CRUD SQL y C Sharp

Código del video :
create database colegios
go
use colegios
go
create table alumnos(
id int identity (1,1),
nombres varchar(100),
edad int,
telefono char(12)
)
--insert
insert into alumnos values('Ariana Lopez',25,85324569)
insert into alumnos values('Nelson Cardenas',20,873456786)
go
select*from alumnos




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 proyec
{
public partial class Form1 : Form
{
SqlConnection cn = new SqlConnection(@"Data Source=ISRAEL-;Initial Catalog=colegios;Integrated Security=True");
public Form1()
{
InitializeComponent();
}

private void Form1_Load(object sender, EventArgs e)
{
getAlumnos();
}
private void getAlumnos()
{
try
{
SqlDataAdapter da = new SqlDataAdapter("select*from alumnos ", cn);
DataTable dt = new DataTable();
da.Fill(dt);
this.dataGridView1.DataSource = dt;

}
catch (InvalidCastException ex)
{
throw ex;
}
finally
{
cn.Close();
}
}
//insert alumnos

private void inserAlumnos()
{
try
{
string a;
a = "insert into alumnos values('" + textBox2.Text + "','" + Convert.ToInt32(textBox3.Text) + "','" + (textBox4.Text) + "')";
SqlCommand cmd = new SqlCommand(a, cn);
cn.Open();
cmd.ExecuteNonQuery();
cn.Close();
MessageBox.Show("Registro guardado correctamente");
getAlumnos();
}
catch (InvalidCastException ex)
{
throw ex;

}

}



private void btnCerrar_Click(object sender, EventArgs e)
{
Close();
}

private void btnGuardar_Click(object sender, EventArgs e)
{
inserAlumnos();

textBox2.Text = "";
}



}

}