An unhandled exception of type 'System.Data.SqlClient.SqlException' occurred in System.Data.dll Additional information: Invalid column name '5th'.

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I am making telephone directory but one error is coming in this line "
cmd.ExecuteNonQuery();" in the delete functionality please tell me what
I am doing wrong and My problem is that if I click the delete button to delete the data from table this error appears and data does not delete from database my c# code is something like this :-

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows.Forms;
using System.Data.SqlClient;

namespace Telephone
{
public partial class Phone : Form
{
SqlConnection con = new SqlConnection(@"Data Source = ZOHAIB\SQLEXPRESS; Initial Catalog = bscs; Integrated Security = True");

public Phone()
{
InitializeComponent();
}

private void Phone_Load(object sender, EventArgs e)
{
Display();
}
private void button1_Click(object sender, EventArgs e)
{
textBox1.Text = "";
textBox2.Clear();
textBox3.Text = "";
textBox4.Clear();
comboBox1.SelectedIndex = -1;
textBox1.Focus();
}

private void button2_Click(object sender, EventArgs e)
{
con.Open();

SqlCommand cmd = new SqlCommand(@"INSERT INTO [5th]

([First Name], [Last Name], [Mobile], [Email], [Catagory])

VALUES ('" + textBox1.Text + "','" + textBox2.Text + "','" + textBox3.Text + "','" + textBox4.Text + "','" + comboBox1.Text + "')", con);
cmd.ExecuteNonQuery();
con.Close();
MessageBox.Show("sucessfuly saved");
Display();
}
void Display()
{
SqlDataAdapter sda = new SqlDataAdapter("select * from [5th]", con);
DataTable dt = new DataTable();
sda.Fill(dt);
dataGridView1.Rows.Clear();
foreach (DataRow item in dt.Rows)
{
int n = dataGridView1.Rows.Add();
dataGridView1.Rows[n].Cells[0].Value = item["First Name"].ToString();
dataGridView1.Rows[n].Cells[1].Value = item[1].ToString();
dataGridView1.Rows[n].Cells[2].Value = item[2].ToString();
dataGridView1.Rows[n].Cells[3].Value = item[3].ToString();
dataGridView1.Rows[n].Cells[4].Value = item[4].ToString();



}

}

private void dataGridView1_MouseClick(object sender, MouseEventArgs e)
{
textBox1.Text= dataGridView1.SelectedRows[0].Cells[0].Value.ToString();
textBox2.Text= dataGridView1.SelectedRows[0].Cells[1].Value.ToString();
textBox3.Text=dataGridView1.SelectedRows[0].Cells[2].Value.ToString();
textBox4.Text = dataGridView1.SelectedRows[0].Cells[3].Value.ToString();
comboBox1.Text = dataGridView1.SelectedRows[0].Cells[4].Value.ToString();

}

private void button3_Click(object sender, EventArgs e)
{
con.Open();

SqlCommand cmd = new SqlCommand(@"DELETE FROM [5th]

WHERE ([5th]='" + textBox1.Text + "')", con);


cmd.ExecuteNonQuery();
con.Close();
MessageBox.Show("Delete sucessfully");
Display();
}
}
}>