Занести данные в таблицу

double a;
a = convert.todouble(textbox.text);

public string connectionString = "Data Source = 'DB.sdf'";
public void AddBrand(string brandName)
{
    try
    {
        SqlCeEngine engine = new SqlCeEngine(connectionString);
        SqlCeConnection connection = new SqlCeConnection(engine.LocalConnectionString);
        connection.Open();
        SqlCeCommand command = connection.CreateCommand();
        command.CommandText = "INSERT INTO brand (brand_name) VALUES (?)";
        command.Parameters.Add("brand_name", brandName);
        command.ExecuteScalar();
        connection.Close();
    }
    catch (Exception ex)
    {
        MessageBox.Show(ex.Message);
        return;
    }
}

using System.Data.SqlClient;
using System.Data.Sql;

public void AddItem(string _itemName, int _itemSize)
{
    try
    {
        Твое подключение
        connection.Open();
        SqlCommand command = connection.CreateCommand();
        command.CommandText = "INSERT INTO Items (ItemName, ItemSize) VALUES (?,?)";
        command.Parameters.Add("ItemName", _itemName);
        command.Parameters.Add("ItemSize", _itemSize);
        command.ExecuteScalar();
        connection.Close();
    }
    catch (Exception ex)
    {
        MessageBox.Show(ex.Message);
        return;
    }
}

AddItem("Кроссовки РИТА",42)

 ItemAdd_Zhel(Convert.ToDouble(textBox2.Text),Convert.ToDouble(textBox3.Text),Convert.ToDouble(textBox4.Text),Convert.ToDouble(textBox5.Text),Convert.ToDouble(textBox5.Text));

public void MetallAdd(double _Mn, double _Si, double _S, double _P, double _Fe)
{
    ...
}

public void ItemAdd_Zhel(double _Mn, double _Si, double _S, double _P, double _Fe)