Error: Object reference not set to an instance of an object

[jdccr]

Hi, I am using .NET Remoting for database access.

I have three components:

1. A class library 'RemoteComponent.dll':

Imports System
Imports System.Data.SqlClient
Imports System.Runtime.Remoting
Imports System.Runtime.Remoting.Channels
Imports System.Runtime.Remoting.Channels.Tcp

<Serializable()> _
Public Class RemoteClass
Inherits MarshalByRefObject

Private oConnection_m As SqlConnection
Private oTransaction_m As SqlTransaction

Private Function ConnectionString() As String
Return "user id=sa;password=;Database=Northwind;Server=localhost;Connect Timeout=30"
End Function

Public Sub BeginTransaction()
Dim oCnn As SqlConnection
Dim oTrx As SqlTransaction

oCnn = New SqlConnection(ConnectionString)
If oCnn.State = ConnectionState.Closed Then
oCnn.Open()
End If
oTrx = oCnn.BeginTransaction

oConnection_m = oCnn
oTransaction_m = oTrx
End Sub

Public Sub CommitTransaction()
oTransaction_m.Commit()
oConnection_m.Close()

oTransaction_m.Dispose()
oTransaction_m = Nothing

oConnection_m.Dispose()
oConnection_m = Nothing
End Sub

Public Overloads Function RunSQL( _
ByVal cSQL As String, _
ByVal cTableName As String) As DataSet

Dim oCmd As SqlCommand = New SqlCommand
Dim oCnn As SqlConnection = Nothing
Dim oAdp As SqlDataAdapter = New SqlDataAdapter
Dim oDs As DataSet = New DataSet

Try
oCnn = oTransaction_m.Connection

With oCmd
.Connection = oCnn
.CommandText = cSQL
.CommandType = CommandType.Text
.Transaction = oTransaction_m
End With

With oAdp
.SelectCommand = oCmd
.Fill(oDs, cTableName)
End With

If IsNothing(oTransaction_m) Then
If Not oCnn Is Nothing And oCnn.State <> ConnectionState.Closed Then
oCnn.Close()
End If
End If

Return oDs

Catch ex As Exception
Throw New System.Exception(ex.Message)
End Try
End Function
End Class

2. A console application 'Listener.exe':

Imports System
imports System.Runtime.Remoting
Imports System.Runtime.Remoting.Channels
Imports System.Runtime.Remoting.Channels.Tcp
Imports System.Runtime.Remoting.Channels.Http

Public Class Listener
Public Shared Function Main(ByVal args() As String) As Integer
Dim channel As TcpChannel = New TcpChannel(8080)
ChannelServices.RegisterChannel(channel)

RemotingConfiguration.RegisterWellKnownServiceType( _
GetType(RemoteComponent.RemoteClass), _
"RemoteComponent.RemoteClass", _
WellKnownObjectMode.SingleCall)

System.Console.WriteLine("Press the enter key to exit...")
System.Console.ReadLine()
Return 0
End Function
End Class

3. A windows application client.exe:

Private Sub Form1_Load(ByVal sender As Object, ByVal e As System.EventArgs) Handles MyBase.Load
Dim chan As TcpChannel = New TcpChannel
ChannelServices.RegisterChannel(chan)

Dim oRemoteClass As RemoteComponent.RemoteClass = CType(Activator.GetObject( _
GetType(RemoteComponent.RemoteClass), _
"tcp://localhost:8080/RemoteComponent.RemoteClass"), RemoteComponent.RemoteClass)

Dim cSQL As String
Dim dsData As DataSet
Dim nI As Integer

cSQL = " select *"
cSQL = cSQL & " from customers"

oRemoteClass.BeginTransaction()
dsData = oRemoteClass.RunSQL(cSQL, "Customers")
oRemoteClass.CommitTransaction()

ChannelServices.UnregisterChannel(chan)
oRemoteClass = Nothing
End Sub

The application client calls the method BeginTransaction that inicialize the private variable oTransaction_m, but when the application client call the method RunSQL the variable oTransaction_m is nothing...

Why the variable oTransaction_m is nothing???

I need test transactions with .NET Remoting!!!

Thanks for your help!

 

[jdccr]

I found the solution!!!

In the .NET documentation I found the following:

SingleCall: Every incoming message is serviced by a new object instance
Singleton: Every incoming message is serviced by the same object instance

So, I changed the instruction:

RemotingConfiguration.RegisterWellKnownServiceType( _
GetType(RemoteComponent.RemoteClass), _
"RemoteComponent.RemoteClass", _
WellKnownObjectMode.SingleCall)​

for the instruction:

RemotingConfiguration.RegisterWellKnownServiceType( _
GetType(RemoteComponent.RemoteClass), _
"RemoteComponent.RemoteClass", _
WellKnownObjectMode.Singleton)​

With Singleton the variable oTransaction_m doesn't death and I can to access this variable in the method RunSQL...

But, I have a question.... Is there any problem with Singleton? What are the advantages or disadvantages in Singleton???

Thanks for your help!!!