Why does this function fail

[vijaysridhara]

Hi,
Can some one help, why does this function fail? The call jumps to directly "Finally" block without waiting for response. I tried chat.SendAsync also, the same result.
` Private Async Sub SendMessage(msg As String)

 Dim iterator As IAsyncEnumerator(Of String) 'Or appropriate type
 txtHistory.AppendText("You: " & msg)
 txtHistory.AppendText(vbCrLf)

 txtHistory.AppendText("Agent: ")
 txtHistory.SelectionStart = txtHistory.TextLength
 txtHistory.ScrollToCaret()
 Try
     iterator = chat.SendAsAsync(New Models.Chat.ChatRole("user"), msg)

     Do While Await iterator.MoveNextAsync()
         Dim item = iterator.Current
         txtHistory.AppendText(item)
     Loop
 Catch ex As Exception
     txtHistory.AppendText(ex.Message)
 Finally
     txtHistory.AppendText(vbCrLf)
     txtHistory.SelectionStart = txtHistory.TextLength
     txtHistory.ScrollToCaret()
     butSend.Enabled = True
 End Try

End Sub`

[awaescher]

First I would recommend not using "Async Sub" which is the VB equivalent of async void antipattern. Such methods cannot be awaited and might cause errors that will not be catched.

Also, I think the code looks strange but I guess you are using iterator.MoveNextAsync() because that's the VB way to handle IAsyncEnumerable<>. If so, this code might be okay, I don't really know. But you are accessing the UI txtHistory.AppendText(item) directly mixed-in with the async calls. It's pretty likely that you might run into a threading issue.

Anyhow, I guess this is too complex to pin this down remotely.

[vijaysridhara]

I feared threading issues, but there were none, nevertheless, I changed code, to make it into a Async Function. Yes IAsyncEnumerable is the code line iterator as IAsyncEnumerator(of String) . I removed the textbox and just put Debug.Print(item). But no luck.

   Dim url As Uri = New Uri("http://localhost:11434")
   Private WithEvents ollama As OllamaApiClient = New OllamaApiClient(url)
   Dim chatter As Chat
   Private Sub cboModels_SelectedIndexChanged(sender As Object, e As EventArgs) Handles cboModels.SelectedIndexChanged
       If cboModels.SelectedIndex = -1 Then Exit Sub
       SetModel()
   End Sub

   Private Function SetModel() As Boolean
       ollama.SelectedModel = cboModels.Text
       ollama.Config.Model = ollama.SelectedModel
       chatter = Nothing
       chatter = New Chat(ollama)
       chatter.Model = cboModels.Text
       Return True
   End Function

Private Async Function SendMessage(msg As String) As Task(Of Boolean)
     Dim iterator As IAsyncEnumerator(Of String) 'Or appropriate type
     Try
         iterator = chatter.SendAsAsync(New Models.Chat.ChatRole("user"), msg)
         Do While Await iterator.MoveNextAsync()
             Dim item = iterator.Current
             Debug.Print(item)
         Loop
     Catch ex As Exception
            Debug.Print(ex.Message)
     End Try
     Return True
 End Function

[vijaysridhara]

On a button click, the send message enters with message, but quickly comes out.

 Private Sub butSend_Click(sender As Object, e As EventArgs) Handles butSend.Click
   Dim t As Task(Of Boolean)
   t = SendMessage(txtRequest.Text.Trim())
End Sub

[awaescher]

That task is never started.

[vijaysridhara]

What does it mean? It never went inside SendMessage?

[awaescher]

If there's not awaiter for a Task, it might be garbage collected before it's executed.

As a workaround you could use .SafeFireAndForget() from here but keep in mind that it is still a workaround.

Please make sure you fully understand this and this.

[vijaysridhara]

Sure, i will read through, btw, I use .NET 8.0... thankx

[Ganesh-403]

@vijaysridhara The behavior here is actually explained by the button click handler rather than SendAsAsync() itself.

You're creating the task:

Dim t As Task(Of Boolean)
t = SendMessage(txtRequest.Text.Trim())

but never awaiting it. While calling an async method does start executing it, the caller immediately returns and any exceptions or completion are no longer observed. Microsoft explicitly recommends awaiting Task-returning methods unless you're intentionally implementing a fire-and-forget pattern. :contentReference[oaicite:0]{index=0}

For a WinForms event handler, the usual pattern is:

Private Async Sub butSend_Click(sender As Object, e As EventArgs) Handles butSend.Click
    Await SendMessage(txtRequest.Text.Trim())
End Sub

and keep:

Private Async Function SendMessage(msg As String) As Task(Of Boolean)
    ...
End Function

This ensures the async operation is awaited, exceptions propagate correctly, and the UI remains responsive. Microsoft also recommends using Async Sub only for event handlers, while other asynchronous methods should return Task/Task(Of T). :contentReference[oaicite:1]{index=1}

If, on the other hand, you intentionally don't want to wait for the operation, then it's better to make that explicit using a fire-and-forget helper (such as SafeFireAndForget) rather than just assigning the returned Task to a variable and ignoring it. :contentReference[oaicite:2]{index=2}

If this solves your problem, feel free to mark it as the accepted answer so others can find it easily.