加载、更新和显示泛型列表类

目标，这个概念有点像购物车，所以当他们向list(Detail)添加物品时，它会将他们添加的物品保存在内存中。当我第一次加载列表(网格)并添加更多行时，这种方法就会起作用。但是如果我设置了第一行，设置了商品和价格，然后决定再添加3行我添加的信息被删除，而不是保持它的值，只是加载更多的行列表，这将重新填充gridview。在过去，我已经用数据表做到了这一点，但我希望能够从中移动并使用这个列表类我还把它设置为viewstate，这样我就可以在整个页面中使用它。

private ListArDocumentdetail Detail
{
    get
    {
        ListArDocumentdetail _detail = new ListArDocumentdetail();
        if (ViewState["Detail"] != null)
        {
            _detail = (ListArDocumentdetail)ViewState["Detail"];    
        }
        return _detail;
     }
     set
     {
        ViewState["Detail"] = value;
     }
}
protected void Page_Load(object sender, EventArgs e)
{
    //creates 2 rows to start off
    CreateRows(2);
}
public void CreateRows(int rowstoadd)
{
    int newtotalrows = Detail.Count + rowstoadd - 1;
    for (int i = Detail.Count; i <= newtotalrows; i++)
    {
        ArDocumentdetail detail = new ArDocumentdetail();
        detail.Lineid = i;
        detail.Itemid = 0;
        detail.Quantity = 1;
        if (Detail.Count > 0)
            Detail.Insert(Detail.Count, detail);
        else
            Detail.Add(detail);
        Detail = Detail;
    }
    gvInvoiceDetail.DataSource = Detail;
    gvInvoiceDetail.DataBind();
    GridViewRow row = gvInvoiceDetail.Rows[gvInvoiceDetail.Rows.Count - 1];
    ImageButton btnAdd = (ImageButton)row.FindControl("btnAdd");
    btnAdd.Visible = true;
}
protected void ibAdd_Click(object sender, ImageClickEventArgs e)
{
    //user can type in how many rows they want to add on to current amount of rows
    //so since grid starts off at 2 and they type 3 the grid refreshes with 5 rows.
    CreateRows(Convert.ToInt32(txtRows.Text));
}
protected void UpdateRow(object sender, EventArgs e)
{
    ImageButton btnUpdate = sender as ImageButton;
    GridViewRow row = btnUpdate.NamingContainer as GridViewRow;
    TextBox txtPrice = (TextBox)row.FindControl("txtPrice");
    TextBox txtQuantity = (TextBox)row.FindControl("txtQuantity");
    DropDownList ddlDescription = (DropDownList)row.FindControl("ddlDescription");
    int index = Detail.FindIndex(f => f.Lineid == row.RowIndex);
    Detail[index].Itemid = Convert.ToInt32(ddlDescription.SelectedValue);
    Detail[index].Price = Convert.ToDecimal(txtPrice.Text);
    Detail[index].Subtotal = Convert.ToDecimal(Detail[index].Price * Convert.ToInt32(txtQuantity.Text));
}