Question

我在asp.net enter image description here

中创建了一个网页

在右侧我有一个ListBox，它在页面加载时绑定数据库中的数据，它在Default.aspx的代码如下所示

  <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>

并在default.aspx.cs页面上，其代码为

 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();
        }
    }

我在page_load上调用 loadSqlList（），以便我可以从ListBox中的数据库中获取列表。现在我有一个提交按钮

<asp:Button ID="btnPreSqlExe" runat="server" Text="Sumbit" 
                onclick="btnPreSqlExe_Click">
 </asp:Button>

在default.aspx.cs页面上提交按钮的代码是

 protected void btnPreSqlExe_Click(object sender, EventArgs e)
    {

        txtQuery.Text = ListOfSql.SelectedItem.Text.ToString();
    }

现在，当我想点击提交按钮时，所选项目的文本应出现在左侧的文本框中当我这样做时，我得到一个错误页面

enter image description here

default.aspx.cs页面的完整代码如下

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

如果未进行选择，则ListOfSql.SelectedItem将为空，因此调用.Text将导致您的错误。

尝试将代码更改为以下内容：

protected void btnPreSqlExe_Click(object sender, EventArgs e)
{
    if(ListOfSql.SelectedItem != null)
    {
        txtQuery.Text = ListOfSql.SelectedItem.Text.ToString();
    }
}

单击提交按钮时，对象引用未设置为对象的实例

1 个答案: