How I show data in textbox (data fetch from database) ?

Trim() use for remove white-spaces but why you used it i can't understand this concept kindly elaborate.
And this code that i attached not working

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Windows.Forms;
using System.Data.SqlClient;
namespace Show
{
    public partial class Form1 : Form
    {
        SqlConnection con = new SqlConnection(@"server = HOME-SAT/SQLEXPRESS;data source = .; initial catalog = test; user id=sa;pwd =pintu ;integrated security = SSPI");
        SqlCommand com=new SqlCommand();
        string str;
     
        public Form1()
        {
            InitializeComponent();
        }

        private void btnlogin_Click(object sender, EventArgs e)
        {
            if (TextBox_user_name.Text == "" || TextBox_password.Text == "")
            {
                MessageBox.Show("Please enter User ID or Password");
                TextBox_user_name.Text = "";
                TextBox_password.Text = "";

            }
            else
            {
                con.Open();
                str = "select * from login where username='" + TextBox_user_name.Text.Trim() + "' and password='" + TextBox_password.Text.Trim() + "'";
                com = new SqlCommand(str, con);
                SqlDataReader reader = com.ExecuteReader();
                while (reader.Read())
                {
                    txt_id.Text = (reader["id"].ToString());
                }
                reader.Close();
                con.Close();
            }
        }
    }
}

Thanks Satyapriya Nayak , i try to fetch data according to your syntax and successfully fetch the data.

Now i faced a new problem the description is that i have 2 records in database. I write username and password and click on button and messagebox occur that show Id against that username and password , it is ok but when i write username and password of second record it cannot show any Id against that username and password , so now please tell me how the problem is resolved ?

using System;
using System.Collections;
using System.Configuration;
using System.Data;
using System.Linq;
using System.Web;
using System.Web.Security;
using System.Web.UI;
using System.Web.UI.HtmlControls;
using System.Web.UI.WebControls;
using System.Web.UI.WebControls.WebParts;
using System.Xml.Linq;
using System.Data.SqlClient;
namespace Displaying_Data
{
    public partial class _Default : System.Web.UI.Page
    {
        string strConnString = ConfigurationManager.ConnectionStrings["ConnectionString"].ConnectionString;
        string str;
        SqlCommand com;

        protected void Page_Load(object sender, EventArgs e)
        {
            SqlConnection con = new SqlConnection(strConnString);
            con.Open();
            str = "select * from emp";
            com = new SqlCommand(str, con);
            SqlDataReader reader = com.ExecuteReader();
            reader.Read();
            textbox1.Text = reader["Name"].ToString(); 

        }
    }
}