Problem maintaining dataset in session variable

J

John Holmes

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 asp.net development but have lots of asp and ado experience.

Thanks,

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

if(!IsPostBack)

{

// create a dataset

BuildDataSet();

// save as session variable

Session["dsAssr"] = dsAssessorData;

}

else

{

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;

dtParcels.Columns.Add(dcID);


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

dcAssessedValue.AllowDBNull = false;

dtParcels.Columns.Add(dcAssessedValue);

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

dcAddress.AllowDBNull = false;

dtParcels.Columns.Add(dcAddress);


DataSet dsAssessorData = new DataSet("Parcels");

dsAssessorData.Tables.Add(dtParcels);

// add a test row

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

rowNew["ParcelID"] = "P34031";

rowNew["Assessed"] = "255000";

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

", " + "Everywhere, WA 98834";

dsAssessorData.Tables[0].Rows.Add(rowNew);

DataGrid1.DataSource = dsAssessorData.Tables[0];

DataGrid1.DataBind();

}

#region Web Form Designer generated code

override protected void OnInit(EventArgs e)

{

//

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

//

InitializeComponent();

base.OnInit(e);

}


/// <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);

}

#endregion

//************* 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.Assessor,false);

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

TextBox2.Text + "'";

SqlConnection cn = new SqlConnection(strCn);

cn.Open();

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"];

while(drAssessor.Read())

{

//*************** 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);

dsAssessorData.Tables[0].Rows.Add(rowNew);

DataGrid1.DataSource = dsAssessorData.Tables[0];

DataGrid1.DataBind();

}

// update session variable

Session["dsAssr"] = dsAssessorData;


TextBox1.Text += TextBox2.Text;

TextBox2.Text = "";

cn.Close();


}


}

}
 
S

Steven Cheng[MSFT]

Hi John Holmes,


Thank you for using Microsoft Newsgroup Services. Based on your desciption,
you are dealing a ASP.NET web page ,the page loaded a DataSet on first
request and stored it into Session, then in the later request(such as
button click post back.), retrieve the dataset from the Session and do some
operation(such as add a row). However, you encountered some errors when run
the page, seems the dataset isn't set to the Session. Please correct me if
my understanding it not quite exact.

After reading the page's code you provided, I found that there is something
incorrect in it:
you define a DataSet in the page class as a public member as below:
public DataSet dsAssessorData;

however, in the BuildDataSet function, you defined another function scope
local DataSet vairable:
private void BuildDataSet()

{

.........................


DataSet dsAssessorData = new DataSet("Parcels");

..........................

}

Also, in the Page_Load , you code like this:
..................
if(!IsPostBack)

{
// create a dataset
BuildDataSet();

// save as session variable
Session["dsAssr"] = dsAssessorData;
}
.......................

the "dsAssessorData" you set to the Session["dsAssr"] is the public class
member DataSet, not the local DataSet variable,
So it is set to "null" which caused the "Null reference" error. You may
set the new DataSet to the public class membe DataSet in BuildDataSet()
function.

In addtion, to make it clear, I've written a simple page showing how to
create a DataSet at first request and then store in Session for later
retrieving, here is the source:

---------------ASPX PAGE ---------------
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN" >
<HTML>
<HEAD>
<title>SessionDataSet</title>
<meta name="GENERATOR" Content="Microsoft Visual Studio .NET 7.1">
<meta name="CODE_LANGUAGE" Content="C#">
<meta name="vs_defaultClientScript" content="JavaScript">
<meta name="vs_targetSchema"
content="http://schemas.microsoft.com/intellisense/ie5">
</HEAD>
<body>
<form id="Form1" method="post" runat="server">
<table width="500" align="center">
<tr>
<td>
<asp:DataGrid id="dgSession" runat="server"></asp:DataGrid>
</td>
<td>
</td>
</tr>
<tr>
<td>
<asp:Button id="btnAdd" runat="server" Text="AddRow"></asp:Button>
</td>
<td>
</td>
</tr>
</table>
</form>
</body>
</HTML>

--------------------------CODE BIND
ASPX.CS-------------------------------------
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;

namespace NewWebApp
{
/// <summary>
/// Summary description for SessionDataSet.
/// </summary>
public class SessionDataSet : System.Web.UI.Page
{
protected System.Web.UI.WebControls.DataGrid dgSession;
protected System.Web.UI.WebControls.Button btnAdd;
protected DataSet dsTemp = null;

private void Page_Load(object sender, System.EventArgs e)
{
// Put user code to initialize the page here
if(!IsPostBack)
{
dsTemp = GetDataSet();
dgSession.DataSource = dsTemp;
dgSession.DataBind();
Session["DATASET"] = dsTemp;
}
else
{
dsTemp = (DataSet)Session["DATASET"];
}

}

protected DataSet GetDataSet()
{
DataSet ds = new DataSet("dsSession");
DataTable tb = new DataTable("tbSession");

tb.Columns.Add("id");
tb.Columns.Add("name");
tb.Columns.Add("email");

for(int i=0;i<20;i++)
{
DataRow newrow = tb.NewRow();
newrow["id"] = i.ToString();
newrow["name"] = "name" + i.ToString();
newrow["email"] = "email" + i.ToString();

tb.Rows.Add(newrow);
}

ds.Tables.Add(tb);

return ds;
}

#region Web Form Designer generated code
override protected void OnInit(EventArgs e)
{
//
// CODEGEN: This call is required by the ASP.NET Web Form Designer.
//
InitializeComponent();
base.OnInit(e);
}

/// <summary>
/// Required method for Designer support - do not modify
/// the contents of this method with the code editor.
/// </summary>
private void InitializeComponent()
{
this.btnAdd.Click += new System.EventHandler(this.btnAdd_Click);
this.Load += new System.EventHandler(this.Page_Load);

}
#endregion

private void btnAdd_Click(object sender, System.EventArgs e)
{
DataTable tb = dsTemp.Tables["tbSession"];
DataRow newrow = tb.NewRow();

Random rnd = new Random();
int rval = rnd.Next();

newrow["id"] = rval.ToString();
newrow["name"] = "name" + rval.ToString();
newrow["email"] = "email" + rval.ToString();

tb.Rows.Add(newrow);

dgSession.DataSource = dsTemp;
dgSession.DataBind();

}
}
}


Please try the preceding suggestion and let me know if you have any
question on it.


Steven Cheng
Microsoft Online Support

Get Secure! www.microsoft.com/security
(This posting is provided "AS IS", with no warranties, and confers no
rights.)
 
J

John Holmes

Thanks, this helped ... john

Steven Cheng said:
Hi John Holmes,


Thank you for using Microsoft Newsgroup Services. Based on your desciption,
you are dealing a ASP.NET web page ,the page loaded a DataSet on first
request and stored it into Session, then in the later request(such as
button click post back.), retrieve the dataset from the Session and do some
operation(such as add a row). However, you encountered some errors when run
the page, seems the dataset isn't set to the Session. Please correct me if
my understanding it not quite exact.

After reading the page's code you provided, I found that there is something
incorrect in it:
you define a DataSet in the page class as a public member as below:
public DataSet dsAssessorData;

however, in the BuildDataSet function, you defined another function scope
local DataSet vairable:
private void BuildDataSet()

{

........................


DataSet dsAssessorData = new DataSet("Parcels");

.........................

}

Also, in the Page_Load , you code like this:
.................
if(!IsPostBack)

{
// create a dataset
BuildDataSet();

// save as session variable
Session["dsAssr"] = dsAssessorData;
}
......................

the "dsAssessorData" you set to the Session["dsAssr"] is the public class
member DataSet, not the local DataSet variable,
So it is set to "null" which caused the "Null reference" error. You may
set the new DataSet to the public class membe DataSet in BuildDataSet()
function.

In addtion, to make it clear, I've written a simple page showing how to
create a DataSet at first request and then store in Session for later
retrieving, here is the source:

---------------ASPX PAGE ---------------
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN" >
<HTML>
<HEAD>
<title>SessionDataSet</title>
<meta name="GENERATOR" Content="Microsoft Visual Studio .NET 7.1">
<meta name="CODE_LANGUAGE" Content="C#">
<meta name="vs_defaultClientScript" content="JavaScript">
<meta name="vs_targetSchema"
content="http://schemas.microsoft.com/intellisense/ie5">
</HEAD>
<body>
<form id="Form1" method="post" runat="server">
<table width="500" align="center">
<tr>
<td>
<asp:DataGrid id="dgSession" runat="server"></asp:DataGrid>
</td>
<td>
</td>
</tr>
<tr>
<td>
<asp:Button id="btnAdd" runat="server" Text="AddRow"></asp:Button>
</td>
<td>
</td>
</tr>
</table>
</form>
</body>
</HTML>

--------------------------CODE BIND
ASPX.CS-------------------------------------
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;

namespace NewWebApp
{
/// <summary>
/// Summary description for SessionDataSet.
/// </summary>
public class SessionDataSet : System.Web.UI.Page
{
protected System.Web.UI.WebControls.DataGrid dgSession;
protected System.Web.UI.WebControls.Button btnAdd;
protected DataSet dsTemp = null;

private void Page_Load(object sender, System.EventArgs e)
{
// Put user code to initialize the page here
if(!IsPostBack)
{
dsTemp = GetDataSet();
dgSession.DataSource = dsTemp;
dgSession.DataBind();
Session["DATASET"] = dsTemp;
}
else
{
dsTemp = (DataSet)Session["DATASET"];
}

}

protected DataSet GetDataSet()
{
DataSet ds = new DataSet("dsSession");
DataTable tb = new DataTable("tbSession");

tb.Columns.Add("id");
tb.Columns.Add("name");
tb.Columns.Add("email");

for(int i=0;i<20;i++)
{
DataRow newrow = tb.NewRow();
newrow["id"] = i.ToString();
newrow["name"] = "name" + i.ToString();
newrow["email"] = "email" + i.ToString();

tb.Rows.Add(newrow);
}

ds.Tables.Add(tb);

return ds;
}

#region Web Form Designer generated code
override protected void OnInit(EventArgs e)
{
//
// CODEGEN: This call is required by the ASP.NET Web Form Designer.
//
InitializeComponent();
base.OnInit(e);
}

/// <summary>
/// Required method for Designer support - do not modify
/// the contents of this method with the code editor.
/// </summary>
private void InitializeComponent()
{
this.btnAdd.Click += new System.EventHandler(this.btnAdd_Click);
this.Load += new System.EventHandler(this.Page_Load);

}
#endregion

private void btnAdd_Click(object sender, System.EventArgs e)
{
DataTable tb = dsTemp.Tables["tbSession"];
DataRow newrow = tb.NewRow();

Random rnd = new Random();
int rval = rnd.Next();

newrow["id"] = rval.ToString();
newrow["name"] = "name" + rval.ToString();
newrow["email"] = "email" + rval.ToString();

tb.Rows.Add(newrow);

dgSession.DataSource = dsTemp;
dgSession.DataBind();

}
}
}


Please try the preceding suggestion and let me know if you have any
question on it.


Steven Cheng
Microsoft Online Support

Get Secure! www.microsoft.com/security
(This posting is provided "AS IS", with no warranties, and confers no
rights.)
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