3 tier architecture in asp.net c#

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C#

ASP.NET

my first class is CustomerDal.cs

using System;
using System.Collections.Generic;
using System.Data;
using System.Data.SqlClient;
using System.Web;

/// <summary>
/// Summary description for Class2
/// </summary>
public class CustomerDal
{
	public CustomerDal()
	{
		//
		// TODO: Add constructor logic here
		//
	}
    public int insert(string customer_id,string first_name,string last_name)  // public int insert(string customer_id,string first_name,string last_name)
    {
        SqlConnection conn = new SqlConnection(System.Configuration.ConfigurationManager.ConnectionStrings["learnConnectionString"].ConnectionString);
        string s1 = "insert into customer(customer_id,first_name,last_name) values(@customer_id,@first_name,@last_name) ";
        SqlCommand cmd = new SqlCommand(s1,conn);
        //set the command type
        cmd.CommandType = CommandType.Text;
        try
        {
            cmd.Parameters.Add(new SqlParameter("customer_id", System.Data.SqlDbType.Int, 4));
           cmd.Parameters["@customer_id"].Direction = System.Data.ParameterDirection.Input;
            cmd.Parameters["@customer_id"].Size = 25;
            cmd.Parameters["@customer_id"].Value = customer_id;

            cmd.Parameters.Add(new SqlParameter("first_name", SqlDbType.VarChar));
            cmd.Parameters["@first_name"].Direction = ParameterDirection.Input;
            cmd.Parameters["@first_name"].Size = 25;
            cmd.Parameters["@first_name"].Value = first_name;

            cmd.Parameters.Add(new SqlParameter("last_name", SqlDbType.VarChar));
            cmd.Parameters["@last_name"].Direction = ParameterDirection.Input;
            cmd.Parameters["@last_name"].Size = 25;
            cmd.Parameters["@last_name"].Value = last_name;
            conn.Open();
           return cmd.ExecuteNonQuery();
        }
        catch
        {
          throw;
        }
        finally
        {
           conn.Close();
           cmd.Dispose();
            conn.Dispose();
        }
    }
}

my second class ic CustomerBal.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;

/// <summary>
/// Summary description for Class1
/// </summary>
public class CustomerBal
{
	public CustomerBal()
	{
		//
		// TODO: Add constructor logic here
		//
	}
    public int insert(string customer_id, string first_name, string last_name)
    {
        CustomerDal cdal = new CustomerDal();
        try
        {
            return cdal.insert(customer_id, first_name, last_name);
        }
        catch 
        {
            throw;
        }
        finally
        {
            cdal=null;
        }
    }
}

and my default.aspx page is

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;

public partial class Default5 : System.Web.UI.Page
{
    protected void Page_Load(object sender, EventArgs e)
    {

    }
    protected void Button1_Click(object sender, EventArgs e)
    {
        CustomerBal Cbal = new CustomerBal();
        try
        {
            if (Cbal.insert(TextBox1.Text, TextBox2.Text, TextBox3.Text) > 0)
            {
                Label1.Text = "Record Inserted Successfully";
            }
            else
            {
                Label1.Text = "Record not Inserted";
            }
        }
        catch(Exception ex)
        {
            Label1.Text = ex.Message;

        }
        finally 
        {
            Cbal = null;
        }
    }
}

and when i run this program than the error is coming
Error:An SqlParameter with ParameterName '@customer_id' is not contained by this SqlParameterCollection.
please give me solution......