The await operator is used to wait for a Promise. It can be used inside an Async block only. The keyword Await makes JavaScript wait until the promise returns a result. It has to be noted that it only makes the async function block wait and not the whole program execution.
Async/await does not do that. Nothing in Javascript does that. You always get a returned promise from an async function and no use of await inside that function changes that. Please read the details of my answer to understand better how the async function works.
The call to the async method starts an asynchronous task. However, because no Await operator is applied, the program continues without waiting for the task to complete. In most cases, that behavior isn't expected.
The most important thing to know about async
and await
is that await
doesn't wait for the associated call to complete. What await
does is to return the result of the operation immediately and synchronously if the operation has already completed or, if it hasn't, to schedule a continuation to execute the remainder of the async
method and then to return control to the caller. When the asynchronous operation completes, the scheduled completion will then execute.
The answer to the specific question in your question's title is to block on an async
method's return value (which should be of type Task
or Task<T>
) by calling an appropriate Wait
method:
public static async Task<Foo> GetFooAsync()
{
// Start asynchronous operation(s) and return associated task.
...
}
public static Foo CallGetFooAsyncAndWaitOnResult()
{
var task = GetFooAsync();
task.Wait(); // Blocks current thread until GetFooAsync task completes
// For pedagogical use only: in general, don't do this!
var result = task.Result;
return result;
}
In this code snippet, CallGetFooAsyncAndWaitOnResult
is a synchronous wrapper around asynchronous method GetFooAsync
. However, this pattern is to be avoided for the most part since it will block a whole thread pool thread for the duration of the asynchronous operation. This an inefficient use of the various asynchronous mechanisms exposed by APIs that go to great efforts to provide them.
The answer at "await" doesn't wait for the completion of call has several, more detailed, explanations of these keywords.
Meanwhile, @Stephen Cleary's guidance about async void
holds. Other nice explanations for why can be found at http://www.tonicodes.net/blog/why-you-should-almost-never-write-void-asynchronous-methods/ and https://jaylee.org/archive/2012/07/08/c-sharp-async-tips-and-tricks-part-2-async-void.html
Avoid async void
. Have your methods return Task
instead of void
. Then you can await
them.
Like this:
private async Task RequestToSendOutputReport(List<byte[]> byteArrays)
{
foreach (byte[] b in byteArrays)
{
while (condition)
{
// we'll typically execute this code many times until the condition is no longer met
Task t = SendOutputReportViaInterruptTransfer();
await t;
}
// read some data from device; we need to wait for this to return
await RequestToGetInputReport();
}
}
private async Task RequestToGetInputReport()
{
// lots of code prior to this
int bytesRead = await GetInputReportViaInterruptTransfer();
}
Best Solution to wait AsynMethod till complete the task is
var result = Task.Run(async() => await yourAsyncMethod()).Result;
just put Wait() to wait until task completed
GetInputReportViaInterruptTransfer().Wait();
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