How do you call an async function from Main on a console app?

Question

How do you call an async function from Main on a console app?

Boruch Tkatch 141

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?

Answer 1

Viorel 88,321

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

Boruch Tkatch 141

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!

Boruch Tkatch 141 Reputation points

2020-12-27T20:34:07.247+00:00
[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.
Boruch Tkatch 141 Reputation points

2020-12-27T20:41:05.467+00:00
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,

Viorel 88,321

Try

Dim Response As Net.Http.HttpResponseMessage = Client.GetAsync("http://example.com").Result;

Answer 2

Mauricio Moccelin 6

@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.

Boruch Tkatch 141

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'.

Answer 3

Hello @Boruch Tkatch

VB.NET does not support async main, here is how to do this then look at the C# example.

VB.NET Source

C# Source

 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

C# version

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;  
  
        }  
  
 }  
}

How do you call an async function from Main on a console app?

2 additional answers

C# version

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