when select drop down items the values should be displayed in the textbox in Asp.net

//function i used for this
private void BindView()
{
    SqlConnection con = new SqlConnection(System.Configuration.ConfigurationManager.ConnectionStrings["sqlCon"].ConnectionString);
    con.Open();
    using (SqlCommand command = con.CreateCommand())
    {
        int productid = Convert.ToInt32(ddproduct.SelectedValue.ToString());
        string query = "SELECT productprice FROM Product_Master WHERE is_activate=1 AND productid=" + productid.ToString();
        command.CommandText = query;
            
        SqlDataReader dr = command.ExecuteReader();
        if (dr.Read())
        {
            txtpayment.Text = dr["productprice"].ToString();
        }
        else
        {

        }
        con.Close();
        dr.Close();
    }
}


//drop down 
protected void ddproduct_SelectedIndexChanged(object sender, EventArgs e)
{
    BindView();
}