I'm trying to implement Asynchronous Fetching in my business object library. Using the following example Rocky provides in his Expert VB 2008 Business Objects book (p417)
Public Shared Sub BeginGetCustomerEdit(ByVal callback _
As EventHandler(Of DataPortalResult(Of CustomerEdit)))
Dim dp = New DataPortal(Of CustomerEdit)
AddHandler dp.FetchCompleted, AddressOf FetchComplete
dp.BeginFetch(New SingleCriteria(Of CustomerEdit, Integer) _
(123, callback))
End Sub
(Replace "CustomerEdit" with my business object type)
Visual Studio gives me the squigly line under CustomerEdit, with the error saying:
'Csla.Server.DataPortalResult' has no type parameters and so cannot have type arguments.
Can anybody please advise as to how they have done this; what am I missing? (I'm using CSLA v3.6.2.0)
Public Shared Sub GetCustomer(ByVal id As Integer, ByVal handler As EventHandler(Of DataPortalResult(Of CustomerEdit)))
Dim dp = New DataPortal(Of CustomerEdit)
AddHandler dp.FetchCompleted, AddressOf FetchCompleted
dp.BeginFetch(New SingleCriteria(Of CustomerEdit, Integer)(id), handler)
End Sub
Private Shared Sub FetchCompleted(ByVal o As Object, ByVal e As DataPortalResult(Of CustomerEdit))
Dim handler = DirectCast(e.UserState, EventHandler(Of DataPortalResult(Of CustomerEdit)))
handler.Invoke(o, e)
End Sub
Oh, I see, sorry.
If you read the error text carefully, it says you are trying to use Csla.Server.DataPortalResult.
You need to be using Csla.DataPortalResult.
You probably have the wrong Imports at the file or project level, so it is using the wrong namespace.
Awesome I got this to work perfectly now. That it, until I tested for how it handles a problem on the database (in this case, I miss-spelled the ConnectionString key). The problem is in the FetchCompleted method:
Private
Shared Sub FetchCompleted(ByVal o As Object, ByVal e As DataPortalResult(Of ArCustomerCollection)) Dim handler = DirectCast(e.UserState, EventHandler(Of DataPortalResult(Of ArCustomerCollection)))handler.Invoke(o, e)
End SubThe problem is that when there is an error getting the data out of the database, then e.UserState in this method is null; resulting in handler being null; resulting in an Object Reference Not Set error on the "handler.Invoke(o,e)" call. I've tried putting in a check like:
If
Not e.Error Is Nothing Then Throw e.Error End IfBut the problem is that this FetchCompleted method does find an appropriate Try..Catch..block on the UI thread, resulting in the "An unhandled exception has occurred in your application" message popping up. Of course if I use the Application_UnhandledException event to catch the exception I can display an appropriate messagebox to the user. But the core problem here is that the callback method in the gui (the one that is supposed to be called by "handler.invoke(o,e)") never gets invoked, and that method contains the code that is meant to reset the state of the GUI after the async call.
So my question is...Is it right that in the FetchCompleted method above the e.UserState is null when e.Error is NOT null? Surely e.UserState should still contain the callback method so that it can be invoked and handle any exception that is in e.error?
This sounds like it could be a bug. Yes, UserState should be
consistent regardless of whether there was an exception on the server.
Rocky
From: shawndewet
[mailto:cslanet@lhotka.net]
Sent: Wednesday, June 03, 2009 2:59 PM
To: rocky@lhotka.net
Subject: Re: [CSLA .NET] Trying to Implement the Book's example of
Asynchronous Fetch
Awesome I got this to work perfectly now. That it, until I tested for
how it handles a problem on the database (in this case, I miss-spelled the
ConnectionString key). The problem is in the FetchCompleted method:
Private Shared Sub FetchCompleted(ByVal
o As Object, ByVal e As
DataPortalResult(Of ArCustomerCollection))
Dim handler = DirectCast(e.UserState,
EventHandler(Of DataPortalResult(Of ArCustomerCollection)))
handler.Invoke(o, e)
End Sub
The problem is that when there is an error getting the data out of the
database, then e.UserState in this method is null; resulting in handler being
null; resulting in an Object Reference Not Set error on the
"handler.Invoke(o,e)" call. I've tried putting in a check like:
If Not e.Error Is
Nothing Then
Throw e.Error
End If
But the problem is that this FetchCompleted method does find an appropriate
Try..Catch..block on the UI thread, resulting in the "An unhandled
exception has occurred in your application" message popping up. Of
course if I use the Application_UnhandledException event to catch the exception
I can display an appropriate messagebox to the user. But the core
problem here is that the callback method in the gui (the one that is supposed
to be called by "handler.invoke(o,e)") never gets invoked, and that
method contains the code that is meant to reset the state of the GUI after the
async call.
So my question is...Is it right that in the FetchCompleted method above the
e.UserState is null when e.Error is NOT null? Surely e.UserState should
still contain the callback method so that it can be invoked and handle any
exception that is in e.error?
I hope so.
From: shawndewet
[mailto:cslanet@lhotka.net]
Sent: Thursday, June 04, 2009 7:08 AM
To: rocky@lhotka.net
Subject: Re: [CSLA .NET] RE: Trying to Implement the Book's example of
Asynchronous Fetch
Any chance of it being resolved in v3.6.3?
shawndewet:
Any chance of it being resolved in v3.6.3?
Yes, absolutely, the fix is in svn now.
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