I want to write an async method with an out
parameter, like this:
public async void Method1()
{
int op;
int result = await GetDataTaskAsync(out op);
}
How do I do this in GetDataTaskAsync
?
This question is related to
c#
async-await
The limitation of the async
methods not accepting out
parameters applies only to the compiler-generated async methods, these declared with the async
keyword. It doesn't apply to hand-crafted async methods. In other words it is possible to create Task
returning methods accepting out
parameters. For example lets say that we already have a ParseIntAsync
method that throws, and we want to create a TryParseIntAsync
that doesn't throw. We could implement it like this:
public static Task<bool> TryParseIntAsync(string s, out Task<int> result)
{
var tcs = new TaskCompletionSource<int>();
result = tcs.Task;
return ParseIntAsync(s).ContinueWith(t =>
{
if (t.IsFaulted)
{
tcs.SetException(t.Exception.InnerException);
return false;
}
tcs.SetResult(t.Result);
return true;
}, default, TaskContinuationOptions.None, TaskScheduler.Default);
}
Using the TaskCompletionSource
and the ContinueWith
method is a bit awkward, but there is no other option since we can't use the convenient await
keyword inside this method.
Usage example:
if (await TryParseIntAsync("-13", out var result))
{
Console.WriteLine($"Result: {await result}");
}
else
{
Console.WriteLine($"Parse failed");
}
Update: If the async logic is too complex to be expressed without await
, then it could be encapsulated inside a nested asynchronous anonymous delegate. A TaskCompletionSource
would still be needed for the out
parameter. It is possible that the out
parameter could be completed before
the completion of the main task, as in the example bellow:
public static Task<string> GetDataAsync(string url, out Task<int> rawDataLength)
{
var tcs = new TaskCompletionSource<int>();
rawDataLength = tcs.Task;
return ((Func<Task<string>>)(async () =>
{
var response = await GetResponseAsync(url);
var rawData = await GetRawDataAsync(response);
tcs.SetResult(rawData.Length);
return await FilterDataAsync(rawData);
}))();
}
This example assumes the existence of three asynchronous methods GetResponseAsync
, GetRawDataAsync
and FilterDataAsync
that are called
in succession. The out
parameter is completed on the completion of the second method. The GetDataAsync
method could be used like this:
var data = await GetDataAsync("http://example.com", out var rawDataLength);
Console.WriteLine($"Data: {data}");
Console.WriteLine($"RawDataLength: {await rawDataLength}");
Awaiting the data
before awaiting the rawDataLength
is important in this simplified example, because in case of an exception the out
parameter will never be completed.
Here's the code of @dcastro's answer modified for C# 7.0 with named tuples and tuple deconstruction, which streamlines the notation:
public async void Method1()
{
// Version 1, named tuples:
// just to show how it works
/*
var tuple = await GetDataTaskAsync();
int op = tuple.paramOp;
int result = tuple.paramResult;
*/
// Version 2, tuple deconstruction:
// much shorter, most elegant
(int op, int result) = await GetDataTaskAsync();
}
public async Task<(int paramOp, int paramResult)> GetDataTaskAsync()
{
//...
return (1, 2);
}
For details about the new named tuples, tuple literals and tuple deconstructions see: https://blogs.msdn.microsoft.com/dotnet/2017/03/09/new-features-in-c-7-0/
Alex made a great point on readability. Equivalently, a function is also interface enough to define the type(s) being returned and you also get meaningful variable names.
delegate void OpDelegate(int op);
Task<bool> GetDataTaskAsync(OpDelegate callback)
{
bool canGetData = true;
if (canGetData) callback(5);
return Task.FromResult(canGetData);
}
Callers provide a lambda (or a named function) and intellisense helps by copying the variable name(s) from the delegate.
int myOp;
bool result = await GetDataTaskAsync(op => myOp = op);
This particular approach is like a "Try" method where myOp
is set if the method result is true
. Otherwise, you don't care about myOp
.
You can do this by using TPL (task parallel library) instead of direct using await keyword.
private bool CheckInCategory(int? id, out Category category)
{
if (id == null || id == 0)
category = null;
else
category = Task.Run(async () => await _context.Categories.FindAsync(id ?? 0)).Result;
return category != null;
}
if(!CheckInCategory(int? id, out var category)) return error
The C#7+ Solution is to use implicit tuple syntax.
private async Task<(bool IsSuccess, IActionResult Result)> TryLogin(OpenIdConnectRequest request)
{
return (true, BadRequest(new OpenIdErrorResponse
{
Error = OpenIdConnectConstants.Errors.AccessDenied,
ErrorDescription = "Access token provided is not valid."
}));
}
return result utilizes the method signature defined property names. e.g:
var foo = await TryLogin(request);
if (foo.IsSuccess)
return foo.Result;
I love the Try
pattern. It's a tidy pattern.
if (double.TryParse(name, out var result))
{
// handle success
}
else
{
// handle error
}
But, it's challenging with async
. That doesn't mean we don't have real options. Here are the three core approaches you can consider for async
methods in a quasi-version of the Try
pattern.
This looks most like a sync Try
method only returning a tuple
instead of a bool
with an out
parameter, which we all know is not permitted in C#.
var result = await DoAsync(name);
if (result.Success)
{
// handle success
}
else
{
// handle error
}
With a method that returns true
of false
and never throws an exception
.
Remember, throwing an exception in a
Try
method breaks the whole purpose of the pattern.
async Task<(bool Success, StorageFile File, Exception exception)> DoAsync(string fileName)
{
try
{
var folder = ApplicationData.Current.LocalCacheFolder;
return (true, await folder.GetFileAsync(fileName), null);
}
catch (Exception exception)
{
return (false, null, exception);
}
}
We can use anonymous
methods to set external variables. It's clever syntax, though slightly complicated. In small doses, it's fine.
var file = default(StorageFile);
var exception = default(Exception);
if (await DoAsync(name, x => file = x, x => exception = x))
{
// handle success
}
else
{
// handle failure
}
The method obeys the basics of the Try
pattern but sets out
parameters to passed in callback methods. It's done like this.
async Task<bool> DoAsync(string fileName, Action<StorageFile> file, Action<Exception> error)
{
try
{
var folder = ApplicationData.Current.LocalCacheFolder;
file?.Invoke(await folder.GetFileAsync(fileName));
return true;
}
catch (Exception exception)
{
error?.Invoke(exception);
return false;
}
}
There's a question in my mind about performance here. But, the C# compiler is so freaking smart, that I think you're safe choosing this option, almost for sure.
What if you just use the TPL
as designed? No tuples. The idea here is that we use exceptions to redirect ContinueWith
to two different paths.
await DoAsync(name).ContinueWith(task =>
{
if (task.Exception != null)
{
// handle fail
}
if (task.Result is StorageFile sf)
{
// handle success
}
});
With a method that throws an exception
when there is any kind of failure. That's different than returning a boolean
. It's a way to communicate with the TPL
.
async Task<StorageFile> DoAsync(string fileName)
{
var folder = ApplicationData.Current.LocalCacheFolder;
return await folder.GetFileAsync(fileName);
}
In the code above, if the file is not found, an exception is thrown. This will invoke the failure ContinueWith
that will handle Task.Exception
in its logic block. Neat, huh?
Listen, there's a reason we love the
Try
pattern. It's fundamentally so neat and readable and, as a result, maintainable. As you choose your approach, watchdog for readability. Remember the next developer who in 6 months and doesn't have you to answer clarifying questions. Your code can be the only documentation a developer will ever have.
Best of luck.
I had the same problem as I like using the Try-method-pattern which basically seems to be incompatible to the async-await-paradigm...
Important to me is that I can call the Try-method within a single if-clause and do not have to pre-define the out-variables before, but can do it in-line like in the following example:
if (TryReceive(out string msg))
{
// use msg
}
So I came up with the following solution:
Define a helper struct:
public struct AsyncOut<T, OUT>
{
private readonly T returnValue;
private readonly OUT result;
public AsyncOut(T returnValue, OUT result)
{
this.returnValue = returnValue;
this.result = result;
}
public T Out(out OUT result)
{
result = this.result;
return returnValue;
}
public T ReturnValue => returnValue;
public static implicit operator AsyncOut<T, OUT>((T returnValue ,OUT result) tuple) =>
new AsyncOut<T, OUT>(tuple.returnValue, tuple.result);
}
Define async Try-method like this:
public async Task<AsyncOut<bool, string>> TryReceiveAsync()
{
string message;
bool success;
// ...
return (success, message);
}
Call the async Try-method like this:
if ((await TryReceiveAsync()).Out(out string msg))
{
// use msg
}
For multiple out parameters you can define additional structs (e.g. AsyncOut<T,OUT1, OUT2>) or you can return a tuple.
You cannot have ref
or out
parameters in async
methods (as was already noted).
This screams for some modelling in the data moving around:
public class Data
{
public int Op {get; set;}
public int Result {get; set;}
}
public async void Method1()
{
Data data = await GetDataTaskAsync();
// use data.Op and data.Result from here on
}
public async Task<Data> GetDataTaskAsync()
{
var returnValue = new Data();
// Fill up returnValue
return returnValue;
}
You gain the ability to reuse your code more easily, plus it's way more readable than variables or tuples.
I think using ValueTuples like this can work. You have to add the ValueTuple NuGet package first though:
public async void Method1()
{
(int op, int result) tuple = await GetDataTaskAsync();
int op = tuple.op;
int result = tuple.result;
}
public async Task<(int op, int result)> GetDataTaskAsync()
{
int x = 5;
int y = 10;
return (op: x, result: y):
}
One nice feature of out
parameters is that they can be used to return data even when a function throws an exception. I think the closest equivalent to doing this with an async
method would be using a new object to hold the data that both the async
method and caller can refer to. Another way would be to pass a delegate as suggested in another answer.
Note that neither of these techniques will have any of the sort of enforcement from the compiler that out
has. I.e., the compiler won’t require you to set the value on the shared object or call a passed in delegate.
Here’s an example implementation using a shared object to imitate ref
and out
for use with async
methods and other various scenarios where ref
and out
aren’t available:
class Ref<T>
{
// Field rather than a property to support passing to functions
// accepting `ref T` or `out T`.
public T Value;
}
async Task OperationExampleAsync(Ref<int> successfulLoopsRef)
{
var things = new[] { 0, 1, 2, };
var i = 0;
while (true)
{
// Fourth iteration will throw an exception, but we will still have
// communicated data back to the caller via successfulLoopsRef.
things[i] += i;
successfulLoopsRef.Value++;
i++;
}
}
async Task UsageExample()
{
var successCounterRef = new Ref<int>();
// Note that it does not make sense to access successCounterRef
// until OperationExampleAsync completes (either fails or succeeds)
// because there’s no synchronization. Here, I think of passing
// the variable as “temporarily giving ownership” of the referenced
// object to OperationExampleAsync. Deciding on conventions is up to
// you and belongs in documentation ^^.
try
{
await OperationExampleAsync(successCounterRef);
}
finally
{
Console.WriteLine($"Had {successCounterRef.Value} successful loops.");
}
}
For developers who REALLY want to keep it in parameter, here might be another workaround.
Change the parameter to an array or List to wrap the actual value up. Remember to initialize the list before sending into the method. After returned, be sure to check value existence before consuming it. Code with caution.
Source: Stackoverflow.com