I'm having troubles passing a dataset between classes. I get the error
System.NullReferenceException
I know this is because something isnt initalised, but what.
dataSet = BusinessFacade.GetArticles()
the part that is giving me trouble is

BusinessFacade.GetArticles()

I know for a fact that the class it is calling has a populated dataSet as i have printed it out in the class it calls, it just seems to not return it.

so

Dim dataSet As New ArticlesDS
Dim BusinessFacade As New ArticlesBF
dataSet = BusinessFacade.GetArticles()

calls


Public Function GetArticles() As ArticlesDS

Dim dataSet As New ArticlesDS
Dim dataCategory As New DataAccess.ConnectToServer

Try
dataCategory.fillDS(dataSet)
Catch e As exception
Dim exception As exception
Throw exception
End Try

Return dataSet



which results in System.NullReferenceException.

Can someone tell me what I'm doing wrong (never used VB).

Does it not give you the actual line number the error is occuring on? I find it odd that if your naming convention is as per normal (ie, classes = Capitalised, therefore BusinessFacade is a class not an object) you're going to get a NullRef from ::GetArticles()

dataCategory could potentially be Null though, but really more info is needed.

I'm guessing, but put:

Dim dataSet As New ArticlesDS
Dim dataCategory As New DataAccess.ConnectToServer

into the class scope (outside the method).

It may be that the object goes out of scope when the method terminates.

As I said, I could be wrong - quicker for you to try than for me to write an app to test it.

*edit* if that works, then it might be a good idea to have your class implement IDisposable interface and provide a Dispose method where you set the dataset to nothing. but that may be superfluous. I'm rusty as all hell with this stuff.

last edited by eu4ia at 21:31:21 19/Sep/05

haha wtf who cares if the object goes out of scope

if someone has strong references to it then it won't be garbage collected. the only way references magically change from object to null is if they are *special*. these references are definitely not special.

Class Admin.aspx.VB

Imports LiquorWorx.DataSchema
Imports LiquorWorx.BusinessFacade

Public Class Admin
Inherits System.Web.UI.Page

#Region " Web Form Designer Generated Code "

'This call is required by the Web Form Designer.
Private Sub InitializeComponent()

End Sub
Protected WithEvents Button1 As System.Web.UI.WebControls.Button
Protected WithEvents Repeater1 As System.Web.UI.WebControls.Repeater

'NOTE: The following placeholder declaration is required by the Web Form Designer.
'Do not delete or move it.
Private designerPlaceholderDeclaration As System.Object

Private Sub Page_Init(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Init
'CODEGEN: This method call is required by the Web Form Designer
'Do not modify it using the code editor.
InitializeComponent()
End Sub

#End Region

Private Sub Page_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load


End Sub

Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click
Dim dataSet As New ArticlesDS
Dim BusinessFacade As New ArticlesBF
dataSet = BusinessFacade.GetArticles()
End Sub
End Class




Class ArticlesBF.VB


Imports System
Imports System.Data
Imports LiquorWorx.DataAccess
Imports LiquorWorx.DataSchema


Public Class ArticlesBF

Public Shared Function GetArticles() As ArticlesDS

Dim dataSet As New ArticlesDS
Dim dataCategory As New DataAccess.ConnectToServer

Try
dataCategory.fillDS(dataSet)
Catch e As exception
Dim exception As exception
Throw exception
End Try

Return dataSet

End Function

End Class



Class ConnectToServer.cs



using System;
using System.Data.SqlClient;
using System.Data;
using LiquorWorx.DataSchema;


namespace LiquorWorx.DataAccess
{
///

/// Summary description for ConnectToServer.
///

public class ConnectToServer
{
string connectString = "Data Source = localhost; Integrated Security = SSPI; Initial Catalog = LiquorWorx";
string sqlCommandString = "SELECT * FROM Articles";
//Commands for connection
SqlConnection liquorWorxConn = new SqlConnection ();
SqlCommand selectCMD = new SqlCommand();

SqlDataAdapter liquorDA = new SqlDataAdapter();


public ConnectToServer()
{
//Initalise default connection strings
liquorWorxConn.ConnectionString = connectString;
//Initalise default command
selectCMD.CommandText = sqlCommandString;
selectCMD.Connection = liquorWorxConn;
liquorDA.SelectCommand = selectCMD;

}

public void fillDS(out ArticlesDS articlesDS)
{
articlesDS = new ArticlesDS();
liquorDA.Fill(articlesDS,"Articles");

/*Test to see data in DataSet
Console.WriteLine("Went into fillDS...");
for (int i=0; i < articlesDS.Tables["Articles"].Rows.Count; i++)
{
System.Console.WriteLine(articlesDS.Tables["Articles"].Rows[i]["Title"].ToString());
System.Console.WriteLine(articlesDS.Tables["Articles"].Rows[i]["Article"].ToString());
System.Console.ReadLine();
}
End of test to see data in DataSet*/
}

}

}



last edited by evil1 at 21:49:04 19/Sep/05

if someone has strong references to it then it won't be garbage collected

Correct. I forgot about that. Just tested it - scope is not the issue.

First you need a connection object to connect to your database.


Const strConn As String = "Provider=SQLOLEDB.1;Password=password;Persist Security Info=True;User ID=sa;Initial Catalog=mydb;Data Source=xxx"

Dim connection As SqlConnection = New SqlConnection(strConn)

Assuming your not using a command object, you need either a datareader object or a dataAdapater object. Datareader is fast, forward only cursor that loops throu a stream of rows. You can execute a select statment or a stored procedure that contains a select statment.

A dataadapater object serves as a bridge between a dataset and a data source for retrieveing and saving data. The dataadapater class represents a set of database commands and a database connection that you use to fill a dataset and update the data source.

Dim command As SqlCommand = New SqlCommand("GetOrders", connection)command.CommandType = CommandType.StoredProcedure

Dim adapter As SqlDataAdapter = New SqlDataAdapter(command)
Dim ds As New Data.DataSet

adapter.Fill(ds)

With ds.Tables(0)
If .Rows.Count > 0 Then


.. and so on.

I've done all that in the C# file. Which is then called by a VB class, which is then called by another VB class, it's when the second call happens that their is a problem.

Dim exception as exception
Throw exception

what are you, high?

Catch e As exception
Dim exception As exception
Throw exception

^ lol

I agree with the comments above.

We actually have a rule here at work that if you've been drinking (ie, for lunch etc) you're not allowed to touch code.
:)

While we are on the topic of VB, can someone please help me with a problem I have. I'd create a new thread but don't like the idea of having 2 VB threads. I mean, 1 thread is more than enough!

OK: A VB client application is trying to use functions that are inside a C/C++ DLL. One particular DLL function expects to receive a c-style string (LPSTR). What should the VB code do to interface with the c/c++ function and be able to successfully send it a string?

The guy who wrote the VB code reckons he needs to pass a BSTR by value or something, but not having a clue about VB, I have no idea what he's talking about. He wants me to change my C++ code to 'cater' for his VB program, but I refuse to do so (the Win32 API doesn't either), so I want to find a way for him to change his code and be C compliant.

If anyone knows a snippet of VB code that can send a c-style string to an LPSTR-receiving C++ function, I'll be very happy. If you also know how a C function should return a string for use by VB, that would be a bonus :)

i think there are two functions to simplify that for him parabol. however, nuts to vb and nuts to .net so I won't be looking in my old msdn stuff for it :)

IIRC you have to use some sort of array (of string or byte or something) otherwise you can use vb's lame variable pointer either VarPtr(whatever) or StrPrt(whatever), or a mixture of the two. Can't really remember and it's different with every situation.

Yep, StrPtr has apparently been removed in VB .NET :/

I've sent the VB coder several links, but he reckons the methods given don't work. I guess if all else fails, I'll 'retire' his VB code and rewrite the frontend myself in a real language.

Stupid uni group projects. Stupid VB.

/bitterness

Of course it has, VB.NET uses System::String

parabol if you are using .NET then it is very easy

<DllImport("foo.dll")> _
Public Shared Function Foo (<MarshalAs(UnmanagedType.LPStr)> Byval str as String)
End Function

if the string is constant or

<DllImport("foo.dll")> _
Public Shared Function Foo (<MarshalAs(UnmanagedType.LPStr)> Byval str as StringBuilder)
End Function

if the string is non-constant

EDIT: damm brackets

oops i lied.. both need to be marked as LP(T)Str
http://msdn.microsoft.com/library/default.asp?url=/library/en-us/cpguide/html/cpcondefaultmarshalingforstrings.asp


last edited by hast at 22:55:18 20/Sep/05