SQLException Occured - asp.net

Question

A first chance exception of type 'System.Data.SqlClient.SqlException' occurred in System.Data.dll
I was trying database connection. But I am getting this error. Please help me.
using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.Data.SqlClient;
namespace OLSWebApp
{
public partial class ItemTypeWebForm : System.Web.UI.Page
{
static string constr = "server=DESKTOP-3N4UH9N; user=sa; pwd=ZEESHAN#123; Initial Catalog=Online Order System";
protected void Page_Load(object sender, EventArgs e)
{
}
protected void SaveButton_Click(object sender, EventArgs e)
{
SqlConnection conn = new SqlConnection(constr);
conn.Open();
string q = "Insert INTO ItemType values ('"+ TypeIdTextBox.Text +"'), ('"+ TypeTextBox.Text +"'),('"+ NameTextBox.Text +"')";
SqlCommand cmd = new SqlCommand(q,conn);
cmd.ExecuteNonQuery();
}
}
}
con.Open() statement generates error..

Answer 1

For SQL Server Connection types please first read this document.
https://www.connectionstrings.com/sql-server/
For your example I think DESKTOP-3N4UH9N is your local PC not the server instance name, isn't it?
Please first find the server instance name by using SQL Server Management Studio (SSMS).
Please try below codes
Standard Security
using System.Data.SqlClient;
SqlConnection conn = new SqlConnection();
conn.ConnectionString =
"Server=myServerAddress; " +
"Database=myDataBase;" +
"User Id=myUsername;" +
"Password=myPassword;"
conn.Open();
Trusted Connection
using System.Data.SqlClient;
SqlConnection conn = new SqlConnection();
conn.ConnectionString =
"Server=myServerAddress;" +
"Database=myDataBase;" +
"Trusted_Connection=True;"
conn.Open();
Connection to a SQL Server instance
using System.Data.SqlClient;
SqlConnection conn = new SqlConnection();
conn.ConnectionString =
"Server=myServerName\myInstanceName;" +
"Database=myDataBase;" +
"User Id=myUsername;" +
"Password=myPassword;"
conn.Open();
Integrated Security
using System.Data.SqlClient;
SqlConnection conn = new SqlConnection();
conn.ConnectionString =
"Data Source=MyLocalSqlServerInstance;" +
"Initial Catalog=MyDatabase;" +
"Integrated Security=SSPI;"
conn.Open();

Answer 2

change this
conn.Open();
string q = "Insert INTO ItemType values ('"+ TypeIdTextBox.Text +"'), ('"+ TypeTextBox.Text +"'),('"+ NameTextBox.Text +"')";
SqlCommand cmd = new SqlCommand(q,conn);
cmd.ExecuteNonQuery();
into this
conn.Open();
string q = "Insert INTO ItemType values (#id, #type ,#name)";
SqlCommand cmd = new SqlCommand(q,conn);
cmd.Parameters.AddWithValue("#id", TypeIdTextBox.Text);
cmd.Parameters.AddWithValue("#type", TypeTextBox.Text);
cmd.Parameters.AddWithValue("#name", NameTextBox.Text);
cmd.ExecuteNonQuery();
conn.Close();
make sure the connection can open without any error