Monthly Archives: September 2013

Which endpoint is actually being used?

Private Function GetEndpointAddress(name As String) As String
Debug.Print("--- GetEndpointAddress ---")
Dim address As String = "Unknown"
Dim appConfig As Configuration = ConfigurationManager.OpenExeConfiguration(ConfigurationUserLevel.None)
Debug.Print("app.config: " & appConfig.FilePath)
Dim serviceModel As ServiceModelSectionGroup = ServiceModelSectionGroup.GetSectionGroup(appConfig)
Dim bindings As BindingsSection = serviceModel.Bindings
Dim endpoints As ChannelEndpointElementCollection = serviceModel.Client.Endpoints
For i As Integer = 0 To endpoints.Count - 1
Dim endpoint As ChannelEndpointElement = endpoints(i)
Debug.Print("Endpoint: " & endpoint.Name & " - " & endpoint.Address.ToString)
If endpoint.Name = name Then
address = endpoint.Address.ToString
End If
Next
Debug.Print("--- GetEndpointAddress ---")
Return address
End Function

Retrieving files from application directory using Reflection

When retrieving a list of files in the directory of the executing application/DLL you can use the following code:

 Dim uri As String = System.IO.Path.GetDirectoryName(System.Reflection.Assembly.GetExecutingAssembly().GetName().CodeBase) Dim local As String = New Uri(uri).LocalPath
 Dim filenames() As String = System.IO.Directory.GetFiles(local, “*.*”)
 

CodeBase returns a URI which means that if you pass the result of GetDirectoryName directly to GetFiles a ‘URI formats are not supported’ error is thrown. Thus we need to convert the URI to a local path.

An interesting way to get around the limitations of a Using block when calling WCF Services

As you may have already experienced there are potential problems with a Using block when interacting with a WCF Service. For example this code can easily produce an undecipherable WCF CommunicationObjectFaultedException  error for no apparent reason:

Using service As New WCF_ServiceReference.MessageClient
'This is the 'payload' code block:
For Each item As Message In MessageList
If service.SendMessage(item) = True Then
MessageList.Remove(item)
End If
Next
End Using

After some research you will discover that the Microsoft recommendation is to wrap the ‘payload’ code in a Try…Catch block instead of a Using block. See ‘Expected Exceptions’ (http://msdn.microsoft.com/en-us/library/aa354510.aspx) for details. This results in the rather longwinded code below:

Dim service As New WCF_ServiceReference.MessageClient
'Exceptions that are thrown from communication methods on a Windows Communication Foundation (WCF) client are either expected or unexpected.
Try
'Here is the 'payload' code block again
For Each item As Message In MessageList
If service.SendMessage(item) = True Then
MessageList.Remove(item)
End If
Next
service.Close()
Catch timex As TimeoutException
'Expected exceptions from communication methods on a WCF client include TimeoutException, CommunicationException, and any derived class of CommunicationException. These indicate a problem during communication that can be safely handled by aborting the WCF client and reporting a communication failure. Because external factors can cause these errors in any application, correct applications must catch these exceptions and recover when they occur.
service.Abort()
Catch commex As CommunicationException
'Code that calls a client communication method must catch the TimeoutException and CommunicationException. One way to handle such errors is to abort the client and report the communication failure.
service.Abort()
Catch ex As Exception
'Typically there is no useful way to handle unexpected errors, so typically you should not catch them when calling a WCF client communication method.
'Throw
Finally
service.Dispose()
End Try

However there is another way to implement this in a manner much more similar to a Using block:

ServiceHelper.Using(Of WCF_ServiceReference.MessageClient)(
Sub(service)
'Here is the 'payload' code block again
For Each item As Message In MessageList 'Note that MessageList is in scope because it is part of the containing class
If service.SendMessage(item) = True Then
MessageList.Remove(item)
End If
Next
End Sub)

This method relies on a ServiceHelper class with a Using function:

Public Shared Sub [Using](Of ServiceT As {ICommunicationObject, IDisposable, New})(action As Action(Of ServiceT))
Dim service = New ServiceT()
Dim serviceclosed As Boolean = False
Try
action(service)
If service.State <> CommunicationState.Faulted Then
service.Close()
serviceclosed = True
End If
Finally
If serviceclosed = False Then
service.Abort()
End If
End Try
End Sub

The using function takes a generic parameter that matches the interfaces supported by a WCF service and an anonymous subroutine (aka lambda). What this means in practice is that we call ServiceHelper.Using passing a WCF service type and a subroutine that consists of the code inside the original Using block. The Using function executes the procedure and, crucially, handles any faulted errors, calling close or abort as required. This has the advantages of being considerably fewer lines of code then in the original procedure, handles errors gracefully, and can be debugged easily.

Note that the code inside the anonymous procedure must not call service.Close or service.Abort as these are handled by the Using procedure.

The only problem with the ServiceHelper class is that we can only pass a subroutine and not a function. So going further we can overload the Using procedure by adding another procedure to our ServiceHelper class to allow for a function that returns a result:

Public Shared Function [Using](Of ServiceT As {ICommunicationObject, IDisposable, New}, TResult)(action As Func(Of ServiceT, TResult)) As TResult
Dim service = New ServiceT()
Dim serviceclosed As Boolean = False
Dim result As TResult
Try
result = action(service)
If service.State <> CommunicationState.Faulted Then
service.Close()
serviceclosed = True
End If
Finally
If serviceclosed = False Then
service.Abort()
End If
End Try
Return result
End Function

Which can be used like this:

Dim result As Boolean = ServiceHelper.Using(Of WCF_ServiceReference.MessageClient, Boolean)(
Function(service) As Boolean
'Here is the 'payload' code block again
For Each item As Message In MessageList 'Note that MessageList is in scope because it is part of the containing class
If service.SendMessage(item) = True Then
MessageList.Remove(item)
End If
Next
Return True
End Function)

Naturally you can carry on adding other Using procedures with different signatures to account for all the variations of the  Action and Func delegates  that are available but the simplest solution is to add a single input parameter, which can take an object of any type, and use that to pass in any required data. This gives us two more procedures to round out our ServiceHelper class:

Public Shared Sub [Using](Of ServiceT As {ICommunicationObject, IDisposable, New}, TParameter)(input As TParameter, action As Action(Of ServiceT, TParameter))
Dim service = New ServiceT()
Dim serviceclosed As Boolean = False
Try
action(service, input)
If service.State <> CommunicationState.Faulted Then
service.Close()
serviceclosed = True
End If
Finally
If serviceclosed = False Then
service.Abort()
End If
End Try
End Sub

Public Shared Function [Using](Of ServiceT As {ICommunicationObject, IDisposable, New}, TParameter, TResult)(input As TParameter, action As Func(Of ServiceT, TParameter, TResult)) As TResult
Dim service = New ServiceT()
Dim serviceclosed As Boolean = False
Dim result As TResult
Try
result = action(service, input)
If service.State <> CommunicationState.Faulted Then
service.Close()
serviceclosed = True
End If
Finally
If serviceclosed = False Then
service.Abort()
End If
End Try
Return result
End Function

Which are used like this:

Dim messages As New List(Of Message)
ServiceHelper.Using(Of WCF_ServiceReference.MessageClient, List(Of Message))(messages,
Sub(service, msglist)
'Here is the 'payload' code block again
For Each item As Message In msglist 'Note that we pass in the message list as a parameter and thus do not rely on the containing class's property
If service.SendMessage(item) = True Then
MessageList.Remove(item)
End If
Next
End Sub)

Dim messages As New List(Of Message)
Dim result As Boolean = ServiceHelper.Using(Of WCF_ServiceReference.MessageClient, List(Of Message), Boolean)(messages,
Function(service, msglist) As Boolean
'Here is the 'payload' code block again
For Each item As Message In msglist 'Note that we pass in the message list as a parameter and thus do not rely on the containing class's property
If service.SendMessage(item) = True Then
MessageList.Remove(item)
End If
Next
Return True
End Function)

Which config is which?

You can use

AppDomain.CurrentDomain.SetupInformation.ConfigurationFile

to determine which app.config file is being used by ConfigurationManager.

Note that if you are writing a service then the service uses its own app.config file and not the one of the program calling it. This can lead to confusion when developing services in Visual Studio using a test driven development methodology as the app.config in the test project is not the one used by the service being tested.