C# 使用c在asp.net上出现致命错误#

我正在创建一个网上商店，我在添加产品时遇到问题，我总是收到一个致命错误。我尝试了不同类型的方法只是为了解决我的问题，但我仍然无法解决它。希望得到一些帮助和解释。多谢各位

     protected void btn_submit_Click(object sender, EventArgs e)
    {
        constructor var = new constructor();
        var.ItemName = txtbx_proName.Text;
        if (rd_car.Checked)
        {
            var.ItemID = '1';
        }
        else if (rd_acc.Checked)
        {
            var.ItemID = '2';
        }
        else if (rd_others.Checked)
        {
            var.ItemID = '3';
        }
        var.Price = Convert.ToDouble(txtbx_price.Text);
        var.Stock = Convert.ToInt32(txtbx_stock.Text);
        var.Description = txtbx_desc.Text;
        //var.Image = image;

        using (MySqlConnection con = new MySqlConnection(ConfigurationManager.ConnectionStrings["DBCon"].ConnectionString))
        {
            try
            {
                con.Open();
                string sql = "INSERT INTO product_tbl (`id`, `product_name`, `product_price`, `product_desc`, `product_type`, `product_stock`) VALUES (NULL,@itemname,@pro_price,@pro_desc,@pro_stock);";
                MySqlCommand cmd = new MySqlCommand(sql, con);
                cmd.Parameters.AddWithValue("@itemname", var.ItemName);
                cmd.Parameters.AddWithValue("@pro_desc", var.Description);
                cmd.Parameters.AddWithValue("@pro_price", var.Price);
                cmd.Parameters.AddWithValue("@pro_type", var.ItemID);

                cmd.ExecuteNonQuery();
                lbl_result.Text = "New Product Added!";

                txtbx_desc.Text = "";
                txtbx_price.Text = "";
                txtbx_proName.Text = "";
                txtbx_stock.Text = "";

            }
            catch (Exception ex)
            {
                lbl_result.Text = "ERROR >> " + ex.Message + "!";
                txtbx_desc.Text = "";
                txtbx_price.Text = "";
                txtbx_proName.Text = "";
                txtbx_stock.Text = "";

            }
            finally
            {
                con.Close();
            }
        }
    }

构造函数代码：

    public class constructor
{
    private string _uname, _pword, _conNum, _desc, _itemName, _email,          _fName, _lName, _address, _categ, _changePword, _answer, _image;
    private int _uid, _stock, _itemId;
    [![enter image description here][1]][1]private double _price;

    public string Image
    {
        get { return _image; }
        set { _image = value; }
    }

    public string Answer
    {
        get { return _answer; }
        set { _answer = value; }
    }

    public string ChangePass
    {
        get { return _changePword; }
        set { _changePword = value; }
    }

    public string Category
    {
        get { return _categ; }
        set { _categ = value; }
    }

    public string Username
    {
        get { return _uname; }
        set { _uname = value; }
    }

    public string Password
    {
        get { return _pword; }
        set { _pword = value; }
    }

    public string Description
    {
        get { return _desc; }
        set { _desc = value; }
    }

    public string ItemName
    {
        get { return _itemName; }
        set { _itemName = value; }
    }

    public string Email
    {
        get { return _email; }
        set { _email = value; }
    }

    public string FirstName
    {
        get { return _fName; }
        set { _fName = value; }
    }

    public string LastName
    {
        get { return _lName; }
        set { _lName = value; }
    }

    public string Address
    {
        get { return _address; }
        set { _address = value; }
    }

    public double Price
    {
        get { return _price; }
        set { _price = value; }
    }

    public int UserId
    {
        get { return _uid; }
        set { _uid = value; }
    }

    public int Stock
    {
        get { return _stock; }
        set { _stock = value; }
    }

    public string ContactNumber
    {
        get { return _conNum; }
        set { _conNum = value; }
    }

    public int ItemID
    {
        get { return _itemId; }
        set { _itemId = value; }
    }
}

---

您的itemId是否在表中自动递增？我想这可能就是原因。如果它是自动递增的，则Id由sql server添加，并且在表中插入itemId auto increment时无法输入Id？？我认为这可能是原因。如果它是自动递增的，则该Id由sql server添加，并且在插入时不能输入Id