Normal v Async calls to a service - vb.net

Question

I have a wcf service reference configured on a client application. It provides a whole series of functions to both retrieve and send data to a web based database. As an example:
Function errorCodesGetAll(ByVal uname As String, ByVal pword As String) As String
and
Function errorCodesGetAllAsync(ByVal uname As String, ByVal pword As String) As System.Threading.Tasks.Task(Of String)
I know that I can populate a rich text box with the first function by using the following code:
RichTextBox1.Text = getCountryList
Private Function getCountryList() As String
Dim svc As New ServiceReference2.ERSAPIServiceClient
svc.Open
Dim str As String = svc.errorCodesGetAll(username, password)
svc.Close()
Return str
End Function
As WCF is still a very new area to me I'm wondering how I would populate the same rich text box but this time using the Async variant of the errorCodesGetAll function?
Thanks for any advice or general pointers as to how the async variants are best used.

Answer 1

Your service will expose a "completed" event as well as the async method, you need to handle that event.
Open the service, wire up the event and call the async method
Private Sub GetCodes()
Dim svc As New ServiceReference2.ERSAPIServiceClient
AddHandler ServiceReference2.errorCodesGetAllCompleted, AddressOf errorCodesGetAllCompletedhandler
ServiceReference2.errorCodesGetAllAsync()
ServiceReference2.Close()
End Sub
Handle the event. This will get called when the service returns. (normally I would not add the "handler" to the end of the method and name it exactly the same as the event, but I thought it might help distinguish the event and the handler)
Private Sub errorCodesGetAllCompletedHandler(ByVal sender As Object, ByVal e As ServiceReference2.errorCodesGetAllEventArgs)
If Not e.Result Is Nothing Then
textbox.text = e.Result
End If
End Sub

Answer 2

Calling the async version of the method is interesting when you're on the UI thread (e.g., on the Click event handler for a button in your form), since that won't "freeze" the UI by blocking the thread, waiting for the networking call to complete.
Since you get the *Async method which returns a Task<T> result, I assume you're using the .NET Framework 4.5. If this is the case, you can take advantage of the Async / Await keywords to call the asynchronous version in a fairly simple way, while still preventing the UI thread from being blocked.
Private Async Sub Button_Click(ByVal sender as Object, ByVal e as EventArgs)
RichTextBox1.Text = Await getCountryList
End Sub
Private Async Function getCountryList() As Task(Of String)
Dim svc As New ServiceReference2.ERSAPIServiceClient
svc.Open
Dim str As String = Await svc.errorCodesGetAllAsync(username, password)
svc.Close()
Return str
End Function