Baffled by Async Await for HttpClient.GetStringAsync

Question

I'm trying to design an approach to downloading XML delivered by a web API. The API allows for multi-threading - so in my usage case, I can use 7 threads at once (I don't understand the technicalities - is this 7 sockets or simultaneous connections?). I haven't factored this into my code below, but would just use a parallel foreach for the 7 simultaneous httpclient queries.

I'm really struggling to understand Await Async and most tutorials seems to be in c#. I'm afraid I'm vb (I do know you can convert, but ruins the learning a little).

I've also read that you shouldn't open up a new instance of httpClient for each GetStringAsync (e.g. via a using statement) and should instead have a global instance and reuse that.

So, two problems - how to design the HttpClient helper class and how to utilise it.

On the first, I have gotten to this stage:

Imports System.Net.Http

Public Class HttpClientHelper

    Private Shared ReadOnly _httpClient As HttpClient = New HttpClient()

    Public Async Function SendRequestToEndpoint(url As String) As Task(Of String)

        Dim retString As String = Await _httpClient.GetStringAsync(url)

        Return retString

    End Function

End Class

On the second, I just got lost! Totally baffled by Await/Async. so can't even get it to compile:

Private Async Sub HttpClientTest(urls As List(Of String))

    'HTTPCLIENT TESTS
    Dim hch As New HttpClientHelper
    Dim MainThreadString As String = Nothing
    For i = 0 To urls.Count - 1

        Await Task.Run(Function()
                           Dim retStr As String = hch.SendRequestToEndpoint(urls(i))
                           'I need to perform actions on main thread variables with retStr - e.g. updating a database
                           MainThreadString = retStr
                       End Function)
        Debug.WriteLine("URL: " & urls(i) & vbCr & MainThreadString)

    Next

End Sub

I just pass in a list of URLs to this method.

This is just a simplified version of the final process. I would essentially be passing the returned xml into another method to update the main database on the main thread.

Sorry if this doesn't make sense, I'm just really struggling to get Async theory!

=========================================================

UPDATE: Following Caius Jard's answer, I updated my code to what I thought would be logical:

Public Class HttpClientHelper

    Private Shared ReadOnly _httpClient As HttpClient = New HttpClient()
    
    Public Async Function SendRequestToEndpoint(url As String) As Task(Of String)

        Dim retString As String = Await _httpClient.GetStringAsync(url)
        Return retString

    End Function

End Class

Public Class 

    Private Sub Main()

        Dim urls As New List(Of String) From {"https://www.google.com", "https://www.bbc.co.uk", "https://stackoverflow.com"}
        HttpClientTest(urls)
        
    End Sub
    
    Private Async Sub HttpClientTest(urls As List(Of String))

        Dim hch As New HttpClientHelper
        For i = 0 To urls.Count - 1

            Dim str As String = Await hch.SendRequestToEndpoint(urls(i))
            Debug.WriteLine(vbCr & vbCr & "URL: " & urls(i) & vbCr & str)

        Next
    End Sub
    
End Class

However, there are still problems:

Nothing is written to the debug console.
Also, using the debugger yields some weird stepping, which I assume is a result of trying to step through asynchronous code?
How come you have to also mark HttpClientTest as async as essentially this method only calls an async Function and isn't async in itself (or does the very act of calling an async method within, albeit with await, render it as async as well?)

I did try:

Await HttpClientTest(urls)

but it wouldn't compile...

Pesky async...

Baffled by Async Await for HttpClient.GetStringAsync

Answers (1)

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