Ggtcf

I am in the process of converting synchronous code to async code. I have a windows form application in VB.net which shows the representation of physical devices which are part of a registrationpost being managed by a postmanager.

So the PostManager class is used to manipulate RegistrationPost objects. These registrationpost, contain multiple devices which need to be connected to with Socket communication in one way or another. These devices conform to a IDevice interface by implementing a "connect" sub/function.

Below a condensed version of the code to get an idea of what I'm trying to do:

Private Sub Form_Load(sender As Object, e As EventArgs) Handles MyBase.Load
 Try
 ...
 Dim pm As New PostManager
 pm.ConnectAndStartPosts
 ...
 Catch ex As Exception
 ...
 End Try
End Sub

Then inside the PostManager Class:

Friend Sub ConnectAndStartPosts()
 ' The global variable "Posts" is a List of RegistrationPost objects
 Parallel.ForEach(Of RegistrationPost)(Posts, Sub(x) x.ConnectAndStartReading())
End Sub

Then inside the RegistrationPost Class:

Public Sub ConnectAndStartReading()
 Parallel.ForEach(Of IDevice)(AllDevices, Sub(x)
 x.Connect()
 ...
 End Sub)
End Sub

Then inside an IDevice:

Public Sub Connect() Implements IDevice.Connect
 ' socket is a global variable of the type Socket(SocketType.Stream, ProtocolType.Tcp)
 socket.Connect(IP, Port)
 ... more code gets executed (if connected, log a message)

 NotifyPropertyChanged("Connected")
End Sub

Ok this is all working but of course I'd like to use async/await to get all the benefits of connecting to an external socket asynchronously. But I'm having trouble understanding how far to "bubble up" the async/await keywords? I'm afraid to get into performance problems because every time you use the async/await keyword, a state machine gets created, so probably it isn't needed to go all the way up?

Would it also be better to drop the Parallel.Foreach and go for a regular ForEach? Or even another approach? See below how this would impact the code example:

' Async/await here as well?
Private async Sub Form_Load(sender As Object, e As EventArgs) Handles MyBase.Load
 Try
 ...
 Dim pm As New PostManager
 await pm.ConnectAndStartPosts
 ...
 Catch ex As Exception
 ...
 End Try
End Sub

Then inside the PostManager Class:

' Leave this a regular 'Sub' or transform to async Function as Task?
Friend Sub ConnectAndStartPosts()
 ' The global variable "Posts" is a List of RegistrationPost objects
 Parallel.ForEach(Of RegistrationPost)(Posts, async Sub(x) await x.ConnectAndStartReading())
End Sub

Then inside the RegistrationPost Class:

' Leave this a regular 'Sub' or transform to async Function as Task?
Public async Function ConnectAndStartReading() as Task
 Parallel.ForEach(Of IDevice)(AllDevices, async Sub(x)
 await x.Connect()
 ... more code, start reading, etc...
 End Sub)
End Function

Then inside an IDevice:

Public async Function Connect() as Task Implements IDevice.Connect
 ' socket is a global variable of the type Socket(SocketType.Stream, ProtocolType.Tcp)
 await socket.ConnectAsync(IP, Port)
 ... more code gets executed (if connected, log a message)

 NotifyPropertyChanged("Connected")
End Function

I'm having trouble understanding how far to "bubble up" the async/await keywords? I'm afraid to get into performance problems because every time you use the async/await keyword, a state machine gets created, so probably it isn't needed to go all the way up?

You should go async all the way. The overhead of async/await, while not zero, is completely lost in the noise as soon as you have real I/O going on.

Would it also be better to drop the Parallel.Foreach and go for a regular ForEach? Or even another approach?

Yes. Parallel is intended for CPU-bound code. For asynchronous concurrency, use Task.WhenAll. I.e., call your new ConnectAsync method for each device (e.g., using LINQ's Select) and then do an Await Task.WhenAll on the resulting tasks. You can do the same thing for registration posts.