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I have a VB Winforms application that uses Await in Form1_Load and works fine. I would now like to switch this to being a console application. How do i call asynchronous subs or functions from Main?
If the function does not return a value, then try Wait:
Task.Delay(4000).Wait()
If it returns a result, then try this:
Dim r = SomeAsyncFunction().Result
That example works, and indeed waits 4 seconds.
Class Program
Public Shared Sub Main()
Task.Delay(4000).Wait()
End Sub
End Class
However, if i try waiting with a sub that uses HttpClient:
Imports System.Net.Http
Class Program
Public Shared Sub Main()
Test.Wait()
End Sub
Private Shared Async Sub Test()
Using Response As HttpResponseMessage = Await (New HttpClient).GetAsync("https://example.com")
Using Content As HttpContent = Response.Content
Debug.WriteLine(Await Content.ReadAsStringAsync)
End Using
End Using
End Sub
End Class
It redlines "Test" in Test.Wait() with an error:
Error BC30491 Expression does not produce a value.
So, i have to change it to a function and return Task:
Private Shared Async Function Test() As Task
Which works. Great!
[Ran out of characters, continued here.]
So, i tried this in my actual project and it failed. The actual project is a WinForms app that can be compiled as an EXE or DLL, simply by changing the project type, and both work fine. I now tried compiling it as a console app, and by changing the signature to Form1_Load, calling the same calls, which is where the issue came up. After a bunch of tests, i have found it fails on Await if i instantiate a form. For example:
Public Shared Sub Main()
Test().Wait()
End Sub
Private Shared Async Function Test() As Task
Dim A As New Form
Await Task.Delay(1)
End Function
It hits the Await and hangs there. However, if i comment out the Dim statement, it works just fine! So, i tried using Wait() instead like you suggested, and that works just fine, though that requires turning it back into a Sub, as there is no return.
Character limit hit again, will continue in next comment.
In my case, however, i am using HttpClient. Wait() does indeed wait:
Public Shared Sub Main()
Dim A As New Form1
A.Test()
End Sub
Private Sub Test()
Dim Client As New Net.Http.HttpClient
Client.GetAsync("http://example.com").Wait()
End Sub
But how do i get the reply? Changing the line to:
Dim Response As Net.Http.HttpResponseMessage = Client.GetAsync("http://example.com").Wait()
Redlines Client.GetAsync("http://example.com").Wait(), with:
BC30491 Expression does not produce a value.
Edit: Oh, i see. I need GetAwaiter.GetResult. That works.
Does this mean i have to get rid of every Async/Await? Hmm, i wonder what the performance hit would be,
Try
Dim Response As Net.Http.HttpResponseMessage = Client.GetAsync("http://example.com").Result;
@Boruch Tkatch I suggest Download .NET 5.0, possible entry points:
public static async void Main()
{
... // await code
}
See this link. I hope I helped you.
Does VB support that? I just created a new .Net Core Console application, went to project properties and changed the Target framework to .NET 5.0 and tried:
Class Program
Public Shared Async Sub Main()
End Sub
End Class
It failed with:
warning BC42356: This async method lacks 'Await' operators and so will run synchronously. Consider using the 'Await' operator to await non-blocking API calls, or 'Await Task.Run(...)' to do CPU-bound work on a background thread.
error BC36934: The 'Main' method cannot be marked 'Async'.
I also tried with code:
Class Program
Public Shared Async Sub Main()
Await Task.Delay(1)
End Sub
End Class
The warning went away, but the error stayed:
error BC36934: The 'Main' method cannot be marked 'Async'.
Hello @Boruch Tkatch
VB.NET does not support async main, here is how to do this then look at the C# example.
Imports System.IO
Module Module1
Sub Main()
MainAsync(New String() {"Hello", "Karen"}).
GetAwaiter().
GetResult()
Dim results = Task.WhenAll(
ExampleOne,
ExampleTwo).
Result
For Each result In results
Console.WriteLine(result)
Next
Console.ReadLine()
End Sub
Public Async Function MainAsync(ByVal args() As String) As Task
Await Task.Delay(3000)
Console.WriteLine($"{args(0)} {args(1)}")
End Function
Async Function ExampleOne() As Task(Of Integer)
Await Task.Delay(2000)
Return 40
End Function
Async Function ExampleTwo() As Task(Of Integer)
Await Task.Delay(5000)
Return 60
End Function
End Module
using System;
using System.Threading.Tasks;
namespace ConsoleAppAsync_csharp
{
class Program
{
static async Task Main(string[] args)
{
await Task.Delay(3000);
Console.WriteLine($"{args[0]} {args[1]}");
var results = Task.WhenAll(ExampleOne(), ExampleTwo()).Result;
foreach (var result in results)
{
Console.WriteLine(result);
}
Console.ReadKey();
}
public static async Task<int> ExampleOne()
{
await Task.Delay(2000);
return 40;
}
public static async Task<int> ExampleTwo()
{
await Task.Delay(5000);
return 60;
}
}
}