Let's say I have a class that implements the IDisposable interface. Something like this:
MyClass uses some unmanaged resources, hence the Dispose() method from IDisposable releases those resources. MyClass should be used like this:
using ( MyClass myClass = new MyClass() ) {
myClass.DoSomething();
}
Now, I want to implement a method that calls DoSomething() asynchronously. I add a new method to MyClass:
Now, from the client side, MyClass should be used like this:
using ( MyClass myClass = new MyClass() ) {
myClass.AsyncDoSomething();
}
However, if I don't do anything else, this could fail as the object myClass might be disposed before DoSomething() is called (and throw an unexpected ObjectDisposedException). So, the call to the Dispose() method (either implicit or explicit) should be delayed until the asynchronous call to DoSomething() is done.
I think the code in the Dispose() method should be executed in a asynchronous way, and only once all asynchronous calls are resolved. I'd like to know which could be the best way to accomplish this.
Thanks.
NOTE: For the sake of simplicity, I haven't entered in the details of how Dispose() method is implemented. In real life I usually follow the Dispose pattern.
UPDATE: Thank you so much for your responses. I appreciate your effort. As chakrit has commented, I need that multiple calls to the async DoSomething can be made. Ideally, something like this should work fine:
using ( MyClass myClass = new MyClass() ) {
myClass.AsyncDoSomething();
myClass.AsyncDoSomething();
}
I'll study the counting semaphore, it seems what I'm looking for. It could also be a design problem. If I find it convenient, I will share with you some bits of the real case and what MyClass really does.
It looks like you're using the event-based async pattern (see here for more info about .NET async patterns) so what you'd typically have is an event on the class that fires when the async operation is completed named DoSomethingCompleted (note that AsyncDoSomething should really be called DoSomethingAsync to follow the pattern correctly). With this event exposed you could write:
var myClass = new MyClass();
myClass.DoSomethingCompleted += (sender, e) => myClass.Dispose();
myClass.DoSomethingAsync();
The other alternative is to use the IAsyncResult pattern, where you can pass a delegate that calls the dispose method to the AsyncCallback parameter (more info on this pattern is in the page above too). In this case you'd have BeginDoSomething and EndDoSomething methods instead of DoSomethingAsync, and would call it something like...
var myClass = new MyClass();
myClass.BeginDoSomething(
asyncResult => {
using (myClass)
{
myClass.EndDoSomething(asyncResult);
}
},
null);
But whichever way you do it, you need a way for the caller to be notified that the async operation has completed so it can dispose of the object at the correct time.
Since C#8.0 you can use IAsyncDisposable.
using System.Threading.Tasks;
public class ExampleAsyncDisposable : IAsyncDisposable
{
public async ValueTask DisposeAsync()
{
// await DisposeAllTheThingsAsync();
}
}
Here is the reference to the official Microsoft documentation.
Async methods usually have a callback allowing you to do do some action upon completition. If this is your case it would be something like this:
// The async method taks an on-completed callback delegate
myClass.AsyncDoSomething(delegate { myClass.Dispose(); });
An other way around this is an async wrapper:
ThreadPool.QueueUserWorkItem(delegate
{
using(myClass)
{
// The class doesn't know about async operations, a helper method does that
myClass.DoSomething();
}
});
I consider it unfortunate that Microsoft didn't require as part of the IDisposable contract that implementations should allow Dispose to be called from any threading context, since there's no sane way the creation of an object can force the continued existence of the threading context in which it was created. It's possible to design code so that the thread which creates an object will somehow watch for the object becoming obsolete and can Dispose at its convenience, and so that when the thread is no longer needed for anything else it will stick around until all appropriate objects have been Disposed, but I don't think there's a standard mechanism that doesn't require special behavior on the part of the thread creating the Dispose.
Your best bet is probably to have all the objects of interest created within a common thread (perhaps the UI thread), try to guarantee that the thread will stay around for the lifetime of the objects of interest, and use something like Control.BeginInvoke to request the objects' disposal. Provided that neither object creation nor cleanup will block for any length of time, that may be a good approach, but if either operation could block a different approach may be needed [perhaps open up a hidden dummy form with its own thread, so one can use Control.BeginInvoke there].
Alternatively, if you have control over the IDisposable implementations, design them so that they can safely be fired asynchronously. In many cases, that will "just work" provided nobody is trying to use the item when it is disposed, but that's hardly a given. In particular, with many types of IDisposable, there's a real danger that multiple object instances might both manipulate a common outside resource [e.g. an object may hold a List<> of created instances, add instances to that list when they are constructed, and remove instances on Dispose; if the list operations are not synchronized, an asynchronous Dispose could corrupt the list even if the object being disposed is not otherwise in use.
BTW, a useful pattern is for objects to allow asynchronous dispose while they are in use, with the expectation that such disposal will cause any operations in progress to throw an exception at the first convenient opportunity. Things like sockets work this way. It may not be possible for a read operation to be exit early without leaving its socket in a useless state, but if the socket's never going to be used anyway, there's no point for the read to keep waiting for data if another thread has determined that it should give up. IMHO, that's how all IDisposable objects should endeavor to behave, but I know of no document calling for such a general pattern.
I wouldn't alter the code somehow to allow for async disposes. Instead I would make sure when the call to AsyncDoSomething is made, it will have a copy of all the data it needs to execute. That method should be responsible for cleaning up all if its resources.
You could add a callback mechanism and pass a cleanup function as a callback.
var x = new MyClass();
Action cleanup = () => x.Dispose();
x.DoSomethingAsync(/*and then*/cleanup);
but this would pose problem if you want to run multiple async calls off the same object instance.
One way would be to implement a simple counting semaphore with the Semaphore class to count the number of running async jobs.
Add the counter to MyClass and on every AsyncWhatever calls increment the counter, on exits decerement it. When the semaphore is 0, then the class is ready to be disposed.
var x = new MyClass();
x.DoSomethingAsync();
x.DoSomethingAsync2();
while (x.RunningJobsCount > 0)
Thread.CurrentThread.Sleep(500);
x.Dispose();
But I doubt that would be the ideal way. I smell a design problem. Maybe a re-thought of MyClass designs could avoid this?
Could you share some bit of MyClass implementation? What it's supposed to do?
Here's a more modern spin on this old question.
The real objective is to track the async Tasks and wait until they finish...
public class MyExample : IDisposable
{
private List<Task> tasks = new List<Task>();
public async Task DoSomething()
{
// Track your async Tasks
tasks.Add(DoSomethingElseAsync());
tasks.Add(DoSomethingElseAsync());
tasks.Add(DoSomethingElseAsync());
}
public async Task DoSomethingElseAsync()
{
// TODO: something else
}
public void Dispose()
{
// Block until Tasks finish
Task.WhenAll(tasks);
// NOTE: C# allows DisposeAsync()
// Use non-blocking "await Task.WhenAll(tasks)"
}
}
Consider turning it into a base class for re-usability.
And sometimes I use a similar pattern for static methods...
public static async Task MyMethod()
{
List<Task> tasks = new List<Task>();
// Track your async Tasks
tasks.Add(DoSomethingElseAsync());
tasks.Add(DoSomethingElseAsync());
tasks.Add(DoSomethingElseAsync());
// Wait for Tasks to complete
await Task.WhenAll(tasks);
}
So, my idea is to keep how many AsyncDoSomething() are pending to complete, and only dispose when this count reaches to zero. My initial approach is:
public class MyClass : IDisposable {
private delegate void AsyncDoSomethingCaller();
private delegate void AsyncDoDisposeCaller();
private int pendingTasks = 0;
public DoSomething() {
// Do whatever.
}
public AsyncDoSomething() {
pendingTasks++;
AsyncDoSomethingCaller caller = new AsyncDoSomethingCaller();
caller.BeginInvoke( new AsyncCallback( EndDoSomethingCallback ), caller);
}
public Dispose() {
AsyncDoDisposeCaller caller = new AsyncDoDisposeCaller();
caller.BeginInvoke( new AsyncCallback( EndDoDisposeCallback ), caller);
}
private DoDispose() {
WaitForPendingTasks();
// Finally, dispose whatever managed and unmanaged resources.
}
private void WaitForPendingTasks() {
while ( true ) {
// Check if there is a pending task.
if ( pendingTasks == 0 ) {
return;
}
// Allow other threads to execute.
Thread.Sleep( 0 );
}
}
private void EndDoSomethingCallback( IAsyncResult ar ) {
AsyncDoSomethingCaller caller = (AsyncDoSomethingCaller) ar.AsyncState;
caller.EndInvoke( ar );
pendingTasks--;
}
private void EndDoDisposeCallback( IAsyncResult ar ) {
AsyncDoDisposeCaller caller = (AsyncDoDisposeCaller) ar.AsyncState;
caller.EndInvoke( ar );
}
}
Some issues may occur if two or more threads try to read / write the pendingTasks variable concurrently, so the lock keyword should be used to prevent race conditions:
public class MyClass : IDisposable {
private delegate void AsyncDoSomethingCaller();
private delegate void AsyncDoDisposeCaller();
private int pendingTasks = 0;
private readonly object lockObj = new object();
public DoSomething() {
// Do whatever.
}
public AsyncDoSomething() {
lock ( lockObj ) {
pendingTasks++;
AsyncDoSomethingCaller caller = new AsyncDoSomethingCaller();
caller.BeginInvoke( new AsyncCallback( EndDoSomethingCallback ), caller);
}
}
public Dispose() {
AsyncDoDisposeCaller caller = new AsyncDoDisposeCaller();
caller.BeginInvoke( new AsyncCallback( EndDoDisposeCallback ), caller);
}
private DoDispose() {
WaitForPendingTasks();
// Finally, dispose whatever managed and unmanaged resources.
}
private void WaitForPendingTasks() {
while ( true ) {
// Check if there is a pending task.
lock ( lockObj ) {
if ( pendingTasks == 0 ) {
return;
}
}
// Allow other threads to execute.
Thread.Sleep( 0 );
}
}
private void EndDoSomethingCallback( IAsyncResult ar ) {
lock ( lockObj ) {
AsyncDoSomethingCaller caller = (AsyncDoSomethingCaller) ar.AsyncState;
caller.EndInvoke( ar );
pendingTasks--;
}
}
private void EndDoDisposeCallback( IAsyncResult ar ) {
AsyncDoDisposeCaller caller = (AsyncDoDisposeCaller) ar.AsyncState;
caller.EndInvoke( ar );
}
}
I see a problem with this approach. As the release of resources is asynchronously done, something like this might work:
MyClass myClass;
using ( myClass = new MyClass() ) {
myClass.AsyncDoSomething();
}
myClass.DoSomething();
When the expected behavior should be to launch an ObjectDisposedException when DoSomething() is called outside the using clause. But I don't find this bad enough to rethink this solution.
I've had to just go old-school. No, you can't use the simplified "using" block. But a Using block is simply syntactic sugar for cleaning up a semi-complex try/catch/finally block. Build your dispose as you would any other method, then call it in a finally block.
public async Task<string> DoSomeStuffAsync()
{
// used to be a simple:
// using(var client = new SomeClientObject())
// {
// string response = await client.OtherAsyncMethod();
// return response;
// }
//
// Since I can't use a USING block here, we have to go old-school
// to catch the async disposable.
var client = new SomeClientObject();
try
{
string response = await client.OtherAsyncMethod();
return response;
}
finally
{
await client.DisposeAsync();
}
}
It's ugly, but it is very effective, and much simpler than many of the other suggestions I've seen.
Related
Can I use IDisposable to do automate job when triggers in exiting the scope?
This case, I am using IDisposable to only do some job in the end of method, not 'disposing' resources.
This is one of my code :
public class ScopeTimer : IDisposable
{
private Stopwatch sw = new Stopwatch();
private string _logMessage;
public ScopeTimer(string logMessage = "")
{
sw.Start();
_logMessage = logMessage;
}
public void SetMessage(string logMessage)
{
_logMessage = logMessage;
}
void IDisposable.Dispose()
{
sw.Stop();
Logging.Logger.Log($"[{_logMessage}] takes {sw.ElapsedMilliseconds} ms.");
}
}
and I am using like this :
public void SomeMethod()
{
using var timer = new ScopeTimer();
// do some job
// when method finished, timer.Dispose() is called
}
At least now, this code looks working fine, but is this a safe way in other many circumstances?
Yes you can do that. Even Stephen Cleary does it in Nito.AsyncEx like:
SemaphoreSlim _syncRoot = new();
...
using var lockHandle = this._syncRoot.Lock();
// or async
using var lockHandle = await this._syncRoot.LockAsync();
But this is opinion based. In my opinion it helps to make code cleaner because you can setup and restore a state and both parts are next to each other. It's easy to see that you don't miss something.
The drawback is that you might create some instances that needs to be freed. That might create some pressure for the GC.
We created a struct (not a class) to wrap such cleanup code:
public readonly struct Finally : IDisposable
{
private readonly Action? _onDispose;
public Finally(Action onDispose)
{
_ = onDispose ?? throw new ArgumentNullException(nameof(onDispose));
this._onDispose = onDispose;
}
public static Finally Create(Action onDispose)
{
return new Finally(onDispose);
}
public void Dispose()
{
// Keep in mind that a struct can always be created using new() or default and
// in that cases _onDispose is null!
this._onDispose?.Invoke();
}
}
You can use this like:
this._childControl.BeginUpdate();
using var #finally = Finally.Create(this._childControl.EndUpdate);
// or
this._member.SomeEvent -= this.OnMemberSomeEvent;
using var #finally = Finally.Create(() => this._member.SomeEvent += this.OnMemberSomeEvent);
Yes the memory allocation for the Finally-struct is unnecessary and the second example creates a delegate. All that stuff needs time and memory. In applications where performance is a real issue it might be better to use the normal try...finally blocks.
But we're also using Linq and this does also need context objects and delegates and nobody cares.
In my opinion, obvious correctness is more important than performance, because a quick but wrong result doesn't help anyone.
I am refactoring older synchronous C# code to use an async library. The current synchronous code makes liberal usage of locks. Outer methods often call inner methods, where both lock on the same objects. These are often "protected objects" defined in the base class and locked upon in base virtual methods and the overrides that call the base. For synchronous code, that's ok as the thread entering the outer/override method lock can also enter the inner/base method one. That is not the case for async / SemaphoreSlim(1,1)s.
I'm looking for a robust locking mechanism I can use in the async world that will allow subsequent downstream calls to the same locking object, to enter the lock, as per the behaviour in synchronous "lock {...}" syntax. The closest I have come is semaphore slim, but it is too restrictive for my needs. It restricts access not only to other threads, but to the same thread requesting entrance in the inner call too. Alternatively, is there a way to know that the thread is already "inside" the semaphore before calling the inner SemaphoreSlim.waitasync()?
Answers questioning the design structure of the inner/outer methods both locking on the same object are welcome (I question it myself!), but if so please propose alternative options. I have thought of only using private SemaphoreSlim(1,1)s, and having inheritors of the base class use their own private semaphores. But it gets tricky to manage quite quickly.
Sync Example: Because the same thread is requesting entrance to the lock in both inner and outer, it lets it in and the method can complete.
private object LockObject = new object();
public void Outer()
{
lock (LockObject)
{
foreach (var item in collection)
{
Inner(item);
}
}
}
public void Inner(string item)
{
lock (LockObject)
{
DoWork(item);
}
}
Async Example: The semaphore doesn't work like that, it will get stuck at the first iteration of inner async because it's just a signal, it doesn't let another one pass until it is released, even if the same thread requests it
protected SemaphoreSlim LockObjectAsync = new SemaphoreSlim(1,1);
public async Task OuterAsync()
{
try
{
await LockObjectAsync.WaitAsync();
foreach (var item in collection)
{
await InnerAsync(item);
}
}
finally
{
LockObjectAsync.Release();
}
}
public async Task InnerAsync(string item)
{
try
{
await LockObjectAsync.WaitAsync();
DoWork(item);
}
finally
{
LockObjectAsync.Release();
}
}
I am in full agreement with Servy here:
Reentrancy like this should generally be avoided even in synchronous code (it usually makes it easier to make mistakes).
Here's a blog post on the subject I wrote a while ago. Kinda long-winded; sorry.
I'm looking for a robust locking mechanism I can use in the async world that will allow subsequent downstream calls to the same locking object, to enter the lock, as per the behaviour in synchronous "lock {...}" syntax.
TL;DR: There isn't one.
Longer answer: An implementation exists, but I wouldn't use the word "robust".
My recommended solution is to refactor first so that the code no longer depends on lock re-entrancy. Make the existing code use SemaphoreSlim (with synchronous Waits) instead of lock.
This refactoring isn't extremely straightforward, but a pattern I like to use is to refactor the "inner" methods into private (or protected if necessary) implementation methods that are always executed under lock. I strongly recommend these inner methods follow a naming convention; I tend to use the ugly-but-in-your-face _UnderLock. Using your example code this would look like:
private object LockObject = new();
public void Outer()
{
lock (LockObject)
{
foreach (var item in collection)
{
Inner_UnderLock(item);
}
}
}
public void Inner(string item)
{
lock (LockObject)
{
Inner_UnderLock(item);
}
}
private void Inner_UnderLock(string item)
{
DoWork(item);
}
This gets more complex if there are multiple locks, but for simple cases this refactoring works well. Then you can replace the reentrant locks with non-reentrant SemaphoreSlims:
private SemaphoreSlim LockObject = new(1);
public void Outer()
{
LockObject.Wait();
try
{
foreach (var item in collection)
{
Inner_UnderLock(item);
}
}
finally
{
LockObject.Release();
}
}
public void Inner(string item)
{
LockObject.Wait();
try
{
Inner_UnderLock(item);
}
finally
{
LockObject.Release();
}
}
private void Inner_UnderLock(string item)
{
DoWork(item);
}
If you have many of these methods, look into writing a little extension method for SemaphoreSlim that returns IDisposable, and then you end up with using blocks that look more similar to the old lock blocks instead of having try/finally everywhere.
The not-recommended solution:
As canton7 suspected, an asynchronous recursive lock is possible, and I have written one. However, that code has never been published nor supported, nor will it ever be. It hasn't been proven in production or even fully tested. But it does, technically, exist.
There is an extended implementation of command pattern to support multi-commands (groups) in C#:
var ctx= //the context object I am sharing...
var commandGroup1 = new MultiItemCommand(ctx, new List<ICommand>
{
new Command1(ctx),
new Command2(ctx)
});
var commandGroup2 = new MultiItemCommand(ctx, new List<ICommand>
{
new Command3(ctx),
new Command4(ctx)
});
var groups = new MultiCommand(new List<ICommand>
{
commandGroup1 ,
commandGroup2
}, null);
Now , the execution is like:
groups.Execute();
I am sharing the same context (ctx) object.
The execution plan of the web app needs to separate
commandGroup1 and commandGroup2 groups in different thread. In specific, commandGroup2 will be executed in a new thread and commandGroup1 in the main thread.
Execution now looks like:
//In Main Thread
commandGroup1.Execute();
//In the new Thread
commandGroup2.Execute();
How can I thread-safely share the same context object (ctx), so as to be able to rollback the commandGroup1 from the new Thread ?
Is t.Start(ctx); enough or do I have to use lock or something?
Some code implementation example is here
The provided sample code certainly leaves open a large number of questions about your particular use-case; however, I will attempt to answer the general strategy to implementing this type of problem for a multi-threaded environment.
Does the context or its data get modified in a coupled, non-atmoic way?
For example, would any of your commands do something like:
Context.Data.Item1 = "Hello"; // Setting both values is required, only
Context.Data.Item2 = "World"; // setting one would result in invalid state
Then absolutely you would need to utilize lock(...) statements somewhere in your code. The question is where.
What is the thread-safety behavior of your nested controllers?
In the linked GIST sample code, the CommandContext class has properties ServerController and ServiceController. If you are not the owner of these classes, then you must carefully check the documentation on the thread-safety of of these classes as well.
For example, if your commands running on two different threads perform calls such as:
Context.ServiceController.Commit(); // On thread A
Context.ServiceController.Rollback(); // On thread B
There is a strong possibility that these two actions cannot be invoked concurrently if the creator of the controller class was not expecting multi-threaded usage.
When to lock and what to lock on
Take the lock whenever you need to perform multiple actions that must happen completely or not at all, or when invoking long-running operations that do not expect concurrent access. Release the lock as soon as possible.
Also, locks should only be taken on read-only or constant properties or fields. So before you do something like:
lock(Context.Data)
{
// Manipulate data sub-properties here
}
Remember that it is possible to swap out the object that Data is pointing to. The safest implementation is to provide a special locking objects:
internal readonly object dataSyncRoot = new object();
internal readonly object serviceSyncRoot = new object();
internal readonly object serverSyncRoot = new object();
for each sub-object that requires exclusive access and use:
lock(Context.dataSyncRoot)
{
// Manipulate data sub-properties here
}
There is no magic bullet on when and where to do the locks, but in general, the higher up in the call stack you put them, the simpler and safer your code will probably be, at the expense of performance - since both threads cannot execute simultaneously anymore. The further down you place them, the more concurrent your code will be, but also more expense.
Aside: there is almost no performance penalty for the actual taking and releasing of the lock, so no need to worry about that.
Assume we have a MultiCommand class that aggregates a list of ICommands and at some time must execute all commands Asynchronously. All Commands must share context. Each command could change context state, but there is no set order!
The first step is to kick off all ICommand Execute methods passing in the CTX. The next step is to set up an event listener for new CTX Changes.
public class MultiCommand
{
private System.Collections.Generic.List<ICommand> list;
public List<ICommand> Commands { get { return list; } }
public CommandContext SharedContext { get; set; }
public MultiCommand() { }
public MultiCommand(System.Collections.Generic.List<ICommand> list)
{
this.list = list;
//Hook up listener for new Command CTX from other tasks
XEvents.CommandCTX += OnCommandCTX;
}
private void OnCommandCTX(object sender, CommandContext e)
{
//Some other task finished, update SharedContext
SharedContext = e;
}
public MultiCommand Add(ICommand cc)
{
list.Add(cc);
return this;
}
internal void Execute()
{
list.ForEach(cmd =>
{
cmd.Execute(SharedContext);
});
}
public static MultiCommand New()
{
return new MultiCommand();
}
}
Each command handles the asynchronous part similar to this:
internal class Command1 : ICommand
{
public event EventHandler CanExecuteChanged;
public bool CanExecute(object parameter)
{
throw new NotImplementedException();
}
public async void Execute(object parameter)
{
var ctx = (CommandContext)parameter;
var newCTX = await Task<CommandContext>.Run(() => {
//the command context is here running in it's own independent Task
//Any changes here are only known here, unless we return the changes using a 'closure'
//the closure is this code - var newCTX = await Task<CommandContext>Run
//newCTX is said to be 'closing' over the task results
ctx.Data = GetNewData();
return ctx;
});
newCTX.NotifyNewCommmandContext();
}
private RequiredData GetNewData()
{
throw new NotImplementedException();
}
}
Finally we set up a common event handler and notification system.
public static class XEvents
{
public static EventHandler<CommandContext> CommandCTX { get; set; }
public static void NotifyNewCommmandContext(this CommandContext ctx, [CallerMemberName] string caller = "")
{
if (CommandCTX != null) CommandCTX(caller, ctx);
}
}
Further abstractions are possible in each Command's execute function. But we won't discuss that now.
Here's what this design does and doesn't do:
It allows any finished task to update the new context on the thread it was first set in the MultiCommand class.
This assumes there is no workflow based state necessary. The post merely indicated a bunch of task only had to run asynchronous rather than in an ordered asynchronous manner.
No currencymanager is necessary because we are relying on each command's closure/completion of the asynchronous task to return the new context on the thread it was created!
If you need concurrency then that implies that the context state is important, that design is similar to this one but different. That design is easily implemented using functions and callbacks for the closure.
As long as each context is only used from a single thread concurrently there is no problem with using it from multiple threads.
I'm trying to follow RAII pattern in my service classes, meaning that when an object is constructed, it is fully initialized. However, I'm facing difficulties with asynchronous APIs. The structure of class in question looks like following
class ServiceProvider : IServiceProvider // Is only used through this interface
{
public int ImportantValue { get; set; }
public event EventHandler ImportantValueUpdated;
public ServiceProvider(IDependency1 dep1, IDependency2 dep2)
{
// IDependency1 provide an input value to calculate ImportantValue
// IDependency2 provide an async algorithm to calculate ImportantValue
}
}
I'm also targeting to get rid of side-effects in ImportantValue getter, to make it thread-safe.
Now users of ServiceProvider will create an instance of it, subscribe to an event of ImportantValue change, and get the initial ImportantValue. And here comes the problem, with the initial value. Since the ImportantValue is calculated asynchronously, the class cannot be fully initialized in constructor. It may be okay to have this value as null initially, but then I need to have some place where it will be calculated first time. A natural place for that could be the ImportantValue's getter, but I'm targeting to make it thread-safe and with no side-effects.
So I'm basically stuck with these contradictions. Could you please help me and offer some alternative? Having value initialized in constructor while nice is not really necessary, but no side-effects and thread-safety of property is mandatory.
Thanks in advance.
EDIT: One more thing to add. I'm using Ninject for instantiation, and as far as I understand, it doesn't support async methods to create a binding. While approach with initiating some Task-based operation in constructor will work, I cannot await its result.
I.e. two next approaches (offered as answers so far) will not compile, since Task is returned, not my object:
Kernel.Bind<IServiceProvider>().ToMethod(async ctx => await ServiceProvider.CreateAsync())
or
Kernel.Bind<IServiceProvider>().ToMethod(async ctx =>
{
var sp = new ServiceProvider();
await sp.InitializeAsync();
})
Simple binding will work, but I'm not awaiting the result of asynchronous initialization started in constructor, as proposed by Stephen Cleary:
Kernel.Bind<IServiceProvider>().To<ServiceProvider>();
... and that's not looking good for me.
I have a blog post that describes several approaches to async construction.
I recommend the asynchronous factory method as described by Reed, but sometimes that's not possible (e.g., dependency injection). In these cases, you can use an asynchronous initialization pattern like this:
public sealed class MyType
{
public MyType()
{
Initialization = InitializeAsync();
}
public Task Initialization { get; private set; }
private async Task InitializeAsync()
{
// Asynchronously initialize this instance.
await Task.Delay(100);
}
}
You can then construct the type normally, but keep in mind that construction only starts the asynchronous initialization. When you need the type to be initialized, your code can do:
await myTypeInstance.Initialization;
Note that if Initialization is already complete, execution (synchronously) continues past the await.
If you do want an actual asynchronous property, I have a blog post for that, too. Your situation sounds like it may benefit from AsyncLazy<T>:
public sealed class MyClass
{
public MyClass()
{
MyProperty = new AsyncLazy<int>(async () =>
{
await Task.Delay(100);
return 13;
});
}
public AsyncLazy<int> MyProperty { get; private set; }
}
One potential option would be to move this to a factory method instead of using a constructor.
Your factory method could then return a Task<ServiceProvider>, which would allow you to perform the initialization asynchronously, but not return the constructed ServiceProvider until ImportantValue has been (asynchronously) computed.
This would allow your users to write code like:
var sp = await ServiceProvider.CreateAsync();
int iv = sp.ImportantValue; // Will be initialized at this point
This is a slight modification to #StephenCleary pattern of async initialization.
The difference being the caller doesn't need to 'remember' to await the InitializationTask, or even know anything about the initializationTask (in fact it is now changed to private).
The way it works is that in every method that uses the initialized data there is an initial call to await _initializationTask. This returns instantly the second time around - because the _initializationTask object itself will have a boolean set (IsCompleted which the 'await' mechanism checks) - so don't worry about it initializing multiple times.
The only catch I'm aware of is you mustn't forget to call it in every method that uses the data.
public sealed class MyType
{
public MyType()
{
_initializationTask = InitializeAsync();
}
private Task _initializationTask;
private async Task InitializeAsync()
{
// Asynchronously initialize this instance.
_customers = await LoadCustomersAsync();
}
public async Task<Customer> LookupCustomer(string name)
{
// Waits to ensure the class has been initialized properly
// The task will only ever run once, triggered initially by the constructor
// If the task failed this will raise an exception
// Note: there are no () since this is not a method call
await _initializationTask;
return _customers[name];
}
// one way of clearing the cache
public void ClearCache()
{
InitializeAsync();
}
// another approach to clearing the cache, will wait until complete
// I don't really see a benefit to this method since any call using the
// data (like LookupCustomer) will await the initialization anyway
public async Task ClearCache2()
{
await InitializeAsync();
}
}
You could use my AsyncContainer IoC container which supports the exact same scenario as you.
It also supports other handy scenarios such as async initializers, run-time conditional factories, depend on async and sync factory functions
//The email service factory is an async method
public static async Task<EmailService> EmailServiceFactory()
{
await Task.Delay(1000);
return new EmailService();
}
class Service
{
//Constructor dependencies will be solved asynchronously:
public Service(IEmailService email)
{
}
}
var container = new Container();
//Register an async factory:
container.Register<IEmailService>(EmailServiceFactory);
//Asynchronous GetInstance:
var service = await container.GetInstanceAsync<Service>();
//Safe synchronous, will fail if the solving path is not fully synchronous:
var service = container.GetInstance<Service>();
I know this is an old question, but it's the first which appears on Google and, quite frankly, the accepted answer is a poor answer. You should never force a delay just so you can use the await operator.
A better approach to an initialization method:
private async Task<bool> InitializeAsync()
{
try{
// Initialize this instance.
}
catch{
// Handle issues
return await Task.FromResult(false);
}
return await Task.FromResult(true);
}
This will use the async framework to initialize your object, but then it will return a boolean value.
Why is this a better approach? First off, you're not forcing a delay in your code which IMHO totally defeats the purpose of using the async framework. Second, it's a good rule of thumb to return something from an async method. This way, you know if your async method actually worked/did what it was supposed to. Returning just Task is the equivalent of returning void on a non-async method.
I have a variation of Stephen Cleary's example of an asynchronous initialization pattern. You could encapsulate the Initialization property and await it in the class methods. In this case, the client code will not need to await the initialization task.
public class ClassWithAsyncInit
{
public ClassWithAsyncInit()
{
Initialization = InitializeAsync();
}
private Task Initialization { get; private set; }
private async Task InitializeAsync()
{
// your async init code
}
public async Task FirstMethod()
{
await Initialization;
// ... other code
}
}
The drawback is that it's not convenient if you have a lot of methods in your class and need to await the Initialization task in each one. But sometimes it is okay. Let's say you have a simple interface for saving some JSON objects:
public IDataSaver
{
void Save(string json);
}
And you need to implement it for a database with the asynchronous initialization logic. Considering that you would have only one public method it makes sense to encapsulate the Initialization property and await it in the Save method:
public class SomeDbDataSaver: IDataSaver
{
protected DatabaseClient DbClient { get; set; }
public SomeDbDataSaver()
{
DbClient = new DatabaseClient();
Initialization = InitializeAsync(); // start off the async init
}
private Task Initialization { get; private set; }
private async Task InitializeAsync()
{
await DbClient.CreateDatabaseIfNotExistsAsync("DatabaseName");
}
public async Task Save(string json)
{
await Initialization;
// ... code for saving a data item to the database
}
}
I have simple method in my C# app, it picks file from FTP server and parses it and stores the data in DB. I want it to be asynchronous, so that user perform other operations on App, once parsing is done he has to get message stating "Parsing is done".
I know it can achieved through asynchronous method call but I dont know how to do that can anybody help me please??
You need to use delegates and the BeginInvoke method that they contain to run another method asynchronously. A the end of the method being run by the delegate, you can notify the user. For example:
class MyClass
{
private delegate void SomeFunctionDelegate(int param1, bool param2);
private SomeFunctionDelegate sfd;
public MyClass()
{
sfd = new SomeFunctionDelegate(this.SomeFunction);
}
private void SomeFunction(int param1, bool param2)
{
// Do stuff
// Notify user
}
public void GetData()
{
// Do stuff
sfd.BeginInvoke(34, true, null, null);
}
}
Read up at http://msdn.microsoft.com/en-us/library/2e08f6yc.aspx
try this method
public static void RunAsynchronously(Action method, Action callback) {
ThreadPool.QueueUserWorkItem(_ =>
{
try {
method();
}
catch (ThreadAbortException) { /* dont report on this */ }
catch (Exception ex) {
}
// note: this will not be called if the thread is aborted
if (callback!= null) callback();
});
}
Usage:
RunAsynchronously( () => { picks file from FTP server and parses it},
() => { Console.WriteLine("Parsing is done"); } );
Any time you're doing something asynchronous, you're using a separate thread, either a new thread, or one taken from the thread pool. This means that anything you do asynchronously has to be very careful about interactions with other threads.
One way to do that is to place the code for the async thread (call it thread "A") along with all of its data into another class (call it class "A"). Make sure that thread "A" only accesses data in class "A". If thread "A" only touches class "A", and no other thread touches class "A"'s data, then there's one less problem:
public class MainClass
{
private sealed class AsyncClass
{
private int _counter;
private readonly int _maxCount;
public AsyncClass(int maxCount) { _maxCount = maxCount; }
public void Run()
{
while (_counter++ < _maxCount) { Thread.Sleep(1); }
CompletionTime = DateTime.Now;
}
public DateTime CompletionTime { get; private set; }
}
private AsyncClass _asyncInstance;
public void StartAsync()
{
var asyncDoneTime = DateTime.MinValue;
_asyncInstance = new AsyncClass(10);
Action asyncAction = _asyncInstance.Run;
asyncAction.BeginInvoke(
ar =>
{
asyncAction.EndInvoke(ar);
asyncDoneTime = _asyncInstance.CompletionTime;
}, null);
Console.WriteLine("Async task ended at {0}", asyncDoneTime);
}
}
Notice that the only part of AsyncClass that's touched from the outside is its public interface, and the only part of that which is data is CompletionTime. Note that this is only touched after the asynchronous task is complete. This means that nothing else can interfere with the tasks inner workings, and it can't interfere with anything else.
Here are two links about threading in C#
Threading in C#
Multi-threading in .NET: Introduction and suggestions
I'd start to read about the BackgroundWorker class
In Asp.Net I use a lot of static methods for jobs to be done. If its simply a job where I need no response or status, I do something simple like below. As you can see I can choose to call either ResizeImages or ResizeImagesAsync depending if I want to wait for it to finish or not
Code explanation: I use http://imageresizing.net/ to resize/crop images and the method SaveBlobPng is to store the images to Azure (cloud) but since that is irrelevant for this demo I didn't include that code. Its a good example of time consuming tasks though
private delegate void ResizeImagesDelegate(string tempuri, Dictionary<string, string> versions);
private static void ResizeImagesAsync(string tempuri, Dictionary<string, string> versions)
{
ResizeImagesDelegate worker = new ResizeImagesDelegate(ResizeImages);
worker.BeginInvoke(tempuri, versions, deletetemp, null, null);
}
private static void ResizeImages(string tempuri, Dictionary<string, string> versions)
{
//the job, whatever it might be
foreach (var item in versions)
{
var image = ImageBuilder.Current.Build(tempuri, new ResizeSettings(item.Value));
SaveBlobPng(image, item.Key);
image.Dispose();
}
}
Or going for threading so you dont have to bother with Delegates
private static void ResizeImagesAsync(string tempuri, Dictionary<string, string> versions)
{
Thread t = new Thread (() => ResizeImages(tempuri, versions, null, null));
t.Start();
}
ThreadPool.QueueUserWorkItem is the quickest way to get a process running on a different thread.
Be aware that UI objects have "thread affinity" and cannot be accessed from any thread other than the one that created them.
So, in addition to checking out the ThreadPool (or using the asynchronous programming model via delegates), you need to check out Dispatchers (wpf) or InvokeRequired (winforms).
In the end you will have to use some sort of threading. The way it basically works is that you start a function with a new thread and it will run until the end of the function.
If you are using Windows Forms then a nice wrapper that they have for this is call the Background Worker. It allows you to work in the background with out locking up the UI form and even provides a way to communicate with the forms and provide progress update events.
Background Worker
.NET got new keyword async for asonchrynous functions. You can start digging at learn.microsoft.com (async). The shortest general howto make function asonchrynous is to change function F:
Object F(Object args)
{
...
return RESULT;
}
to something like this:
async Task<Object> FAsync(Object args)
{
...
await RESULT_FROM_PROMISE;
...
return RESULT;
}
The most important thing in above code is that when your code approach await keyword it return control to function that called FAsync and make other computation until promissed value has been returned and procede with rest of code in function FAsync.