Asp.net 将新对象添加到ArrayList中，当前会重写最后一个对象

我正在将产品添加到ArrayList中，以使用中继器显示

   Protected Sub Add_Click(sender As Object, e As EventArgs) Handles Add.Click
    Dim ProductID As Integer = ddProduct.SelectedValue
    Dim Qty As Integer = QtyBox.Text
    Dim TempProduct As OrderEntry = fillProduct(ProductID, Qty)
    OrderList.Add(TempProduct)
    Orders.DataSource = OrderList
    Orders.DataBind()
   End Sub

但是，当我添加一个新对象时，它会用一个新对象替换旧对象，并且我假设它引用了旧对象，而不是创建一个新对象。我在哪里创建一个新的

Imports System.Data
Imports System.Data.SqlClient


Partial Class placeOrder
Inherits System.Web.UI.Page
Dim OrderList As New ArrayList
Protected Sub Page_Load(sender As Object, e As EventArgs) Handles Me.Load


    If Not Page.IsPostBack() Then
        dataFill()
    End If

End Sub

Private Sub dataFill()
    fillProduct()

End Sub
Private Sub fillProduct()
    Dim conn As New SqlConnection(ConfigurationManager.ConnectionStrings("class_readonly").ConnectionString)
    'Here bad connection string is a problem
    conn.Open()

    'Dim cmd As SqlCommand = conn.CreateCommand()
    Dim sqlCmd As New SqlCommand()
    With sqlCmd
        .Connection = conn
        .CommandText = "SELECT distinct ProductID, Name FROM dbo.Product;"
    End With

    Dim objDataAdapter As New SqlDataAdapter()
    Dim objDataSet As New DataSet()
    objDataAdapter.SelectCommand = sqlCmd

    objDataAdapter.Fill(objDataSet)
    ddProduct.DataSource = objDataSet.Tables(0)
    ddProduct.DataBind()
End Sub

Protected Sub Add_Click(sender As Object, e As EventArgs) Handles Add.Click
    Dim ProductID As Integer = ddProduct.SelectedValue
    Dim Qty As Integer = QtyBox.Text
    Dim TempProduct As OrderEntry = fillProduct(ProductID, Qty)
    OrderList.Add(TempProduct)
    Orders.DataSource = OrderList
    Orders.DataBind()
    dataFill()


End Sub

Private Function fillProduct(ByVal ProductID As Integer, ByVal Qty As Integer) As OrderEntry
    Dim conn As New SqlConnection(ConfigurationManager.ConnectionStrings("class_readonly").ConnectionString)
    'Here bad connection string is a problem
    conn.Open()

    'Dim cmd As SqlCommand = conn.CreateCommand()
    Dim sqlCmd As New SqlCommand()
    With sqlCmd
        .Connection = conn
        .CommandText = "SELECT * FROM dbo.Product WHERE ProductID=@ProductID ;"
        .Parameters.Add(New SqlParameter("@ProductID", SqlDbType.Int, 4)).Value = ProductID
    End With
    Dim objDataAdapter As New SqlDataAdapter()
    Dim objDataSet As New DataSet()
    objDataAdapter.SelectCommand = sqlCmd

    objDataAdapter.Fill(objDataSet)

    Dim dtRow As DataRow = objDataSet.Tables(0).Rows(0)
    Dim Total_Cost As Decimal = Decimal.Multiply(Qty, dtRow("Wholesale_Price"))
    Dim Total_Price As Decimal = Decimal.Multiply(Qty, dtRow("Retail_Price"))
    Dim Profit As Decimal = Total_Price - Total_Cost
    Dim productInfo As New OrderEntry(dtRow("ProductID"), Qty, Total_Cost, Total_Price, Profit, dtRow("Name"))

    sqlCmd.Dispose()
    conn.Dispose()
    Return productInfo
End Function

End Class

---

好的，这是一个ASP.NET网页。单击按钮后，它会将页面发回服务器。OrderList不会在调用之间持久化，每次单击都会重新初始化。尝试在按钮单击过程中将OrderList添加到会话变量，如下所示：

OrderList.Add(TempProduct)
Session("OrderList") = OrderList

在页面加载事件中，插入：

OrderList = Session("OrderList")

---

此外，数据填充过程也是多余的。删除该过程并用fillProduct替换对dataFill的调用。这不是一个错误，但它不整洁。

正如@OneFineDay所指出的，如果数组中的每个元素都是相同的类型，那么就使用“List of”类型的对象，而不是“ArrayList”

这就是你们班的样子。您正在单击“添加”将列表保存到会话变量中。在Page_Load中，您将检索此会话变量，将其强制转换为（OrderEntry的）列表，并将其分配回页面级别的列表变量。我还删除了不必要的数据填充过程。确保您理解这些概念，而不仅仅是复制和粘贴

Imports System.Data
Imports System.Data.SqlClient

Partial Class placeOrder
Inherits System.Web.UI.Page

Private m_myOrderList As List(Of OrderEntry)

Protected Sub Page_Load(sender As Object, e As EventArgs) Handles Me.Load
    If Not Page.IsPostBack() Then fillProduct()
    m_myOrderList = CType(Session("MyOrderList"), List(Of OrderEntry))
End Sub

Private Sub fillProduct()
    Dim conn As New SqlConnection(ConfigurationManager.ConnectionStrings("class_readonly").ConnectionString)
    'Here bad connection string is a problem
    conn.Open()

    'Dim cmd As SqlCommand = conn.CreateCommand()
    Dim sqlCmd As New SqlCommand()
    With sqlCmd
        .Connection = conn
        .CommandText = "SELECT distinct ProductID, Name FROM dbo.Product;"
    End With

    Dim objDataAdapter As New SqlDataAdapter()
    Dim objDataSet As New DataSet()
    objDataAdapter.SelectCommand = sqlCmd

    objDataAdapter.Fill(objDataSet)
    ddProduct.DataSource = objDataSet.Tables(0)
    ddProduct.DataBind()
End Sub

Protected Sub Add_Click(sender As Object, e As EventArgs) Handles Add.Click
    If IsNothing(m_myOrderList) = True Then m_myOrderList = New List(Of OrderEntry)
    Dim ProductID As Integer = ddProduct.SelectedValue
    Dim Qty As Integer = QtyBox.Text
    Dim TempProduct As OrderEntry = fillProduct(ProductID, Qty)
    m_myOrderList.Add(TempProduct)
    Session("MyOrderList") = m_myOrderList
    Orders.DataSource = m_myOrderList
    Orders.DataBind()
  End Sub

Private Function fillProduct(ByVal ProductID As Integer, ByVal Qty As Integer) As OrderEntry
    Dim conn As New SqlConnection(ConfigurationManager.ConnectionStrings("class_readonly").ConnectionString)
    'Here bad connection string is a problem
    conn.Open()

    'Dim cmd As SqlCommand = conn.CreateCommand()
    Dim sqlCmd As New SqlCommand()
    With sqlCmd
        .Connection = conn
        .CommandText = "SELECT * FROM dbo.Product WHERE ProductID=@ProductID ;"
        .Parameters.Add(New SqlParameter("@ProductID", SqlDbType.Int, 4)).Value = ProductID
    End With
    Dim objDataAdapter As New SqlDataAdapter()
    Dim objDataSet As New DataSet()
    objDataAdapter.SelectCommand = sqlCmd

    objDataAdapter.Fill(objDataSet)

    Dim dtRow As DataRow = objDataSet.Tables(0).Rows(0)
    Dim Total_Cost As Decimal = Decimal.Multiply(Qty, dtRow("Wholesale_Price"))
    Dim Total_Price As Decimal = Decimal.Multiply(Qty, dtRow("Retail_Price"))
    Dim Profit As Decimal = Total_Price - Total_Cost
    Dim productInfo As New OrderEntry(dtRow("ProductID"), Qty, Total_Cost, Total_Price, Profit, dtRow("Name"))

    sqlCmd.Dispose()
    conn.Dispose()
    Return productInfo
End Function

End Class

---

你能给我们看一下fillProduct的功能吗？是的！我更新了最上面的帖子。

列表（OrderEntry）

现在比

ArrayList

更受欢迎。我开始发现这是一个“System.NullReferenceException”类型的异常，发生在App_Web_yc1evu5x.dll中，但没有在OrderList上的用户代码错误中处理。添加（TempProduct）这需要一些修改。我发布了另一个答案。这也是一个优秀的视频，概述了.NET从ArrayList等非类型集合到List（of T）等通用集合的演变。前者实际上是遗留代码。非常感谢你！成功了。你能解释一下，如果IsNothing（m_myOrderList）=True，那么m_myOrderList=New List（Of OrderEntry）List（Of T）是一个引用类型，这意味着它是一个指向值的指针，而不是存储值本身。没有New关键字的引用类型的变量声明将默认为Nothing。Nothing是空引用，这意味着它不指向任何值。请注意，我已经在页面顶部的变量声明中删除了New关键字。在实际需要之前，不需要保留内存。如果省略按钮单击过程的第一行，当您添加第一项时，您将在m_myOrderList.Add（TempProduct）上看到一个空引用，因为m_myOrderList引用指向“Nothing”。Nothing和对空列表的引用之间有很大区别。因此，该行检查m_myOrderList是否没有指向任何内容，如果是，则使用m_myOrderList=新列表（OrderEntry的）使其指向某个内容（空列表）。在将第一个TempProduct对象添加到列表之前，它将不指向任何内容。我还要补充一点，如果您需要使用非类型化列表，因为列表中的元素具有不同的类型，那么您可以始终使用list（of Object）。