How to use Async/Await in Silverlight 5 (VB) - vb.net

Question

I'm trying to call a web service and have my code wait for that service to return a result (or timeout). My project is Silverlight 5 with Web Services using .NET 4.0 and I'm running this project under VS 2012 with the Microsoft.Bcl.Async.1.0.16\lib\sl4\Microsoft.Threading.Tasks.dll ... Task.Extensions.dll ... and Extensions.Silverlight.dll.
This is how I've been doing it and it's working, but I'm trying to figure out how to change my code so that I can use the Async/Await process. The web service reference is configured to return ObservableCollection and Generic.Dictionary with Reuse types in all referenced assemblies.
Some of my code I need to convert to Async/Await:
Private _Units As Collections.ObjectModel.ObservableCollection(Of DC.SL.Services.WebServiceUnit.Units)
Public Property Units() As Collections.ObjectModel.ObservableCollection(Of DC.SL.Services.WebServiceUnit.Units)
Get
Return _Units
End Get
Set(ByVal value As Collections.ObjectModel.ObservableCollection(Of DC.SL.Services.WebServiceUnit.Units))
_Units = value
OnPropertyChanged(New PropertyChangedEventArgs("Units"))
End Set
End Property
Public Sub ReadUnits()
Try
' Client is required
If Not Me.Client Is Nothing Then
' User is required
If Not Me.User Is Nothing Then
' Must be a real Client
If Me.Client.ClientID > 0 Then
' My have a sites
If Not Me.Site Is Nothing Then
' Call the web service relative to where this application is running
Dim webServiceURI As New Uri("../WebServices/Unit.svc", UriKind.RelativeOrAbsolute)
Dim webServiceAddress As New EndpointAddress(webServiceURI)
' Setup web Service proxy
Dim wsUnits As New DC.SL.Services.WebServiceUnit.UnitClient
wsUnits.Endpoint.Address = webServiceAddress
' Add event handler so we can trap for web service completion
AddHandler wsUnits.LoadsCompleted, AddressOf LoadUnitsCompleted
' Call web service to get Sites the user has access to
wsUnits.LoadsAsync(Me.Client, Me.Site.SiteID, Me.Size.SizeID, Me.RentalType.RentalTypeID, Me.UnitState)
End If
End If
End If
End If
Catch ex As Exception
Dim Problem As New DC.SL.Tools.Errors(ex)
End Try
End Sub
Private Sub LoadUnitsCompleted(ByVal sender As Object, ByVal e As DC.SL.Services.WebServiceUnit.LoadsCompletedEventArgs)
Try
If Not IsNothing(e.Result) Then
Me.Units = e.Result
If Me.Units.Count > 0 Then
Me.Unit = Me.Units.Item(0)
End If
End If
Catch ex As Exception
Dim Problem As New DC.SL.Tools.Errors(ex)
End Try
End Sub
Still not getting this to work ... here is what I have now, but the problem remains ... UI thread execution continues and does NOT wait for the Web Service call to finish.
Calling code:
ReadUnitsAsync().Wait(3000)
Here is the updated code:
Public Async Function ReadUnitsAsync() As Task(Of Boolean)
Dim Results As Object = Await LoadReadUnitsAsync()
Return True
End Function
Public Function LoadReadUnitsAsync() As Task(Of System.Collections.ObjectModel.ObservableCollection(Of DC.SL.Services.WebServiceUnit.Units))
LoadReadUnitsAsync = Nothing
Dim tcs = New TaskCompletionSource(Of System.Collections.ObjectModel.ObservableCollection(Of DC.SL.Services.WebServiceUnit.Units))
' Client is required
If Not Me.Client Is Nothing Then
' User is required
If Not Me.User Is Nothing Then
' Must be a real Client associated
If Me.Client.ClientID > 0 Then
' Only get associated sites IF we don't have any defined
If Not Me.Site Is Nothing Then
' Call the web service relative to where this application is running
Dim webServiceURI As New Uri("../WebServices/Unit.svc", UriKind.RelativeOrAbsolute)
Dim webServiceAddress As New EndpointAddress(webServiceURI)
' Setup Site web Service proxy
Dim wsUnits As New DC.SL.Services.WebServiceUnit.UnitClient
wsUnits.Endpoint.Address = webServiceAddress
' Add event handler so we can trap for web service completion
AddHandler wsUnits.LoadUnitsCompleted, Sub(s, e)
If e.Error IsNot Nothing Then
tcs.TrySetException(e.Error)
ElseIf e.Cancelled Then
tcs.TrySetCanceled()
Else
tcs.TrySetResult(e.Result)
End If
End Sub
'' Set Busy Status
'BusyStack.Manage(ProcessManager.StackAction.Add, "ReadUnits", Me.IsWorking, Me.IsWorkingMessage)
' Call web service to get Sites the user has access to
wsUnits.LoadUnitsAsync(Me.Client, Me.Site.SiteID, Me.Size.SizeID, Me.RentalType.RentalTypeID, Me.UnitState)
Return tcs.Task
End If
End If
End If
End If
End Function
So here is the final code (abbreviated) that seems to be working to my goals (aka waiting for a Web Service to finish before proceeding).
Public Class UIUnits
Implements INotifyPropertyChanged, IDataErrorInfo
Public Event PropertyChanged As PropertyChangedEventHandler Implements INotifyPropertyChanged.PropertyChanged
Public Async Sub OnPropertyChanged(ByVal e As PropertyChangedEventArgs)
Dim propertyEventHandler As PropertyChangedEventHandler = PropertyChangedEvent
Try
If propertyEventHandler IsNot Nothing Then
RaiseEvent PropertyChanged(Me, e)
Select Case e.PropertyName
Case "Size"
Await ReadUnitsAsync()
DoSomethingElseAfterWebServiceCallCompletes()
Case Else
End Select
End If
Catch ex As Exception
Dim problem As New DC.SL.Tools.Errors(ex)
End Try
End Sub
...
Private _Units As Collections.ObjectModel.ObservableCollection(Of DC.SL.Services.WebServiceUnit.Units)
Public Property Units() As Collections.ObjectModel.ObservableCollection(Of DC.SL.Services.WebServiceUnit.Units)
Get
Return _Units
End Get
Set(ByVal value As Collections.ObjectModel.ObservableCollection(Of DC.SL.Services.WebServiceUnit.Units))
_Units = value
OnPropertyChanged(New PropertyChangedEventArgs("Units"))
End Set
End Property
...
Public Async Function ReadUnitsAsync() As Task(Of Boolean)
Me.Units = Await LoadReadUnitsAsync()
Return True
End Function
...
Public Function LoadReadUnitsAsync() As Task(Of System.Collections.ObjectModel.ObservableCollection(Of DC.SL.Services.WebServiceUnit.Units))
LoadReadUnitsAsync = Nothing
Dim tcs = New TaskCompletionSource(Of System.Collections.ObjectModel.ObservableCollection(Of DC.SL.Services.WebServiceUnit.Units))
' Client is required
If Not Me.Client Is Nothing Then
' User is required
If Not Me.User Is Nothing Then
' Must be a real Client associated
If Me.Client.ClientID > 0 Then
' Only get associated sites IF we don't have any defined
If Not Me.Site Is Nothing Then
' Call the web service relative to where this application is running
Dim webServiceURI As New Uri("../WebServices/Unit.svc", UriKind.RelativeOrAbsolute)
Dim webServiceAddress As New EndpointAddress(webServiceURI)
' Setup web Service proxy
Dim wsUnits As New DC.SL.Services.WebServiceUnit.UnitClient
wsUnits.Endpoint.Address = webServiceAddress
' Add event handler so we can trap for web service completion
AddHandler wsUnits.LoadUnitsCompleted, Sub(s, e)
If e.Error IsNot Nothing Then
tcs.TrySetException(e.Error)
ElseIf e.Cancelled Then
tcs.TrySetCanceled()
Else
tcs.TrySetResult(e.Result)
End If
End Sub
' Call web service
wsUnits.LoadUnitsAsync(Me.Client, Me.Site.SiteID, Me.Size.SizeID, Me.RentalType.RentalTypeID, Me.UnitState)
Return tcs.Task
End If
End If
End If
End If
End Function

Answer 1

In this case it's probably easiest to convert from the lowest level and work your way up. First, you need to define your own TAP-friendly extension methods on your service. VS will generate these for you if you are doing desktop development, but unfortunately it will not do this for Silverlight.
The MSDN docs describe how to wrap EAP (EAP is the pattern that uses *Async methods with matching *Completed events). If you have APM methods, it's even easier to wrap those into TAP (APM is the pattern that uses Begin*/End* method pairs).
Once you have a wrapper, e.g., LoadUnitsTaskAsync, change your method to call that instead of LoadUnitsAsync and Await the result. This will require your ReadUnits method to be Async, so change it to a Task-returning Function (and change its name from ReadUnits to ReadUnitsAsync). Next change all callers of ReadUnitsAsync so they Await its result. Repeat until you reach an actual event handler, which may be Async Sub (do not use Async Sub for any intermediate methods; use Async Function ... As Task instead).

Answer 2

I used this library to achieve async await in my Silverlight 5 project