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!!!