Develop Reference

How can I read return value from Async method in C#?

My async method is as below:
public async Task<List<object>> handleSummaryOfWallets()
{
string token = giveMeToken("URL AND CREDS");
Channel channel = new Channel("NANANANA GIROUD", ChannelCredentials.Insecure);
OMGadminAPI.OMGadminAPIClient client = new OMGadminAPI.OMGadminAPIClient(channel);
var summaryBalancesParams = new OMGadminAPIGetCurrenciesSummariesParams();
summaryBalancesParams.AdminAuthTokenSecret = token;
List<object> summariesCurrenciesOMGadmin = new List<object>();
using (var call = client.GetCurrenciesSummaries(summaryBalancesParams))
{
while (await call.ResponseStream.MoveNext())
{
OMGadminAPICurrencySummary currencySummary = call.ResponseStream.Current;
summariesCurrenciesOMGadmin.Add(currencySummary);
Console.WriteLine(summariesCurrenciesOMGadmin);
}
return summariesCurrenciesOMGadmin;
}
}
As you can see, above async method returns list of objects. I call this method as below:
var listOfBalances = balances.handleSummaryOfWallets().Wait();
and it gives me error:
Error CS0815: Cannot assign void to an implicitly-typed variable
From the error, I understand that this is not correct way to call async method. But I need to read ready list of objects from async fetched data. Its request-response, no real stable stream. So I need to generate this list only once per request. I'm using gRPC framework for RPC calls.
Please help me fetch this data and make ready to use.

The Task.Wait method waits for the Task to complete execution. It returns void. That is the reason why the exception.
Now to overcome the exception and to read the return value, one way is as mentioned in other answer and the comments; await the call as below:
public async void TestAsync()
{
var listOfBalances = await handleSummaryOfWallets();
}
Note that your calling method should also be async method now.
As you are calling Wait in your code, it looks that you want the result immediately; you have nothing else left to do that does not depend on result. In that case, you may choose to stop async chain by calling Wait. But you need to do some changes as below:
public void TestAsync()
{
var task = handleSummaryOfWallets();//Just call the method which will return the Task<List<object>>.
task.Wait();//Call Wait on the task. This will hold the execution until complete execution is done.
var listOfBalances = task.Result;//Task is executed completely. Read the result.
}
Note that calling method is no longer async. Other explanation is given in code-comments.
Other short alternative to above code is as below:
public void TestAsync()
{
var listOfBalances = handleSummaryOfWallets().Result;
}

Just use await while calling your method
var listOfBalances = await balances.handleSummaryOfWallets();

Async/await, run blocking the UI without obvious reason

I have been reasearching once again the async tasks. No mater how i set the Tasks, my application suffers from UI freeze all the time. I have the following code for downloading the string from a webpage:
internal string DownloadString(string URL)
{
var result = LoadCompanyContracts(URL);
return result.Result;
}
internal async Task<string> LoadCompanyContracts(string URL)
{
Task<string> task2 = Task<string>.Factory.StartNew(() =>
{
for (int i = 0; i <= 10000000; i++) Console.WriteLine(i);
WebClient wc = new WebClient();
string tmp = wc.DownloadString(new Uri(URL));
return tmp;
});
return task2.Result;
}
When i execute this task and during the for loop the UI of my application is freezing. Even though i believe that this code should not freeze the UI i am not able to find a solution. I have tried many different options and really want to use tasks instead of threads or events with webclient async.
Info: I am using .net 4.5 for my project. The difference in my code is that these functions are inside a class library(don't know if it matters).
Is it possible to run this code without blocking the user interface with async await by calling the DownloadString function from my code? If not what are the alternatives(any good nuget packages)?

The async keyword doesn't make something run asynchronously, it enables you to use await to await an already asynchronous operation. You need to use
DownloadStringTaskAsync to truly download in an asynchronous manner:
internal async Task<string> LoadCompanyContracts(string URL)
{
....
using(var wc = new WebClient())
{
string tmp = await wc.DownloadStringTaskAsync(new Uri(URL));
return tmp;
}
}
await by itself returns execution in the original execution context (ie the UI thread). This may or may not be desirable, which is why library code typically uses ConfigureAwait(false); and lets the final user of the library to decide how to await:
string tmp = await wc.DownloadStringTaskAsync(new Uri(URL))
.ConfigureAwait(false);
Finally, there's no point in awaiting if you are going to call .Result from the top-level function. There is no point in using await at all if you don't want to do use the method's result in your code. LoadCompanyContracts could be just:
internal Task<string> LoadCompanyContracts(string URL)
{
....
using(var wc = new WebClient())
{
return wc.DownloadStringTaskAsync(new Uri(URL))
.ConfigureAwait(false);
}
}
Oops
Typically, you don't need to use await at all if you just return the result of an asynchronous operation. The method could just return wc.DownloadStringTaskAsync(..); BUT that would cause the method to return and dispose the WebClient before download finishes. Avoiding the using block isn't a solution either, as it will let an expensive object like WebClient alive longer than necessary.
That's why HttpClient is preferable to WebClient: a single instance supports multiple concurrent calls, which means you can have just one instance eg as a field and reuse it, eg:
HttpClient _myClient =new HttpClient();
internal Task<string> LoadCompanyContractsAsync(string URL)
{
....
return _myClient.GetStringAsync(new Uri(URL))
.ConfigureAwait(false);
}
}
You could get rid of your DownloadString since it doesn't do anything on top of LoadCompanyContracts. If it does use the result of LoadCompanyContracts, it should be rewritten as:
internal async Task<string> DownloadString(string URL)
{
var result = await LoadCompanyContracts(URL);
//Do something with the result
return result;
}
EDIT
The original answer used DownloadStringAsync which is a legacy method that raises an event when download completes. The correct method is DownloadStringTaskAsync
EDIT 2
Since we are talking about a UI, the code can be made asynchronous all the way to the top event handler by using the async void syntax for the handler, eg async void Button1_Click, eg:
async void LoadCustomers_Click(...)
{
var contracts=await LoaCompanyContracts(_companyUrls);
txtContracts>Text=contracts;
}
In this case we want to return to the original thread, so we don't use ConfigureAwait(false);

Calling Async in a Sync method

I've been reading examples for a long time now, but unfortunately I've been unable to apply the solutions to the code I'm working with. Some quick Facts/Assorted Info:
1) I'm new to C#
2) The code posted below is modified from Amazon Web Services (mostly stock)
3) Purpose of code is to compare server info to offline already downloaded info and create a list of need to download files. This snip is for the list made from the server side, only option with AWS is to call async, but I need this to finish before moving forward.
public void InitiateSearch()
{
UnityInitializer.AttachToGameObject(this.gameObject);
//these are the access key and secret access key for credentials
BasicAWSCredentials credentials = new BasicAWSCredentials("secret key", "very secret key");
AmazonS3Config S3Config = new AmazonS3Config()
{
ServiceURL = ("url"),
RegionEndpoint = RegionEndpoint.blahblah
};
//Setting the client to be used in the call below
AmazonS3Client Client = new AmazonS3Client(credentials, S3Config);
var request = new ListObjectsRequest()
{
BucketName = "thebucket"
};
Client.ListObjectsAsync(request, (responseObject) =>
{
if (responseObject.Exception == null)
{
responseObject.Response.S3Objects.ForEach((o) =>
{
int StartCut = o.Key.IndexOf(SearchType) - 11;
if (SearchType == o.Key.Substring(o.Key.IndexOf(SearchType), SearchType.Length))
{
if (ZipCode == o.Key.Substring(StartCut + 12 + SearchType.Length, 5))
{
AWSFileList.Add(o.Key + ", " + o.LastModified);
}
}
}
);
}
else
{
Debug.Log(responseObject.Exception);
}
});
}
I have no idea how to apply await to the Client.ListObjectsAsync line, I'm hoping you all can give me some guidance and let me keep my hair for a few more years.

You can either mark your method async and await it, or you can call .Wait() or .Result() on the Task you're given back.

I have no idea how to apply await to the Client.ListObjectsAsync line
You probably just put await in front of it:
await Client.ListObjectsAsync(request, (responseObject) => ...
As soon as you do this, Visual Studio will give you an error. Take a good look at the error message, because it tells you exactly what to do next (mark InitiateSearch with async and change its return type to Task):
public async Task InitiateSearchAsync()
(it's also a good idea to add an Async suffix to follow the common pattern).
Next, you'd add an await everywhere that InitiateSearchAsync is called, and so on.

I'm assuming Client.ListObjectsAsync returns a Task object, so a solution for your specific problem would be this:
public async void InitiateSearch()
{
//code
var collection = await Client.ListObjectsAsync(request, (responseObject) =>
{
//code
});
foreach (var item in collection)
{
//do stuff with item
}
}
the variable result will now be filled with the objects. You may want to set the return type of InitiateSearch() to Task, so you can await it too.
await InitiateSearch(); //like this
If this method is an event handler of some sort (like called by the click of a button), then you can keep using void as return type.
A simple introduction from an unpublished part of the documentation for async-await:
Three things are needed to use async-await:
The Task object: This object is returned by a method which is executed asynchronous. It allows you to control the execution of the method.
The await keyword: "Awaits" a Task. Put this keyword before the Task to asynchronously wait for it to finish
The async keyword: All methods which use the await keyword have to be marked as async
A small example which demonstrates the usage of this keywords
public async Task DoStuffAsync()
{
var result = await DownloadFromWebpageAsync(); //calls method and waits till execution finished
var task = WriteTextAsync(#"temp.txt", result); //starts saving the string to a file, continues execution right await
Debug.Write("this is executed parallel with WriteTextAsync!"); //executed parallel with WriteTextAsync!
await task; //wait for WriteTextAsync to finish execution
}
private async Task<string> DownloadFromWebpageAsync()
{
using (var client = new WebClient())
{
return await client.DownloadStringTaskAsync(new Uri("http://stackoverflow.com"));
}
}
private async Task WriteTextAsync(string filePath, string text)
{
byte[] encodedText = Encoding.Unicode.GetBytes(text);
using (FileStream sourceStream = new FileStream(filePath, FileMode.Append))
{
await sourceStream.WriteAsync(encodedText, 0, encodedText.Length);
}
}
Some thing to note:
You can specify a return value from an asynchronous operations with Task. The await keyword waits till the execution of the method finishes, and returns the string.
the Task object contains the status of the execution of the method, it can be used as any other variable.
if an exception is thrown (for example by the WebClient) it bubbles up at the first time the await keyword is used (in this example at the line string result (...))
It is recommended to name methods which return the Task object as MethodNameAsync
For more information about this take a look at http://blog.stephencleary.com/2012/02/async-and-await.html.

Calling HTTPRequest takes too long for large number of calls

I have to call 100,000 urls and I don't need to wait for the response. I have searched a lot . some people say there is no way to call http request without waiting for the response, also there are few answers for questions like mine that say to set Method="POST".
The following is the source code based on all of them. I have tried to call the urls asynchronously using WhenAll.
The problem is this that when I see CPU usage in task manager, it is fully busy for 140 seconds and within that time the system is almost unusable.
protected async void btnStartCallWhenAll_Click(object sender, EventArgs e)
{
// Make a list of web addresses.
List<string> urlList = SetUpURLList(Convert.ToInt32(txtNoRecordsToAdd.Text));
// One-step async call.
await ProcessAllURLSAsync(urlList);
}
private async Task ProcessAllURLSAsync(List<string> urlList)
{
// Create a query.
IEnumerable<Task<int>> CallingTasksQuery =
from url in urlList select ProcessURLAsync(url);
// Use ToArray to execute the query and start the Calling tasks.
Task<int>[] CallingTasks = CallingTasksQuery.ToArray();
// Await the completion of all the running tasks.
int[] lengths = await Task.WhenAll(CallingTasks);
int total = lengths.Sum();
}
private async Task<int> ProcessURLAsync(string url)
{
await CallURLAsync(url);
return 1;
}
private async Task CallURLAsync(string url)
{
// Initialize an HttpWebRequest for the current URL.
var webReq = (HttpWebRequest)WebRequest.Create(url);
webReq.Method="POST";
// Send the request to the Internet resource and wait for the response.
Task<WebResponse> responseTask = webReq.GetResponseAsync() ;
}
private List<string> SetUpURLList(int No)
{
List<string> urls = new List<string>
{
};
for (int i = 1; i <= No; i++)
urls.Add("http://msdn.microsoft.com/library/windows/apps/br211380.aspx");
return urls;
}
By the way the compiler hints that "this async method lacks 'await' operators and will run synchronously consider using the await operator to await non-blocking api calls or await task.run(..) to do cpu bound work on background thread" for this line:
private async Task CallURLAsync(string url).
I don't know if this affect my problem or not but after searching same problem in other questions, they said that I need to disable compiler message before this line.

Want to understand async

I've used async coding a little bit but I don't really fully understand how to use it -- though I understand the concept and why I need it.
Here's my set up:
I have a Web API that I will call from my ASP.NET MVC app and my Web API will call DocumentDB. In code samples, I see a lot of await keywords while sending queries to DocumentDB.
I'm confused if I need to make my Index action method in my MVC app async?
I'm also confused if my CreateEmployee() method in my Web API should be async?
What is the right way to use async in this scenario?
Here's my code (This code is currently giving me errors because my MVC action method is not async)
---- ASP.NET MVC App Code ----
public ActionResult Index()
{
Employee emp = new Employee();
emp.FirstName = "John";
emp.LastName = "Doe";
emp.Gender = "M";
emp.Ssn = "123-45-6789";
using (var client = new HttpClient())
{
client.BaseAddress = new Uri("http://myWebApi.com");
client.DefaultRequestHeaders.Accept.Clear();
client.DefaultRequestHeaders.Accept.Add(new MediaTypeWithQualityHeaderValue("application/json"));
HttpResponseMessage response = await client.PostAsJsonAsync("hr/create/newemployee", emp);
if (response.IsSuccessStatusCode)
{
emp = await response.Content.ReadAsAsync<Employee>();
}
}
// Display employee info
return View(emp);
}
---- Web API Code ----
private static readonly string endPointUrl = ConfigurationManager.AppSettings["EndPointUrl"];
private static readonly string authorizationKey = ConfigurationManager.AppSettings["AuthorizationKey"];
private static readonly string databaseId = ConfigurationManager.AppSettings["DatabaseId"];
private static DocumentClient client;
public static async Task<Employee> CreateEmployee(Employee emp)
{
try
{
//Create a Document client
using (client = new DocumentClient(new Uri(endPointUrl), authorizationKey))
{
//Get the database
var database = await GetDatabaseAsync();
//Get the Document Collection
var collection = await GetCollectionAsync(database.SelfLink, "Employees");
await client.CreateDocumentAsync(collection.SelfLink, emp);
// Further process employee
}
}
catch
{
// Handle error
}
return employee;
}
private static async Task<DocumentCollection> GetCollectionAsync(string dbLink, string id)
{
DocumentCollection collection = client.CreateDocumentCollectionQuery(dbLink).Where(c => c.Id == id).ToArray().FirstOrDefault();
return collection;
}
private static async Task<Database> GetDatabaseAsync()
{
Database database = client.CreateDatabaseQuery().Where(db => db.Id == databaseId).ToArray().FirstOrDefault();
return database;
}

Here's my explanation
class MainClass
{
public static async Task<String> AsyncMethod(int delay) {
await Task.Delay (TimeSpan.FromSeconds(delay));
return "The method has finished it's execution after waiting for " + delay + " seconds";
}
public static async Task Approach1(int delay)
{
var response = await AsyncMethod (delay); // await just unwraps Task's result
Console.WriteLine (response);
}
public static Task Approach2(int delay)
{
return AsyncMethod(delay).ContinueWith(message => Console.WriteLine(message)); // you could do the same with
}
public static void Main (string[] args)
{
var operation1 = Approach1 (3);
var operation2 = Approach2 (5);
Task.WaitAll (operation1, operation2);
Console.WriteLine("All operations are completed")
}
}
Eventually both Approach1 and Approach2 are identical pieces of code.
The async/await is syntactic sugar around Task API. It takes your async method splits it into parts before await, and after await. The "before" part is executed immediately. The "after" part is getting executed when await operation is completed. You are able to track the second part of operation via the Task API since you get a reference to a Task.
In general async allows to treat a method call as a some sort of long operation that you can reference via the Task API and wait until it is finished and continue with another piece of code. Either via ContinueWith call of via using await in general it's the same.
Before async/await/Task concepts people were using callbacks, but handling errors was as easy as hell, the Task is similar to a concept of callback except that it is able allow handling exceptions more easily.
In general all this Task/async/await mantra is close to concept of promises if it happen that you've worked with jQuery/JavaScript there's a similar concept here's a nice question explaining how it's done there "jQuery deferreds and promises - .then() vs .done()"
Edit: I've just found out that .NET lacks implementation of then functionality similar to one found in jQuery/JavaScript.
The difference between ContinueWith and Then is that Then is able to compose task, and to execute them sequentially while ContinueWith is not, it is able only to launch task in parallel, but it can be easily implemented via the await construct. Here is my updated code containing the whole shebang:
static class Extensions
{
// Implementation to jQuery-like `then` function in .NET
// According to: http://blogs.msdn.com/b/pfxteam/archive/2012/08/15/implementing-then-with-await.aspx
// Further reading: http://blogs.msdn.com/b/pfxteam/archive/2010/11/21/10094564.aspx
public static async Task Then(this Task task, Func<Task> continuation)
{
await task;
await continuation();
}
public static async Task<TNewResult> Then<TNewResult>(
this Task task, Func<Task<TNewResult>> continuation)
{
await task;
return await continuation();
}
public static async Task Then<TResult>(
this Task<TResult> task, Func<TResult,Task> continuation)
{
await continuation(await task);
}
public static async Task<TNewResult> Then<TResult, TNewResult>(
this Task<TResult> task, Func<TResult, Task<TNewResult>> continuation)
{
return await continuation(await task);
}
}
class MainClass
{
public static async Task<String> AsyncMethod1(int delay) {
await Task.Delay (TimeSpan.FromSeconds(delay));
return "The method has finished it's execution after waiting for " + delay + " seconds";
}
public static Task<String> AsyncMethod2(int delay)
{
return Task.Delay (TimeSpan.FromSeconds (delay)).ContinueWith ((x) => "The method has finished it's execution after waiting for " + delay + " seconds");
}
public static async Task<String> Approach1(int delay)
{
var response = await AsyncMethod1 (delay); // await just unwraps Task's result
return "Here is the result of AsyncMethod1 operation: '" + response + "'";
}
public static Task<String> Approach2(int delay)
{
return AsyncMethod2(delay).ContinueWith(message => "Here is the result of AsyncMethod2 operation: '" + message.Result + "'");
}
public static void Main (string[] args)
{
// You have long running operations that doesn't block current thread
var operation1 = Approach1 (3); // So as soon as the code hits "await" the method will exit and you will have a "operation1" assigned with a task that finishes as soon as delay is finished
var operation2 = Approach2 (5); // The same way you initiate the second long-running operation. The method also returns as soon as it hits "await"
// You can create chains of operations:
var operation3 = operation1.ContinueWith(operation1Task=>Console.WriteLine("Operation 3 has received the following input from operation 1: '" + operation1Task.Result + "'"));
var operation4 = operation2.ContinueWith(operation2Task=>Console.WriteLine("Operation 4 has received the following input from operation 2: '" + operation2Task.Result + "'"));
var operation5 = Task.WhenAll (operation3, operation4)
.Then(()=>Task.Delay (TimeSpan.FromSeconds (7)))
.ContinueWith((task)=>Console.WriteLine("After operation3 and 4 have finished, I've waited for additional seven seconds, then retuned this message"));
Task.WaitAll (operation1, operation2); // This call will block current thread;
operation3.Wait (); // This call will block current thread;
operation4.Wait (); // This call will block current thread;
operation5.Wait (); // This call will block current thread;
Console.WriteLine ("All operations are completed");
}
}

you can only use await inside a method if that method is async and async methods need to return Task, Task<T> or void although void returning async methods are reserved for event handlers because the exceptions thrown within them are swallowed and you cannot await their completion or chain subsequent tasks.
I think your Index action needs to be async and return a Task<ActionResult> and your CreateEmployee method needs to be async as well as it is using await inside it.
See Best Practices in Asynchronous Programming for some guidelines on when and how to use async-await

async await
They are tricky to understand.
First of all, in your methods in Web API, you are using async without await. I'm sure you are getting some errors / warning there right?
--
async await are used to return the working thread back to the caller when you are waiting for I/O to be finished. So, yes, you do want to use it both in your MVC and Web API side. Please make sure you understand this sentence before moving on.
--
The thing about async / await is that, if you use it, you have to use it ALL the way through the calling functions, or else it doesn't make sense (and you'll get errors / warning too). This means that whatever library you are using must support it. In this case "DocumentClient". By convention, the methods that support it will end in "Async" and it will return a Task which you can await.
--
So your short answer:
use async await from the very beginning (your controller), and try to make it await whatever long operations it calls. If that is also your code, you should be able to await from there ... and await from there ... until you finally call something that is not your code. If you can await that code that is not yours, then you are set. If you cannot, then you should not use async await form the very beginning.
(no way this made sense)