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Problem maintaining dataset in session variable

P: n/a
I have a web interface where the user types in ID's one at a time. After an
ID is typed in, a button is clicked and the button click event has code that
does a query and returns a data reader and then appends the data to a
dataset that is built in the Page_Load code in the if(!isPostBack) block.
When I try to add a row in the button click event code I get an error saying
that "Object reference not set to an instance of an object". I'm saving the
dataset in a Session object and then restoring it in the else clause of the
if(!isPostBack) block. I've pasted code below, please help. I'm relatively
new at development but have lots of asp and ado experience.


John Holmes
--------------------------------- code ----------------------------------
using System;

using System.Collections;

using System.ComponentModel;

using System.Data;

using System.Drawing;

using System.Web;

using System.Web.SessionState;

using System.Web.UI;

using System.Web.UI.WebControls;

using System.Web.UI.HtmlControls;

using ConnectionLibrary;

using System.Data.SqlClient;

namespace Affidavits


/// <summary>

/// Summary description for WebForm1.

/// </summary>

public class WebForm1 : System.Web.UI.Page


protected System.Web.UI.WebControls.TextBox TextBox1;

protected System.Web.UI.WebControls.TextBox TextBox2;

protected System.Web.UI.WebControls.DataGrid DataGrid1;

protected System.Web.UI.HtmlControls.HtmlButton btnAddParcel;

public DataSet dsAssessorData;
private void Page_Load(object sender, System.EventArgs e)


// Put user code to initialize the page here



// create a dataset


// save as session variable

Session["dsAssr"] = dsAssessorData;




DataSet dsAssessorData = new DataSet("Parcels");

dsAssessorData = (DataSet) Session["dsAssr"];



private void BuildDataSet()


DataTable dtParcels = new DataTable("tabParcels");

// add columns

DataColumn dcID = new DataColumn("ParcelID",typeof(string));

dcID.Unique = true;

dcID.AllowDBNull = false;

DataColumn dcAssessedValue = new DataColumn("Assessed", typeof(int));

dcAssessedValue.AllowDBNull = false;


DataColumn dcAddress = new DataColumn("Address", typeof(string));

dcAddress.AllowDBNull = false;

DataSet dsAssessorData = new DataSet("Parcels");


// add a test row

DataRow rowNew = dsAssessorData.Tables[0].NewRow();

rowNew["ParcelID"] = "P34031";

rowNew["Assessed"] = "255000";

rowNew["Address"] = "88493" + " " + "Nowhere Road" +

", " + "Everywhere, WA 98834";


DataGrid1.DataSource = dsAssessorData.Tables[0];



#region Web Form Designer generated code

override protected void OnInit(EventArgs e)



// CODEGEN: This call is required by the ASP.NET Web Form Designer.




/// <summary>

/// Required method for Designer support - do not modify

/// the contents of this method with the code editor.

/// </summary>

private void InitializeComponent()


this.btnAddParcel.ServerClick += new
System.EventHandler(this.btnAddParcel_ServerClick) ;

this.Load += new System.EventHandler(this.Page_Load);



//************* error occurs in this click event

private void btnAddParcel_ServerClick(object sender, System.EventArgs e)


// connect to database and retrieve data

string strCn =
SCConnection.GetConnectionString(SCConnection.Asse ssor,false);

string strSql = "select * from ExciseAffidavitView where [parcel number] =
'" +

TextBox2.Text + "'";

SqlConnection cn = new SqlConnection(strCn);


SqlCommand cmdGetAssrData = new SqlCommand(strSql,cn);

SqlDataReader drAssessor = cmdGetAssrData.ExecuteReader();

// I tried adding this code, but it didn't help

//DataSet dsAssessorData = new DataSet("Parcels");

//dsAssessorData = (DataSet) Session["dsAssr"];



//*************** error occurs on the following line

DataRow rowNew = dsAssessorData.Tables[0].NewRow();

rowNew["ParcelID"] = drAssessor.GetString(0);

rowNew["Assessed"] = drAssessor.GetString(15);

rowNew["Address"] = drAssessor.GetString(3) + " " + drAssessor.GetString(4)

", " + drAssessor.GetString(5);


DataGrid1.DataSource = dsAssessorData.Tables[0];



// update session variable

Session["dsAssr"] = dsAssessorData;
TextBox1.Text += TextBox2.Text;

TextBox2.Text = "";


Jul 19 '05 #1
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2 Replies

P: n/a

Ray at work

"John Holmes" <jo****> wrote in message

using System.Collections;

Jul 19 '05 #2

P: n/a
Hello John,

Thanks for posting in the group.

Based on my understanding, now the question is: You stored a dataset in a
session in application. However, you got "Object reference not set
to an instance of an object" when the page is executing.

After reviewing your code, I found that in the else block, dsAssessorData
is defined as a local variable. So it will not be valid beyond the else
block. That could cause some potenial erros if we still want to use this
variable later.

BTW, in the future, it would be best to post these questions in the
following newsgroup.


All .NET ASP.NET issues, configuration and other questions are posted in
the newsgroup above.

The reason why we recommend posting appropriately is you will get the most
qualified pool of respondents, and other partners who the newsgroups
regularly can either share their knowledge or learn from your interaction
with us. Also, this is to make sure that the responders can better track
the problem. Thank you for your understanding.

Thanks again for using Microsoft MSDN Newsgroups.

Have a nice day!

Best regards,
Yanhong Huang
Microsoft Community Support

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Jul 19 '05 #3

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