I have a small MVC 5 application that calls a web service, and receives a JSON response. I deserialise the response into my own type and it gets passed on to the view and data is displayed by razor.
The controller handler:
public async Task<ActionResult> Search(string q)
{
var vm = new SearchResultViewModel(await _searchService.GetDataAsync(q));
return View(vm);
}
The search service method:
public async Task<ISearchResult> GetDataAsync(string q)
{
var fullRequest = new UriBuilder(RequestUri) {Query = "q=" + q};
var result = await _client.GetAsync(fullRequest.ToString()).ConfigureAwait(false);
if (result.IsSuccessStatusCode)
{
var jsonResponse = await result.Content.ReadAsStringAsync().ConfigureAwait(false);
// How should I call this?
return JsonConvert.DeserializeObject<SearchResult>(jsonResponse);
}
return new SearchResult
}
My question: How should I call JsonConvert.DeserializeObject? It's an inherently CPU bound operation, so is it ok to call synchronously (and block the thread) since I can't return until it's done anyway? If there's a problem with deserialisation, a cancellation token couldn't be used.
If I should call asynchronously, should I use Task.Factory.StartNew() as suggested by intellisense, as a replacement for the deprecated JsonConvert.DeserializeObjectAsync()? This Channel 9 video suggests (at 58mins) that this isn't such a good idea. Perhaps another option, such as Task.Run()? Possibly a bad idea since it might cause SyncContext issues?
Any pointers gratefully received!
Your code is good as is. DeserializeObject will run inside a thread-pool thread since you are using ConfigureAwait(false).
Your overall method (GetDataAsync) would still be asynchronous since it will return to the caller on the first await.
Related
(Edited to add more detail about every call I'm making)
I have a Xamarin Forms application connecting to a .Net Core 2.2 web service hosted in Azure App Services.
In my view model I have a call like this:
private async Task GetItems() {
var result = await itemsListFactory.GetItemsAsync()
}
Which calls this:
public async Task<IEnumerable<IItemInfo>> GetItemsAsync() {
return await ItemList.GetItemListAsync();
}
Which calls this (CSLA business object):
public static async Task<ItemList> GetItemListAsync() {
return await DataPortal.FetchAsync<ItemList>();
}
Which calls this:
[Fetch]
private async void DataPortal_Fetch() {
var rlce = RaiseListChangedEvents;
RaiseListChangedEvents = false;
IsReadOnly = false;
using (var ctx = Dal.DalFactory.GetManager()) {
var dal = ctx.GetProvider<IItemDal>();
List<ItemDto> list = null;
list = await dal.FetchAsync();
foreach (var item in list) {
Add(DataPortal.FetchChild<ItemInfo>(item));
}
}
IsReadOnly = true;
RaiseListChangedEvents = rlce;
}
Which calls:
public async Task<List<ItemDto>> FetchAsync() {
var resultSet = new List<ItemDto>();
var connectionManager = ServiceLocator.Current.GetInstance<IAzureConnectionManager>();
using (var conn = await connectionManager.GetOpenConnectionAsync()) {
/* Reading from DB */
}
return resultSet;
}
The implementation of the AzureConnectionManager looks like this:
public async Task<SqlConnection> GetOpenConnectionAsync()
{
var accessToken = await new AzureServiceTokenProvider().GetAccessTokenAsync("https://database.windows.net/");
var connection = new SqlConnection(dbconnection) {
AccessToken = accessToken
};
await connection.OpenAsync();
return connection;
}
However, the first time I make this call (e.g. first call of the day, or after not using the service for a while) I get no results back. Any subsequent calls seem to work just fine. My guess is this has something to do with the service having to take a few "extra steps" to return data due to inactivity.
This suspicion seems to be confirmed whenever I debug the web service and set breakpoints in my view model as well as the server-side code. Whenever the service's call returns with no records it's almost as if it's returning early from the server, because it returns to the view model with no data, and then my debugger hops back onto the server after it's received the access token. So, it's as if my code decided not to wait for the GetAccessTokenAsync and OpenAsync to finish what they had to do before returning to the client.
I can fix this by adding a .Result to GetAccessTokenAsync() and .Wait() to OpenAsync() like this:
public async Task<SqlConnection> GetOpenConnectionAsync()
{
var accessToken = new AzureServiceTokenProvider().GetAccessTokenAsync("https://database.windows.net/").Result;
var connection = new SqlConnection(dbconnection) {
AccessToken = accessToken
};
connection.OpenAsync().Wait();
return connection;
}
But this feels like a hack.
I doubt this is the way I'm supposed to fix this, but maybe it is. At the very least I'd like to just understand what's going on here if this is the correct way to handle this situation.
The await operator suspends evaluation of the enclosing async method until the asynchronous operation represented by its operand completes. When the asynchronous operation completes, the await operator returns the result of the operation, if any. When the await operator is applied to the operand that represents already completed operation, it returns the result of the operation immediately without suspension of the enclosing method. The await operator doesn't block the thread that evaluates the async method. When the await operator suspends the enclosing async method, the control returns to the caller of the method.
Official Document on this
So if we look at the what the documents say about Async/Await you'll notice that
When the await operator is applied to the operand that represents already completed operation, it returns the result of the operation immediately without suspension of the enclosing method.
More then likely OpenAsync(); Is seen as a operand that's already completed as you might not be awaiting your Returns, So the Operation Runs retrieve's your data but because your not suspending anything in OpenAsync It might assume the operand is already completed on the first instance and just continue then the data is loaded so on your second attempt you have the data to work with, as its already populated on the first try.
So i'd like to see a bit more code actually.
However one thing I will say is that .Wait() is Bad If you have to wait for a result and force that wait the better way to do this is .GetAwaiter().GetResult() I can link you a Seminar that explains in details about this. But in Essence .Wait() Throws exceptions into the void and make them extremly difficult to track(Or at-least far more difficult then you'd want them to be)
"Also note in no way am I anywhere near a Expert in Async/Await so feel free to correct me"
Im quite new to writing controllers for asp.net and Im trying to return IQueryable, but I cant seem to get the call for the content to return.
This is my controller:
// GET: api/RumsaRooms
[EnableQuery]
public IQueryable<RumsaRoom> GetRooms()
{
return db.Rooms;
}
and this is my client call:
public async Task<IQueryable<T>> GetAllOf<T>()
{
var typeName = typeof(T).Name;
var result = await _client.GetAsync($"api/{typeName}");
if (!result.IsSuccessStatusCode)
{
var exception = await result.Content.ReadAsStringAsync();
}
//This method never returns
var rooms = await result.Content.ReadAsAsync<IQueryable<T>>();
return rooms;
}
I have enabled multipleactiveresultsets in the connectionstring.
The StatusCode is 200.
The method that GetAllOf() looks like this:
private async Task<bool> LoadEntities()
{
var rooms = (await _rumsaClient.GetAllOf<RumsaRoom>()).ToList();
RoomsCollection = new ObservableCollection<RumsaRoom>(rooms);
return true;
}
LoadAllEntities is called in the constructor of my viewmodel.
If I change the call to this it works:
var rooms = await result.Content.ReadAsAsync<List<T>>();
Is it not possible to ReadAsAsync to a IQueryable?
Thanks
Erik
Your problem is almost certainly in this code:
LoadAllEntities is called in the constructor of my viewmodel.
I explain why this deadlock happens in detail on my blog. It doesn't have anything to do with ReadAsAsync or IQueryable. It has to do with calling Wait or Result on an asynchronous task.
In summary:
Tasks returned by async methods are only completed when that method completes.
await by default captures a "context" and uses that "context" to resume the async method.
On ASP.NET, this "context" is an instance of AspNetSynchronizationContext, which only allows one thread in at a time.
When the code calls Wait/Result, it will block the thread (which is still in the ASP.NET request context), waiting for the task to complete.
When the await is ready to resume the method, it does so in the captured context, and waits for the context to be free.
Since await cannot complete the method until the context is free, and the context is in use by a thread waiting until the method completes, you end up with a deadlock.
The proper way to solve this is to not block on asynchronous code; use await instead. This principle is called "async all the way", and is described in my MSDN article on async best practices. Since you're trying to call asynchronous code from a constructor, you may also find my blog post on async constructors helpful, which explains some alternative approaches.
This question already has answers here:
How to call asynchronous method from synchronous method in C#?
(17 answers)
Closed 6 years ago.
I have the below method:
public string RetrieveHolidayDatesFromSource() {
var result = this.RetrieveHolidayDatesFromSourceAsync();
/** Do stuff **/
var returnedResult = this.TransformResults(result.Result); /** Where result gets used **/
return returnedResult;
}
private async Task<string> RetrieveHolidayDatesFromSourceAsync() {
using (var httpClient = new HttpClient()) {
var json = await httpClient.GetStringAsync(SourceURI);
return json;
}
}
The above does not work and seems to not return any results properly. I am not sure where I am missing a statement to force the await of a result? I want the RetrieveHolidayDatesFromSource() method to return a string.
The below works fine but it is synchronous and I believe it can be improved upon? Note that the below is synchronous in which I would like to change to Asynchronous but am unable to wrap my head around for some reason.
public string RetrieveHolidayDatesFromSource() {
var result = this.RetrieveHolidayDatesFromSourceAsync();
/** Do Stuff **/
var returnedResult = this.TransformResults(result); /** This is where Result is actually used**/
return returnedResult;
}
private string RetrieveHolidayDatesFromSourceAsync() {
using (var httpClient = new HttpClient()) {
var json = httpClient.GetStringAsync(SourceURI);
return json.Result;
}
}
Am I missing something?
Note: For some reason, when I breakpoint the above Async Method, when it gets to the line var json = await httpClient.GetStringAsync(SourceURI) it just goes out of breakpoint and I can't go back into the method.
Am I missing something?
Yes. Asynchronous code - by its nature - implies that the current thread is not used while the operation is in progress. Synchronous code - by its nature - implies that the current thread is blocked while the operation is in progress. This is why calling asynchronous code from synchronous code literally doesn't even make sense. In fact, as I describe on my blog, a naive approach (using Result/Wait) can easily result in deadlocks.
The first thing to consider is: should my API be synchronous or asynchronous? If it deals with I/O (as in this example), it should be asynchronous. So, this would be a more appropriate design:
public async Task<string> RetrieveHolidayDatesFromSourceAsync() {
var result = await this.DoRetrieveHolidayDatesFromSourceAsync();
/** Do stuff **/
var returnedResult = this.TransformResults(result); /** Where result gets used **/
return returnedResult;
}
As I describe in my async best practices article, you should go "async all the way". If you don't, you won't get any benefit out of async anyway, so why bother?
But let's say that you're interested in eventually going async, but right now you can't change everything, you just want to change part of your app. That's a pretty common situation.
In that case, the proper approach is to expose both synchronous and asynchronous APIs. Eventually, after all the other code is upgraded, the synchronous APIs can be removed. I explore a variety of options for this kind of scenario in my article on brownfield async development; my personal favorite is the "bool parameter hack", which would look like this:
public string RetrieveHolidayDatesFromSource() {
return this.DoRetrieveHolidayDatesFromSourceAsync(sync: true).GetAwaiter().GetResult();
}
public Task<string> RetrieveHolidayDatesFromSourceAsync() {
return this.DoRetrieveHolidayDatesFromSourceAsync(sync: false);
}
private async Task<string> DoRetrieveHolidayDatesFromSourceAsync(bool sync) {
var result = await this.GetHolidayDatesAsync(sync);
/** Do stuff **/
var returnedResult = this.TransformResults(result);
return returnedResult;
}
private async Task<string> GetHolidayDatesAsync(bool sync) {
using (var client = new WebClient()) {
return sync
? client.DownloadString(SourceURI)
: await client.DownloadStringTaskAsync(SourceURI);
}
}
This approach avoids code duplication and also avoids any deadlock or reentrancy problems common with other "sync-over-async" antipattern solutions.
Note that I would still treat the resulting code as an "intermediate step" on the path to a properly-asynchronous API. In particular, the inner code had to fall back on WebClient (which supports both sync and async) instead of the preferred HttpClient (which only supports async). Once all the calling code is changed to use RetrieveHolidayDatesFromSourceAsync and not RetrieveHolidayDatesFromSource, then I'd revisit this and remove all the tech debt, changing it to use HttpClient and be async-only.
public string RetrieveHolidayDatesFromSource() {
var result = this.RetrieveHolidayDatesFromSourceAsync().Result;
/** Do stuff **/
var returnedResult = this.TransformResults(result.Result); /** Where result gets used **/
return returnedResult;
}
If you add .Result to the async call, it will execute and wait for the result to arrive, forcing it to be synchronous
UPDATE:
private static string stringTest()
{
return getStringAsync().Result;
}
private static async Task<string> getStringAsync()
{
return await Task.FromResult<string>("Hello");
}
static void Main(string[] args)
{
Console.WriteLine(stringTest());
}
To address the comment: This works without any problems.
I want to read a XML file from the Web with following method.
public static async void Load_WinPhone(string URL)
{
HttpClient client = new HttpClient();
var httpResponseMessage = await client.GetAsync(new Uri(URL));
if (httpResponseMessage.StatusCode == System.Net.HttpStatusCode.OK)
{
var xmlStream = await httpResponseMessage.Content.ReadAsStreamAsync();
XDocument Xdoc = XDocument.Load(xmlStream);
var query = from data in Xdoc.Descendants("article")
select new MyClass
{
Title = data.Element("title").Value
}
foreach (MyClass x in query)
{
AnotherClass.List.Add(x);
}
}
This Works, but after the method finished the AnotherClass.List is still empty.
I think it is because of the async, I tried this in the console without the async and it worked fine.
But now i want to to this on a Windows Phone 8.1 and the list stays empty.
Can someone explain me why or even have a workaround for this?
Yes, that's how await works - from the point of view of the caller, it's basically the same thing as a return. So when you call this method, it most likely returns on the first await - long before AnotherClass.List is modified.
The main problem you have is that your method is async void - you're throwing away all the information about the method's execution. Instead, you want to return Task - this allows you to await the method or bind a continuation to it.
Whenever you break the await chain, you also break the synchronicity of the code. Most of the time (especially in UI), you want to await all the way to the top - usually, the only thing that's async void is the event handlers, and even then it's only because event handlers must return void.
Overall, multi-threading and asynchronous code is a rather big topic - http://www.albahari.com/threading/ is a great start on understanding most of the fundamentals, as well as ways to handle it well in C#.
I have a webservice that loads up some plugins (dlls) and calls their Process method. One of the plugins takes a list of members and ensures that they are all included in a MailChimp list.
Here is the code that adds the users to the MailChimp group.
private async Task AddMCUsers(List<Member> _memberList)
{
using (var http = new HttpClient())
{
var creds = Convert.ToBase64String(Encoding.ASCII.GetBytes("user:password");
http.DefaultRequestHeaders.Authorization = new System.Net.Http.Headers.AuthenticationHeaderValue("Basic", creds);
string memberURI = string.Format(#"{0}lists/{1}/members", _baseURI, _memberGroupId);
var jss = new JavaScriptSerializer();
foreach (var user in _memberlist)
{
var _addStatus = "";
try
{
var content = jss.Serialize(new MCPost()
{
email_address = user.Email,
status = "subscribed",
merge_fields = new MCMergeFields()
{
FNAME = user.Firstname,
LNAME = user.Lastname
}
});
using(var result = await http.PostAsync(memberURI, new StringContent(content,Encoding.UTF8, "application/json")))
{
var resultText = await result.Content.ReadAsStringAsync();
if(result.IsSuccessStatusCode)
{
_addStatus = "Success";
var _returnedUser = jss.Deserialize<MCMember>(resultText);
//Store new user's id
user.ServiceId = _returnedUser.id;
}
else
{
_addStatus = "Fail";
}
}
}
catch {
_addStatus = "Error";
}
LogEvent("Add User - " + _addStatus, string.Format("Id: {0} - {1} {2} (Account: {3}) : {4}", user.Id, user.Firstname, user.Lastname, user.AccountId, user.Email));
}
}
}
In normal procedural code, this wouldn't be a problem. However, the only Post method available on the httpClient was PostAsync. Being fairly new to the async/await stuff, I'm not sure the ramifications on the rest of my code ... particularly as it relates to my attempt to reuse the httpClient instead of instantiating a new one for each http call.
I'm not sure what happens with await when its wrapped in a foreach like I have. Will I run into issues with reusing the httpClient to make repeated calls when running asynchronously?
My other question is, what is actually going to be returned. IOW, my understanding is that await returns a Task. However, here, I'm looping through the list and making multiple calls to await PostAsync. My method returns a Task. But which task gets returned? If my calling method needs to wait for completion before moving on, what does its call look like?
private void Process()
{
//Get List
var task = AddMCUsers(list);
task.Wait();
//Subsequent processing
}
I've read that you should use Async all the way. Does this mean my calling method should look more like this?
public async Task Process()
{
//Get list
...
await AddMCUsers(list);
//Other processing
}
Thanks to whatever help you can offer on this.
In normal procedural code, this wouldn't be a problem.
The whole point of async/await is to write asynchronous code in a way that looks practically identical to "normal" synchronous code.
Being fairly new to the async/await stuff, I'm not sure the ramifications on the rest of my code ... particularly as it relates to my attempt to reuse the httpClient instead of instantiating a new one for each http call.
HttpClient was intended to be reused; in fact, it can be used for any number of calls simultaneously.
I'm not sure what happens with await when its wrapped in a foreach like I have.
One way to think of it is that await "pauses" the method until its operation completes. When the operation completes, then the method continues executing. I have an async intro that goes into more detail.
Will I run into issues with reusing the httpClient to make repeated calls when running asynchronously?
No, that's fine.
IOW, my understanding is that await returns a Task.
await takes a Task. It "unwraps" that task and returns the result of the task (if any). If the task completed with an exception, then await raises that exception.
My method returns a Task. But which task gets returned?
The Task returned from an async method is created by the async state machine. You don't have to worry about it. See my intro for more details.
If my calling method needs to wait for completion before moving on, what does its call look like? ... I've read that you should use Async all the way. Does this mean my calling method should look more like this?
Yes, it should look like your second snippet:
public async Task ProcessAsync()
{
//Get list
...
await AddMCUsers(list);
//Other processing
}
The only thing I changed was the Async suffix, which is recommended by the Task-based Asynchronous Pattern.
in your code you should be fine with reusing the HttpClient. What async / await is allow the code to release the execution thread to prevent locking a cpu thread while waiting for the web response. It also releases control back to the caller. When releasing code back to the caller it means that if your Process function does not await your AddMCUsers, Process could finish before AddMCUsers (useful in fire and forget situations to not await a method).
What async/await do not do is affect the logical flow of an individual method. When you await an async web call the execution is paused and then resumed at the same point once the web call returns. There is also thread context tracking and the code resumes in the same context (ie. UI thread or background thread depending on the parent) by default, but this can be changed if needed.
At some point in your code you may want to have a method that blocks until your async code competes and that is where you will want your Task.Wait() call to block execution. If all you use is awaits then it is possible for your program to end before your task competes. See the code example below.
class Program
{
static void Main(string[] args)
{
Task waitForMe = Task.Run(() => waitAsync());
}
static async Task waitAsync()
{
await Task.Delay(5000);
}
}
in the sample with out a Task.Wait call to block the Main method the program will end before the 5 second wait is complete. Having a main method of the following will cause the program to wait for 5 seconds before exiting:
static void Main(string[] args)
{
Task waitForMe = Task.Run(() => waitAsync());
waitForMe.Wait();
}