Add(task);
task.ThreadId = i;
task.OnStatusChange += new TaskerPlugin.EventHandler(ChangeStatus);
task.OnActionComplete += new TaskerPlugin.EventHandler(ReportComplete);
task.OnActionSuccess += new TaskerPlugin.EventHandler(ReportSuccess);
task.OnActionFailure += new TaskerPlugin.EventHandler(ReportFailure);
task.OnActionAttempt += new TaskerPlugin.EventHandler(ReportAttempt);
task.OnActionError += new TaskerPlugin.EventHandler(ReportError);
task.OnActionCancelled += new TaskerPlugin.EventHandler(ReportCancellation);
task.OnActionBegin += new TaskerPlugin.EventHandler(ReportStartOfTask);
task.OnActionEnd += new TaskerPlugin.EventHandler(ReportEndOfTask);
// Do work from plugin
// This is where I'd like to call different
// functions possibly inside Start()
task.Start();
}
return true;
}
catch (Exception)
{
return false;
}
}
Current code calling the function:
private void CreateThreads(int threadCount)
{
// taskMan is a class variable to queue more jobs in the future
taskMan = new TaskManager(PLUGIN_FOLDER)
{
PluginType = PLUGIN_TYPE,
UseProxies = (_config.IpSolution == "Proxies" || _config.IpSolution == "Proxy URL")
};
taskMan.AddThreads(threadCount);
}
I want to eventually just call a function to add a job to it with a predefined number of threads:
private void AddJob(string pluginName, string methodName, List<string> args)
I'd prefer not just using a string list to put all of my arguments in, but I don't really know of another way of doing it. Maybe a list of objects which I then cast later? Both these ideas are very messy...
I am assuming that AddJob is the overloaded method that you need to call with different parameters.
You might have to tweak your PluginHandler.CreateInstance(PluginType) method to do something like this while creating the task, this would allow you to execute any method you need in the task that you create..
Task task = new Task(() =>
{
classInstance.YourMethod("some param1", some other param2));
}
Further with some reflection..
var classInstance = new YourClass();
Type type = classInstance.GetType();
MethodInfo methodInfo = type.GetMethod("AddJob");
object[] parametersArray = new object[] { "some param1", "some parma2" };
methodInfo.Invoke(methodInfo, parametersArray);
and finally,
Task task = new Task(() =>
{
var classInstance = new YourClass();
Type type = classInstance.GetType();
MethodInfo methodInfo = type.GetMethod("AddJob");
object[] parametersArray = new object[] { "some param1", "some parma2" };
methodInfo.Invoke(classInstance, parametersArray);
}
In case the AddJob method is present in the current class itself, there could be little changes to the code.
I am writing a code in c# to do image processing. I want to use threading and i suppose threading in WPF application is little different. I tried to run thread but it only works when the function is void(), i.e does not take any arguments.
However, my function is taking 3 arguemnts like this
frame_extract.Frame_Processing(_colorFrame_, widht, height);
So therefore the following does not work
depth_Threads = new System.Threading.Thread(**) since ** takes on void() type.
perhaps i am missing something, but my question is how can i work with threading for functions that take arguments.
Maybe you could use the TPL.
It should then be something like:
Task.Factory.StartNew(() => frame_extract.Frame_Processing(_colorFrame_, widht, height));
But be aware that you might have to marshal to the ui-thread.
If you want to create the thread in the ui thread and want the new thread to interact with mentioned ui thread, something like the following should work:
var task = new Task(() => frame_extract.Frame_Processing(_colorFrame_, widht, height));
task.Start(TaskScheduler.FromCurrentSynchronizationContext());
That should work.
I'm not 100% sure if that's what you want, but I think you need to do this :
depth_Threads = new System.Threading.Thread(()=>frame_extract.Frame_Processing(_colorFrame_, widht, height));
It depends on what values you are passing in. Sometimes if you are using objects, they are locked to a given thread in which case you need to create duplicates prior and then pass the duplicates into the new thread.
Do the following. Your method should receive single object argument like this void SomeVoid(object obj). Make an object array with all variables that you want to pass to the method SomeVoid like this object[] objArr = { arg1, arg2, arg3 }; and then call the Start method of thread object with objArr argument since the Start() method receives one object parameter. Now back to your method, cast and obj received from Start method to an object array like this object arr = obj as object[]; and then you can access those 3 arguments like this arr[0] arr[1] and arr[2]
You can use the ParameterizedThreadStart class.
1) Create a class who will holds your three arguments
public class FrameProcessingArguments
{
public object ColorFrame { get; set; }
public int Width { get; set; }
public int Height { get; set; }
}
2) Modify your Frame_Processing method to take as parameter an instance of Object and inside it, cast that instance as a FrameProcessingArguments
if (arguments == null) throw new NullArgumentException();
if(arguments.GetType() != typeof(FrameProcessingArguments)) throw new InvalidTypeException();
FrameProcessingArguments _arguments = (FrameProcessingArguments) arguments;
3) Create and start your thread
FrameProcessingArguments arguments = new FrameProcessingArguments()
{
ColorFrame = null,
Width = 800,
Height = 600
}
Thread thread = new Thread (new ParameterizedThreadStart(frame_extract.Frame_Processing));
// You can also let the compiler infers the appropriate delegate creation syntax:
// and use the short form : Thread thread = new Thread(frame_extract.Frame_Processing);
thread.Start (arguments);
So I encountered a strange issue today - I had a simple creation of an instance inside the critical section of a lock, and it would throw a null reference exception when I manually dragged the next line to execute. To illustrate:
public class SearchEngineOptimizationParser
{
protected static ConcurrentDictionary<string, SearchEngineOptimizationInfo> _referralInformation = null;
protected static DateTime _lastRecordingDate;
protected static object _lockRecordingObject = new object();
protected static Dictionary<string, string> _searchProviderLookups = null;
static SearchEngineOptimizationParser()
{
_referralInformation = new ConcurrentDictionary<string, SearchEngineOptimizationInfo>();
_lastRecordingDate = DateTime.Now;
_searchProviderLookups = new Dictionary<string, string>();
_searchProviderLookups.Add("google.com", "q");
_searchProviderLookups.Add("yahoo.com", "p");
_searchProviderLookups.Add("bing.com", "q");
}
public SearchEngineOptimizationParser()
{
}
public virtual void ParseReferrer(Uri requestUrl, NameValueCollection serverVariables, ISession session)
{
string corePath = requestUrl.PathAndQuery.SmartSplit('?')[0].ToLower();
string referrer = serverVariables["HTTP_REFERER"];
if (!string.IsNullOrWhiteSpace(referrer))
{
NameValueCollection queryString = HttpUtility.ParseQueryString(referrer);
string dictionaryKey = session.AffiliateID + "|" + corePath;
foreach (var searchProvider in _searchProviderLookups)
{
if (referrer.Contains(searchProvider.Key))
{
if (queryString[searchProvider.Value] != null)
{
string keywords = queryString[searchProvider.Value];
SearchEngineOptimizationInfo info = new SearchEngineOptimizationInfo
{
Count = 1,
CorePath = corePath,
AffiliateId = session.AffiliateID,
Keywords = keywords
};
_referralInformation.AddOrUpdate(dictionaryKey, info, (key, oldValue) =>
{
oldValue.Count++;
return oldValue;
});
break;
}
}
}
}
if (DateTime.Now > _lastRecordingDate.AddHours(1))
{
lock (_lockRecordingObject)
{
if (DateTime.Now > _lastRecordingDate.AddHours(1))
{
SearchEngineKeywordRepository repository = new SearchEngineKeywordRepository();
List<KeyValuePair<string, SearchEngineOptimizationInfo>> currentInfo = _referralInformation.ToList();
Action logData = () =>
{
foreach (var item in currentInfo)
repository.LogKeyword(item.Value);
};
Thread logThread = new Thread(new ThreadStart(logData));
logThread.Start();
_lastRecordingDate = DateTime.Now;
_referralInformation.Clear();
}
}
}
}
EDIT: Updated Real Object
public class SearchEngineKeywordRepository
{
public virtual void LogKeyword(SearchEngineOptimizationInfo keywordInfo)
{
LogSearchEngineKeywords procedure = new LogSearchEngineKeywords();
procedure.Execute(keywordInfo.CorePath, keywordInfo.AffiliateId, keywordInfo.Keywords, keywordInfo.Count);
}
}
The general pattern being that I want to do this 'something' only every hour (in the context of a website application that gets a lot of traffic). I would breakpoint my first if statement, and then step the next line to execute inside the second if statement. When doing so, the act of initializing the SomeObject instance would cause a null reference exception. It had a completely 100% default constructor - I didn't even specify one.
However, when I let the code go through naturally, it would execute without problem. For some reason, it seems that when I skipped over the lock call into the critical section to just test run that code, it caused all kinds of errors.
I'm curious to know why that is; I understand the lock keyword is just syntactic sugar for a Monitor.Enter(o) try / finally block, but that seems to be that when invoking the constructor, something else was happening.
Anyone have any ideas?
EDIT: I've added the actual code to this. I'm able to reproduce this at will, but I still don't understand why this is happening. I've tried copying this code to another solution and the problem does not seem to occur.
I've tried to reproduce your situation, but as I expected I could not. I've tried both the 2.0 and 4.0 runtime, in 32 and 64 bit mode (debugging sometimes behaves differently under x64).
Is the code shown a simplification? Have you checked all your assumptions? I understand you're skipping 3 lines of code, both the if statements and the lock? In that case, even setting the lock object to null does not cause the exception you describe.
(Having _lockRecordingObject set to null causes an ArgumentNullException when leaving the lock scope)