How do I eliminate the error ---system.data.sqlclient.sqlexception: 'conversion failed when converting the nvarchar value '58XX' to data type int.'

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I have to display the rows where class=classno where classno is taken from previous page and displayed here as a label .

The code is bellow:

What I have tried:

Class_detail.aspx

ASP.NET

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

<!DOCTYPE html>

<html xmlns="http://www.w3.org/1999/xhtml">
<head runat="server">
    <title></title>
</head>
<body>
    <form id="form1" runat="server">
        <div>
            <hr />
             <asp:Label ID="LabelClass" runat="server" Text="class Details" BorderColor="White"></asp:Label>
           
            <hr /> 
            <br />
            <br />
            <asp:GridView ID="GridView3" runat="server"></asp:GridView>
        </div>

    </form>
</body>
</html>

Class_details.aspx.cs

C#

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.Web.UI.WebControls;
using System.Data.SqlClient;
using System.Data;
using System.Configuration;

namespace Websearch
{
    public partial class Class_Details : System.Web.UI.Page
    {
        protected void Page_Load(object sender, EventArgs e)
        {
            String MainCon = 
           ConfigurationManager.ConnectionStrings["MyConn"].ConnectionString;
            SqlConnection sqlcon = new SqlConnection(MainCon);
            sqlcon.Open();
            LabelClass.Text = Request.QueryString["Classno"].ToString();
            int Classnum = Convert.ToInt32(LabelClass.Text);
            SqlCommand sqlcom1 = new SqlCommand();
            String sqlquerry1 = "Select Class, Item from [customer_class_item] where 
            Class = " + Classnum ;
            sqlcom1.CommandText = sqlquerry1;
            sqlcom1.Connection = sqlcon;
            sqlcom1.Parameters.AddWithValue("Class", LabelClass.Text);
            DataTable dt1 = new DataTable();
            SqlDataAdapter sd1 = new SqlDataAdapter(sqlcom1);
            sd1.Fill(dt1);
            GridView3.DataSource = dt1;
            GridView3.DataBind();

Posted 15-Feb-21 0:26am

Ananya19

Updated 15-Feb-21 22:00pm

Richard Deeming

v2

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Comments

Maciej Los 15-Feb-21 6:31am

If classNo is a text, why do you convert it to int?
int Classnum = Convert.ToInt32(LabelClass.Text);

Richard Deeming 16-Feb-21 4:04am

Whilst in this specific case you're probably safe, since Classnum is an integer, using string concatenation to build a query can and will lead to SQL Injection[^] vulnerabilities, and massive fines when your database is breached.

NEVER use string concatenation to build a SQL query. ALWAYS use a parameterized query.

Everything you wanted to know about SQL injection (but were afraid to ask) | Troy Hunt[^]
How can I explain SQL injection without technical jargon? | Information Security Stack Exchange[^]
Query Parameterization Cheat Sheet | OWASP[^]

using (SqlConnection sqlcon = new SqlConnection(MainCon))
using (SqlCommand sqlcom1 = new SqlCommand("SELECT Class, Item FROM [customer_class_item] WHERE Class = @Class", sqlcon))
{
    sqlcom1.Parameters.AddWithValue("@Class", Classnum);
    
    DataTable dt1 = new DataTable();
    SqlDataAdapter sd1 = new SqlDataAdapter(sqlcom1);
    sd1.Fill(dt1);
    
    GridView3.DataSource = dt1;
    GridView3.DataBind();
}

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OriginalGriff · Answer 1 · 2021-02-15T00:32:00

You can't. You are sending non-numeric data to a numeric field, so SQL is trying to convert it to an integer and rightly failing: "58XX" is not a number.

If you want to send data to numeric fields you need to validate it first, and tell the user when he made a mistake instead of blindly assuming he can type!