VB.net - How can a definition at module level throw an exception - vb.net

Question

I was beginning to think that I was getting good at VB.net, but not this one has me stumped.
Code looks something like this
Public Class MyServer
.....
Public myMQTTclient = New MqttClient("www.myserv.com")
.....
Private Sub Ruptela_Server(sender As Object, e As EventArgs) Handles
MyBase.Load
<some code>
StartMQTT()
<some more code>
MQTT_Publish(.....)
End Sub
Public Function StartMQTT()
' Establish a connection
Dim code As Byte
Try
code = myMQTTclient.Connect(MQTT_ClientID)
Catch ex As Exception
<error handling code>
End Try
Return code
End Function
Public Sub MQTT_Publish(ByVal DeviceID As String, ByVal Channel As String, ByVal ChannelType As String, ByVal Value As String, ByVal Unit As String)
Dim myTopic As String = "MyTopic"
Dim myPayload As String = "My Payload"
Dim msgId As UShort = myMQTTclient.Publish(myTopic, Encoding.UTF8.GetBytes(myPayload), MqttMsgBase.QOS_LEVEL_EXACTLY_ONCE, False)
End Sub
As this stands it works 100% OK. The coding may seem a bit odd, but the intent is as follows :
a) create an object 'myMQTTclient' at module level so it has scope throughout the module
b) run StartMQTT() - It can still see the object.
c) within main program call MQTT_Publish many times - I can still see the object
Now the issue is this... it all goes well until "www.myserv.com" fails DNS, then the underlying winsock code throws an exception.
So ... I'm thinking - no problem - just wrap the declaration in a try block or check that www.myserv.com exists before launching the declaration.
Ah, but you can't put code at module level, it has to be in a sub or function.
Hmmm... now I'm stumped. There has to be a 'proper' way to do this, but I'll be darned if I can figure it out.
Anyone able to help ?

Answer 1

I'd follow the advice from #djv about declaring it just as you need it. To wrap that in a Try... Catch block you can do that in an Init method.
Public Class MyServer
Implements IDisposable ' As per djv recommendation which I second...
Private myMQQTclient As MqttClient
Public Sub Init()
Try
myMQQTClient = New MqttClient("<your url>")
Catch ex As Exception
' Do whatever
End Try
End Sub
' more code and implement the Dispose method...
End Class
You can then go on and implement the IDisposble interface to ensure that you release the resources.

Answer 2

Ok, I am not sure if I have the right library. But I found this Nuget package: OpenNETCF.MQTT which seems to have the class you are using.
I would do it this way
Public Class MyServerClass
Implements IDisposable
Public myMQTTclient As MQTTClient
'Private Sub Ruptela_Server(sender As Object, e As EventArgs) Handles MyBase.Load
' ' <some code>
' StartMQTT()
' ' <some more code>
' ' MQTT_Publish(.....)
'End Sub
Public Sub New(brokerHostName As String)
myMQTTclient = New MQTTClient(brokerHostName)
End Sub
Public Function StartMQTT()
' Establish a connection
Dim code As Byte
Try
code = myMQTTclient.Connect(MQTT_ClientID)
Catch ex As Exception
'<error handling code>
End Try
Return code
End Function
Public Sub MQTT_Publish(ByVal DeviceID As String, ByVal Channel As String, ByVal ChannelType As String, ByVal Value As String, ByVal Unit As String)
Dim myTopic As String = "MyTopic"
Dim myPayload As String = "My Payload"
Dim msgId As UShort = myMQTTclient.Publish(myTopic, Encoding.UTF8.GetBytes(myPayload), MqttMsgBase.QOS_LEVEL_EXACTLY_ONCE, False)
End Sub
#Region "IDisposable Support"
Private disposedValue As Boolean
Protected Overridable Sub Dispose(disposing As Boolean)
If Not disposedValue Then
If disposing Then
myMQTTclient.Dispose()
End If
End If
disposedValue = True
End Sub
Public Sub Dispose() Implements IDisposable.Dispose
Dispose(True)
End Sub
#End Region
End Class
And now you can see the usage when IDisposable is implemented, in a Using block:
Module Module1
Sub Main()
Using myserver As New MyServerClass("www.myserv.com")
myserver.StartMQTT()
myserver.MQTT_Publish(...)
End Using
End Sub
End Module
This makes it so your object is only in scope in the Using, and the object's Dispose method will automatically be called on End Using
I don't know what the base class was originally and why this was declared Private Sub Ruptela_Server(sender As Object, e As EventArgs) Handles MyBase.Load. It seems like it was possibly a form? You should keep your server code separate from Form code if that was the case. I suppose you could paste the Using into your form load, but then you would be blocking your UI thread. The referenced library has Async support so it might be a good idea to leverage that if coming from a UI.
I've made many assumptions, so I'll stop to let you comment and see how close relevant my answer is.