When running a .Net MAUI app using the Windows simulator, I have the following code (stripped down to include the relevant statements):
task = Task.Run( async () => await do_something( cancellation_token ), cancellation_token );
The do_something() method calls Geolocation.Default.GetLocationAsync, as shown below:
do_something( CancellationToken ct )
{
try
{
location = await Geolocation.Default.GetLocationAsync( geo_location_request, ct );
}
catch ( Exception )
{
/*
At this point, an exception is raised:
Microsoft.Maui.ApplicationModel.PermissionException: Permission request must be invoked on main thread
at Microsoft.Maui.ApplicationModel.Permissions.LocationWhenInUse.RequestLocationPermissionAsync()
*/
}
}
As per the comments above, an exception is raised that states the call to Geolocation.Default.GetLocationAsync() must be invoked on the main thread.
However, when using the Android simulator, no exception is raised and everything works fine. I did not see anything mentioned in the documentation that the call to Geolocation.Default.GetLocationAsync() must be invoked on the main thread.
Apologies if such information does exist and I missed it -- if that is the case, please provide the relevant link.
Questions
Is this an undocumented "feature"?
Is this a bug in the Windows simulator?
I can get around the issue (by invoking the task on the MainThread), but I want to know if it is really necessary to do so given that it works on Android, but not on Windows.
Thanks in advance.
The system automatically checks permissions when you get the location, whereas in Windows the location permission is checked on the main thread. For more details, you can refer to the following issue:https://github.com/Xamarin/Essentials/pull/1777
Related
I have an Android app that uses NDK - a regular Android Java app with regular UI and C++ core. There are places in the core where I need to call Java methods, which means I need a JNIEnv* for that thread, which in turn means that I need to call JavaVM->AttachCurrentThread() to get the valid env.
Previously, was just doing AttachCurrentThread and didn't bother to detach at all. It worked fine in Dalvik, but ART aborts the application as soon as a thread that has called AttachCurrentThread exits without calling DetachCurrentThread. So I've read the JNI reference, and indeed it says that I must call DetachCurrentThread. But when I do that, ART aborts the app with the following message:
attempting to detach while still running code
What's the problem here, and how to call DetachCurrentThread properly?
Dalvik will also abort if the thread exits without detaching. This is implemented through a pthread key -- see threadExitCheck() in Thread.cpp.
A thread may not detach unless its call stack is empty. The reasoning behind this is to ensure that any resources like monitor locks (i.e. synchronized statements) are properly released as the stack unwinds.
The second and subsequent attach calls are, as defined by the spec, low-cost no-ops. There's no reference counting, so detach always detaches, no matter how many attaches have happened. One solution is to add your own reference-counted wrapper.
Another approach is to attach and detach every time. This is used by the app framework on certain callbacks. This wasn't so much a deliberate choice as a side-effect of wrapping Java sources around code developed primarily in C++, and trying to shoe-horn the functionality in. If you look at SurfaceTexture.cpp, particularly JNISurfaceTextureContext::onFrameAvailable(), you can see that when SurfaceTexture needs to invoke a Java-language callback function, it will attach the thread, invoke the callback, and then if the thread was just attached it will immediately detach it. The "needsDetach" flag is set by calling GetEnv to see if the thread was previously attached.
This isn't a great thing performance-wise, as each attach needs to allocate a Thread object and do some internal VM housekeeping, but it does yield the correct behavior.
I'll try a direct and practical approach (with sample code, without use of classes) answering this question for the occasional developer that came up with this error in android, in cases where they had it working and after a OS or framework update (Qt?) it started to give problems with that error and message.
JNIEXPORT void Java_com_package_class_function(JNIEnv* env.... {
JavaVM* jvm;
env->GetJavaVM(&jvm);
JNIEnv* myNewEnv; // as the code to run might be in a different thread (connections to signals for example) we will have a 'new one'
JavaVMAttachArgs jvmArgs;
jvmArgs.version = JNI_VERSION_1_6;
int attachedHere = 0; // know if detaching at the end is necessary
jint res = jvm->GetEnv((void**)&myNewEnv, JNI_VERSION_1_6); // checks if current env needs attaching or it is already attached
if (JNI_EDETACHED == res) {
// Supported but not attached yet, needs to call AttachCurrentThread
res = jvm->AttachCurrentThread(reinterpret_cast<JNIEnv **>(&myNewEnv), &jvmArgs);
if (JNI_OK == res) {
attachedHere = 1;
} else {
// Failed to attach, cancel
return;
}
} else if (JNI_OK == res) {
// Current thread already attached, do not attach 'again' (just to save the attachedHere flag)
// We make sure to keep attachedHere = 0
} else {
// JNI_EVERSION, specified version is not supported cancel this..
return;
}
// Execute code using myNewEnv
// ...
if (attachedHere) { // Key check
jvm->DetachCurrentThread(); // Done only when attachment was done here
}
}
Everything made sense after seeing the The Invocation API docs for GetEnv:
RETURNS:
If the current thread is not attached to the VM, sets *env to NULL, and returns JNI_EDETACHED. If the specified version is not supported, sets *env to NULL, and returns JNI_EVERSION. Otherwise, sets *env to the appropriate interface, and returns JNI_OK.
Credits to:
- This question Getting error "attempting to detach while still running code" when calling JavaVm->DetachCurrentThread that in its example made it clear that it was necessary to double check every time (even though before calling detach it doesn't do it).
- #Michael that in this question comments he notes it clearly about not calling detach.
- What #fadden said: "There's no reference counting, so detach always detaches, no matter how many attaches have happened."
I'm call a .net 4.0 dll from a vb6 app using com interop. In .net I create an xps document, via a xaml fixed document and save it to disk. This causes and memory leak and I've found a great solution here.
Saving a FixedDocument to an XPS file causes memory leak
The solution above, that worked for me, involves this line of code:
Dispatcher.CurrentDispatcher.Invoke(DispatcherPriority.SystemIdle, new DispatcherOperationCallback(delegate { return null; }), null);
What exactly is happening with this line of code. Is that by setting the delegate to null this disposes the Dispatcher object?
While it initially appears that the supplied code does nothing, it actually has a non-obvious side effect that is resolving your problem. Let's break it down into steps:
Dispatcher.CurrentDispatcher Gets the dispatcher for the current thread.
Invoke synchronously executes the supplied delegate on the dispatcher's thread (the current one)
DispatcherPriority.SystemIdle sets the execution priority
new DispatcherOperationCallback(delegate { return null; }) creates a delegate that does nothing
null is passed as an argument to the delegate
All together, this looks like it does nothing, and indeed it actually does do "nothing". The important part is that it waits until the dispatcher for the current thread has cleared any scheduled tasks that are higher priority than SystemIdle before doing the "nothing". This allows the scheduled cleanup work to happen before you return to your vb6 app.
I have looking at many threads about the exception "cannot pass a GCHandle across AppDomains" but I still don't get it....
I'm working with an RFID Reader which is driven by a DLL. I don't have source code for this DLL but only a sample to show how to use it.
The sample works great but I have to copy some code in another project to add the reader to the middleware Microsoft Biztalk.
The problem is that the process of Microsoft Biztalk works in another AppDomain. The reader handle events when a tag is read. But when I run it under Microsoft Biztalk I got this annoying exception.
I can't see any solution on how to make it work...
Here is some code that may be interesting :
// Let's connecting the result handlers.
// The reader calls a command-specific result handler if a command is done and the answer is ready to send.
// So let's tell the reader which functions should be called if a result is ready to send.
// result handler for reading EPCs synchronous
Reader.KSRWSetResultHandlerSyncGetEPCs(ResultHandlerSyncGetEPCs);
[...]
var readerErrorCode = Reader.KSRWSyncGetEPCs();
if (readerErrorCode == tKSRWReaderErrorCode.KSRW_REC_NoError)
{
// No error occurs while sending the command to the reader. Let's wait until the result handler was called.
if (ResultHandlerEvent.WaitOne(TimeSpan.FromSeconds(10)))
{
// The reader's work is done and the result handler was called. Let's check the result flag to make sure everything is ok.
if (_readerResultFlag == tKSRWResultFlag.KSRW_RF_NoError)
{
// The command was successfully processed by the reader.
// We'll display the result in the result handler.
}
else
{
// The command can't be proccessed by the reader. To know why check the result flag.
logger.error("Command \"KSRWSyncGetEPCs\" returns with error {0}", _readerResultFlag);
}
}
else
{
// We're getting no answer from the reader within 10 seconds.
logger.error("Command \"KSRWSyncGetEPCs\" timed out");
}
}
[...]
private static void ResultHandlerSyncGetEPCs(object sender, tKSRWResultFlag resultFlag, tKSRWExtendedResultFlag extendedResultFlag, tKSRWEPCListEntry[] epcList)
{
if (Reader == sender)
{
// Let's store the result flag in a global variable to get access from everywhere.
_readerResultFlag = resultFlag;
// Display all available epcs in the antenna field.
Console.ForegroundColor = ConsoleColor.White;
foreach (var resultListEntry in epcList)
{
handleTagEvent(resultListEntry);
}
// Let's set the event so that the calling process knows the command was processed by reader and the result is ready to get processed.
ResultHandlerEvent.Set();
}
}
You are having a problem with the gcroot<> helper class. It is used in the code that nobody can see, inside that DLL. It is frequently used by C++ code that was designed to interop with managed code, gcroot<> stores a reference to a managed object. The class uses the GCHandle type to add the reference. The GCHandle.ToIntPtr() method returns a pointer that the C++ code can store. The operation that fails is GCHandle.FromIntPtr(), used by the C++ code to recover the reference to the object.
There are two basic explanations for getting this exception:
It can be accurate. Which will happen when you initialized the code in the DLL from one AppDomain and use it in another. It isn't clear from the snippet where the Reader class object gets initialized so there are non-zero odds that this is the explanation. Be sure to keep it close to the code that uses the Reader class.
It can be caused by another bug, present in the C++ code inside the DLL. Unmanaged code often suffers from pointer bugs, the kind of bug that can accidentally overwrite memory. If that happens with the field that stores the gcroot<> object then nothing goes wrong for a while. Until the code tries to recover the object reference again. At that point the CLR notices that the corrupted pointer value no longer matches an actual object handle and generates this exception. This is certainly the hard kind of bug to solve since this happens in code you cannot fix and showing the programmer that worked on it a repro for the bug is very difficult, such memory corruption problems never repro well.
Chase bullet #1 first. There are decent odds that Biztalk runs your C# code in a separate AppDomain. And that the DLL gets loaded too soon, before or while the AppDomain is created. Something you can see with SysInternals' ProcMon. Create a repro of this by writing a little test program that creates an AppDomain and runs the test code. If that reproduces the crash then you'll have a very good way to demonstrate the issue to the RFID vendor and some hope that they'll use it and work on a fix.
Having a good working relationship with the RFID reader vendor to get to a resolution is going to be very important. That's never not a problem, always a good reason to go shopping elsewhere.
i am making an application in silverlight 3.0. In that application i have one public method as
public void DrawWavform()
{
Line[,] line = new Line[10,200];
line[i,j]= new Line();//i am getting error here of invalid thread access
//some operation
}
In application i am creating different threads as per user inputs and calling DrawWaveform method from that newly created thread. I want to parallel opeation.Please suggest me solution.Thanks in advance.
Any operation changing the GUI must be executed on the UI thread. This can be done using the dispatcher:
Deployment.Current.Dispatcher.BeginInvoke( () =>
{
// update ui
});
or
(SomeDependencyObject).Dispatcher.BeginInvoke( () => { /* ... */ } );
Anyway, this code should be used very rarely and contain only UI related code. Executing expensive operations on the UI thread will cause your application to hang.
I'm using Visual C++ 6.0. I'm not sure of the service pack level of the visual studio installation, but the OS is Win 2K SP4. The failing code is part of a DLL.
Here's the code:
EIO::OpenConnection()
{
m_Client = new CSocket();
if(m_Client->Create() == 0) {
delete m_Client;
m_Client = NULL;
return CAsyncSocket::GetLastError();
}
if (!m_Client->Connect((LPCTSTR)m_IPAddress, 7)) {
delete m_Client;
m_Client = NULL;
return CAsyncSocket::GetLastError();
}
...<stuff>...
}
This compiles without error on my build system and executes without either of the calls to m_Client methods failing. When I move this DLL to the production system (Win 2K, not sure of service pack level yet), the call to m_Client->Connect() returns an error, so it goes into the IF block. CAsyncSocket::GetLastError() then the debugger to open and report an 0xC0000005 access violation. I don't understand this stuff enough to get anything out of the disassembly.
I've also tried CSocket::GetLastError() and m_Client->GetLastError() with the same results.
I'm fairly certain that m_Client->Connect() fails because of some security policy that's on the production machine that's absent on the development system, but I'd like to get the actual error code so I can help the IT guy narrow his search.
I haven't yet tried forcing a call to GetLastError() on my build system to see if I get an access violation there.
The GetLastError() method most likely calls WSAGetLastError().
But for WSAGetLastError() to work, WSACleanup() must not have been called yet.
I'm guessing that when you delete m_Client that exactly this happens.
Try calling GetLastError() before you delete the m_Client object.