Creating and Storing Values in a Datatable Using Session Variable

Question

I'm having a problem storing and displaying values from a datatable. My goal is to add a value to a datatable, reload the page, and then display the datatable with the added value and give the ability to enter more. Currently, I can't even get the data table to show up. I am sure that it is an issue with saving the data in a session, but I simply cannot get the saving/loading from session to work.

I appericate any help you guys can give me, and I apologize if it's something really blindingly simple. Seems I just need a second opinion on it.

This is my current webpage,

<%@ Page Language="C#" AutoEventWireup="true" CodeBehind="WebForm1.aspx.cs" Inherits="WebApplication1.WebForm1" %>

<!DOCTYPE html>

<html xmlns="http://www.w3.org/1999/xhtml">
<head runat="server">
    <title></title>
</head>
<body>
    <form id="form1" runat="server">
    <div>
    <p><asp:TextBox ID="txtName" runat="server"></asp:TextBox><asp:TextBox ID="txtEmail" runat="server"></asp:TextBox><asp:TextBox ID="txtAddress" runat="server"></asp:TextBox><asp:TextBox ID="txtInfo" runat="server"></asp:TextBox></p>
    <p>
        <asp:Button ID="btnAdd" runat="server" Text="Add" OnClick="btnAdd_Click"/></p>
    </div>
    <div>
        <table border="1">
            <asp:Repeater ID="rptrData" runat="server">
                <HeaderTemplate>
                    <tr>
                        <td>Name</td>
                        <td>Email</td>
                        <td>Address</td>
                        <td>Info</td>
                    </tr>
                </HeaderTemplate>
                <ItemTemplate>
                    <tr>
                        <td><%#Eval("Name") %></td>
                        <td><%#Eval("Email") %></td>
                        <td><%#Eval("Address") %></td>
                        <td><%#Eval("Info") %></td>
                    </tr>
                </ItemTemplate>
                <AlternatingItemTemplate>
                    <tr>
                        <td><%#Eval("Name") %></td>
                        <td><%#Eval("Email") %></td>
                        <td><%#Eval("Address") %></td>
                        <td><%#Eval("Info") %></td>
                    </tr>
                </AlternatingItemTemplate>
            </asp:Repeater>
        </table>
    </div>
    </form>
</body>
</html>

And my current code behind,

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.Net.Mail;
using System.Net;
using System.Data;
using System.Collections;

namespace WebApplication1
{
    public partial class WebForm1 : System.Web.UI.Page
    {
        public DataTable MyDT = new DataTable();
        protected void Page_Load(object sender, EventArgs e)
        {
            if (!Page.IsPostBack)
            {
                DataTable MyDT = (DataTable)Session["MyDT"];
                rptrData.DataSource = MyDT;
                rptrData.DataBind();
            }
        }
        protected void btnAdd_Click(object sender, EventArgs e)
        {
            //DataTable MyDT = new DataTable();
            MyDT.Columns.Add("Name");
            MyDT.Columns.Add("Address");
            MyDT.Columns.Add("Email");
            MyDT.Columns.Add("Info");
            string Name = txtName.Text;
            string Address = txtAddress.Text;
            string Email = txtEmail.Text;
            string Info = txtInfo.Text;

            DataRow dtRow = MyDT.NewRow();
            dtRow["Name"] = Name;
            dtRow["Address"] = Address;
            dtRow["Email"] = Email;
            dtRow["Info"] = Info;

            Session.Add("MyDT", MyDT);
        }  
    }
}

You only want to add the columns to the datatable once, i recommend in page load, not everytime you click the add button. — Taylor Brown
– Taylor Brown, Commented Mar 21, 2016 at 20:18

Adam Heeg · Accepted Answer · 2016-03-21 20:32:51Z

1

Your code has a few basic issues. If you see what I changed and compare it to what you had you should be able to figure out some of the problems.

public DataTable MyDT;
protected void Page_Load(object sender, EventArgs e)
{
    if (!IsPostBack)
    {
        MyDT = new DataTable();
        MyDT.Columns.Add("Name");
        MyDT.Columns.Add("Address");
        MyDT.Columns.Add("Email");
        MyDT.Columns.Add("Info");
        Session["MyDT"] = MyDT;
    }
    else
    {
        // Here we load the session datatable to the variable you declared at the top.
        // It will be ready for us in the button click event.  We know this because
        // of asp.net page lifecycle which will run the page_load before the button
        // click event.

        MyDT = (DataTable)Session["MyDT"];
    }

    // You can bind the repeater here also if you want.  Doesn't really make a huge difference
}



protected void btnAdd_Click(object sender, EventArgs e)
{
    string Name = txtName.Text;
    string Address = txtAddress.Text;
    string Email = txtEmail.Text;
    string Info = txtInfo.Text;

    DataRow dtRow = MyDT.NewRow();
    dtRow["Name"] = Name;
    dtRow["Address"] = Address;
    dtRow["Email"] = Email;
    dtRow["Info"] = Info;
    MyDT.Rows.Add(dtRow);

    rptrData.DataSource = MyDT;
    rptrData.DataBind();
}

You'll notice I don't even have to save the datatable back to the session variable. The DataTable is a reference type so any changes I make to the variable MyDT are being saved to the same spot the Session variable is pointing to.

I also removed the new declaration at the top if your class for MyDT. You don't need that.

Finally you'll see that the columns are created a single time when the page first loads. As someone commented in your question recreating the columns is not a good idea or needed.

edited Mar 21, 2016 at 20:32

answered Mar 21, 2016 at 20:26

Adam Heeg

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3 Comments

Patrick O Over a year ago

Hey, thanks a lot for your help. The code looks a lot cleaner, I've learned a lot from it. I am having one problem, it's throwing "System.NullReferenceException: Object reference not set to an instance of an object." because when it attempts to load the new datarow"dtRow" both dtRow and MyDT are null.

Patrick O Over a year ago

Hey! Seemed to have fixed it, sorry about that. Thank you a lot, I really appreciate it.

Adam Heeg Over a year ago

Are you sure you have the else statement in Page_Load which assigns the Session["MyDT"] to the table? If you do have that then my next question is what browser are you using? Also, set a break point and test your Session class to make sure it is saving data. Your security settings may be blocking that.

Taylor Brown · Accepted Answer · 2016-03-21 20:11:02Z

I have a solution for this in VB, don't have time to convert to C# myself, but hopefully this will help.

The idea is to create the table first when loading the page and store it in the session var. Then to add to it, create a new DataRow and insert them into the table, update the session var, and rebind to the grid.

Protected Sub Page_Load(ByVal sender As Object, ByVal e As System.EventArgs) Handles Me.Load

    If Not IsPostBack Then

        CreateDetailTable()

    End If

End Sub


''' <summary>
''' create the detail table and store in a session var
''' </summary>
''' <remarks></remarks>

Private Sub CreateDetailTable()

    Dim dt As New DataTable

    dt.Columns.Add("McsStatusId")
    dt.Columns.Add("McsHeaderId")
    dt.Columns.Add("SeqNo")
    dt.Columns.Add("Status")
    dt.Columns.Add("DateTime")
    dt.Columns.Add("Timezone")
    dt.Columns.Add("Location")
    dt.Columns.Add("State")
    dt.Columns.Add("Country")
    dt.Columns.Add("Comments")
    dt.Columns.Add("TransmittedFlag")
    dt.Columns.Add("TransmissionNo")
    dt.Columns.Add("AddUserId")
    dt.Columns.Add("AddTime")
    dt.Columns.Add("LastUserId")
    dt.Columns.Add("LastTime")
    dt.Columns.Add("DeleteFlag")

    Session(hdnSessionVar.Value) = dt

End Sub


''' <summary>
''' add detail record to the table stored in the session, then use this table to refresh the grid
''' </summary>
''' <remarks></remarks>

Private Function AddDetailToGrid() As Boolean

    Dim rtn As Boolean = False

    Try

        Dim u As New ClassUserProfile
        u.GetUserFromSession()

        Dim dt As DataTable = TryCast(Session(hdnSessionVar.Value), DataTable)
        Dim dr As DataRow

        dr = dt.NewRow

        dr("McsStatusId") = 0 'filler
        dr("McsHeaderId") = hdnMcsHeaderId.Value 'filler
        dr("SeqNo") = 0 'filler
        dr("Status") = ddlStatus.SelectedItem.Text.Trim
        dr("DateTime") = txtStatusDate.Text.Trim & " " & txtStatusTime.Text.Trim
        dr("Timezone") = ddlTimezones.SelectedItem.Text.Trim
        dr("Location") = txtStatusLocation.Text.Trim
        dr("State") = txtStatusState.Text.Trim
        dr("Country") = txtStatusCountry.Text.Trim
        dr("Comments") = txtStatusComments.Text.Trim
        dr("TransmittedFlag") = 0 'these two fields should not be able to be added by user, they will be updated in a DB job that runs, and
        dr("TransmissionNo") = 0 'once they are set this status detail record will no longer be editable
        dr("AddUserId") = 0 'filler
        dr("AddTime") = DateTime.Now 'filler
        dr("LastUserId") = 0 'filler
        dr("LastTime") = DateTime.Now  'filler
        dr("DeleteFlag") = 0 'filler

        If hdnSeqNo.Value = "" Then
            dt.Rows.Add(dr)
        Else
            dt.Rows.InsertAt(dr, hdnSeqNo.Value)
        End If

        Session(hdnSessionVar.Value) = dt 'update the session var

        'bind the grid
        gridMcsStatus.DataSource = dt
        gridMcsStatus.DataBind()

        rtn = True

    Catch ex As Exception

        mbStatus.ShowErrorMsg("There was an error adding to the grid: " & ex.ToString)

    End Try

    Return rtn

End Function

''' <summary>
''' bind the session var containing the table data to the grid
''' </summary>
''' <remarks></remarks>

Private Sub BindStatusDetailGrid()

    Using dt As DataTable = TryCast(Session(hdnSessionVar.Value), DataTable)

        gridMcsStatus.DataSource = dt
        gridMcsStatus.DataBind()

    End Using

End Sub

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