Object reference not set to an instance of an object while clicking on submit button - asp.net

Question

I have created a webpage in asp.net
on right hand side I have an ListBox which binds data from database when page loads, Its code at Default.aspx is as below
<asp:ListBox ID="ListOfSql" runat="server"
SelectionMode="Single" DataTextField="sql_name" DataValueField="sql_text"
style="margin-left: 0px; width:auto; height:300px" EnableViewState="true">
</asp:ListBox>
and at default.aspx.cs page its code is
private void loadSqlList()
{
if (!IsPostBack)
{
conn.Open();
DataTable dt = new DataTable();
DataSet ds = new DataSet();
string preSql = "select sql_name, sql_text from cn_sql_log order by sql_name";
OracleDataAdapter da = new OracleDataAdapter(preSql, conn);
da.Fill(ds);
ListOfSql.DataSource = ds;
ListOfSql.DataTextField = "Sql_Name";
ListOfSql.DataValueField = "sql_Text";
ListOfSql.DataBind();
ListOfSql.SelectedIndex = 0;
conn.Close();
}
}
I am calling loadSqlList() on page_load so that I can get list from database in ListBox.
Now I have an submit button
<asp:Button ID="btnPreSqlExe" runat="server" Text="Sumbit"
onclick="btnPreSqlExe_Click">
</asp:Button>
on default.aspx.cs page submit button's code is
protected void btnPreSqlExe_Click(object sender, EventArgs e)
{
txtQuery.Text = ListOfSql.SelectedItem.Text.ToString();
}
Now when I want that on clicking on submit button then text of selected item should appear in textbox which is at left hand side
when I do so I am getting an error page
complete code of default.aspx.cs page is as below
using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.Data.OracleClient;
using System.Data;
using System.Data.SqlClient;
using System.Configuration;
using System.Windows.Forms;
using System.IO;
using System.Threading;
public partial class _Default : System.Web.UI.Page
{
System.Data.OracleClient.OracleConnection conn = new OracleConnection("Data Source = orcl; user id=*****;password = *****; unicode = true;");
//OracleConnection conn = new OracleConnection("Data Source=orcl;Persist Security Info=True;User ID=system;password = ravi_123;Unicode=True");
DataTable dt = new DataTable();
protected void Page_Load(object sender, EventArgs e)
{
if (!IsPostBack)
{
}
loadSqlList();
lblLastRefresh.Text = DateTime.Now.ToShortTimeString();
lblDate.Text = DateTime.Now.ToShortDateString();
Response.Cache.SetAllowResponseInBrowserHistory(false);
//============================================= Checking user logged in or not ===================================
//if (Session["txtUserName"] == null)
//{
// Response.Write("<Script Language ='JavaScript'> alert('Session Expired, please login again')</script>");
// conn.Close();
// Response.Redirect("login.aspx");
//}
//============================================= Checking user logged in or not (END) ===================================
//=====================User Ip Tracer =======================
string UserIp = Request.ServerVariables["http_x_forwarded_for"];
string UserHost = Request.ServerVariables["http_X_forwarded_for"];
string userMacAdd = Request.ServerVariables["http_x_forwarde_for"];
if (string.IsNullOrEmpty(UserIp))
{
UserIp = Request.ServerVariables["Remote_ADDR"];
UserHost = System.Net.Dns.GetHostName();
}
LblLoc.Text = UserIp;
LblHostName.Text = UserHost;
Response.Cache.SetCacheability(HttpCacheability.NoCache);
Response.Cache.SetNoStore();
Response.AddHeader("Pragma", "no-cache");
//=====================User Ip Tracer code end =======================
if (!IsPostBack)
{
}
}
protected void onclick_logout(object sender, EventArgs e)
{
conn.Close();
Session.Clear();
Response.Redirect("login.aspx");
}
protected void exportToExcel_Click(object sender, EventArgs e)
{
try
{
conn.Open();
string sql;
sql = txtQuery.Text.ToString();
OracleDataAdapter da = new OracleDataAdapter(sql, conn);
//================ User Details Data Insertion in DataBase Ends here ===============
da.Fill(dt);
ExportTableData(dt);
Response.Write("<Script>alert('Ip Locked in DB')</Script>");
}
catch (Exception ex)
{
lblError.Text = ex.Message.ToString();
}
}
protected void btnClear_clik(object sender, EventArgs e)
{
txtQuery.Text = string.Empty;
}
private void ExportTableData(DataTable dtdata)
{
string attach = "attachment;filename="+ListOfSql.SelectedItem.Text.ToString()+".xls";
Response.ClearContent();
Response.AddHeader("content-disposition", attach);
Response.ContentType = "application/ms-excel";
if (dtdata != null)
{
foreach (DataColumn dc in dtdata.Columns)
{
Response.Write(dc.ColumnName + "\t");
}
Response.Write(System.Environment.NewLine);
foreach (DataRow dr in dtdata.Rows)
{
for (int i = 0; i < dtdata.Columns.Count; i++)
{
Response.Write(dr[i].ToString() + "\t");
}
Response.Write("\n");
}
Response.End();
}
}
// References for this page
// http://forums.asp.net/t/1768549.aspx
protected void btnPreSqlExe_Click(object sender, EventArgs e)
{
txtQuery.Text = ListOfSql.SelectedItem.Text.ToString();
}
private void loadSqlList()
{
if (!IsPostBack)
{
conn.Open();
DataTable dt = new DataTable();
DataSet ds = new DataSet();
string preSql = "select sql_name, sql_text from cn_sql_log order by sql_name";
OracleDataAdapter da = new OracleDataAdapter(preSql, conn);
da.Fill(ds);
ListOfSql.DataSource = ds;
ListOfSql.DataTextField = "Sql_Name";
ListOfSql.DataValueField = "sql_Text";
ListOfSql.DataBind();
ListOfSql.SelectedIndex = 0;
conn.Close();
}
}
}

Answer 1

If a selection isn't made then ListOfSql.SelectedItem will be null, and therefore calling .Text on that will result in your error.
Try changing your code to something like this:
protected void btnPreSqlExe_Click(object sender, EventArgs e)
{
if(ListOfSql.SelectedItem != null)
{
txtQuery.Text = ListOfSql.SelectedItem.Text.ToString();
}
}