ASP.NET填充列表框问题

本文关键字：问题 列表 填充 NET ASP | 更新日期: 2023-09-27 18:21:38

目前，我正在尝试创建一个ASP.NET页面，该页面将根据您选择的类别按钮在列表框中列出某个类别中的书籍，然后我有另外两个按钮（一个用于DESC顺序，一个用于ASC顺序）。现在的问题是，当我在点击虚构按钮并填充列表框后点击ASC或DESC按钮时，它会擦除列表框。

我早些时候发布了一个类似的问题，得到了一些修复，但当我按下ASC或DESC按钮时，它仍然在清除列表框。

我对ASP.NET很陌生，所以非常欢迎简单或"新手友好"的解释和代码示例/修复！

提前感谢！

下方的代码

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;
public partial class partin : System.Web.UI.Page
{
private List<String> books = new List<String>();
void Page_PreRender()
{
    Item_Listbox.DataSource = books;
    Item_Listbox.DataBind();   
}
int SortASC(string x, string y)
{
    return String.Compare(x, y);
}
int SortDESC(string x, string y)
{
    return String.Compare(x, y) * -1;
}
protected void Page_Load(object sender, EventArgs e)
{
    if (!IsPostBack)
    {
        Header_Label.Text = "Welcome! Please select a book category.";
        Item_Listbox.DataSource = books;
        Item_Listbox.DataBind();
    }
}
protected void Fiction_Click(object sender, EventArgs e)
{
        Header_Label.Text = "Fiction Section";
        books.Add("Title: The Old Man and The Sea | Decription: An epic novel. | Price: 10 USD | Quantity: 3");
        books.Add("Title: A Game of Thrones | Decription: A tale of fire and ice. | Price: 15 USD | Quantity: 6");
        books.Add("Title: Dracula | Decription: A book about vampires. | Price: 5 USD | Quantity: 7");
        books.Add("Title: Twilight | Decription: An awful book. | Price: Free | Quantity: 1000");
        Item_Listbox.DataSource = books;
        Item_Listbox.DataBind();
}

protected void Non_Fiction_Click(object sender, EventArgs e)
{
    Header_Label.Text = "Non-Fiction Section";

}
protected void Self_Help_Click(object sender, EventArgs e)
{
    Header_Label.Text = "Self Help Section";

}
protected void Sort_Command(object sender, CommandEventArgs e)
{
    if (e.CommandName == "Sort")
    {
        switch (e.CommandArgument.ToString())
        {
            case "ASC":
                books.Sort(SortASC);
                break;
            case "DESC":
                books.Sort(SortDESC);
                break;
        }
    }
    Item_Listbox.DataSource = books;
    Item_Listbox.DataBind();  
}

}

ASPX代码：

    &nbsp;<asp:Panel 
        ID="Navigation" runat="server" Height="276px" Width="197px" 
        CssClass="Navigation" BorderColor="Black" BorderStyle="Double">
    <asp:Button ID="Fiction" runat="server" Text="Fiction" Width="145px" 
            CssClass="Nav_buttons" onclick="Fiction_Click" />
    <asp:Button ID="Non_Fiction" runat="server" Text="Non-Fiction" Width="145px" 
            CssClass="Nav_buttons" onclick="Non_Fiction_Click" />
    <asp:Button ID="Self_Help" runat="server" Text="Self Help" Width="145px" 
            CssClass="Nav_buttons" onclick="Self_Help_Click" />
    <asp:Button ID="New_Item" runat="server" Text="Add New Item" Width="145px" CssClass="Nav_buttons" />        
</asp:Panel>
<asp:Panel ID="Books_Panel" runat="server" CssClass="Books_Panel" Height="409px" 
        BorderStyle="Double">
        <asp:Label ID="Header_Label" runat="server" 
            style="top: 79px; left: 693px; position: absolute; height: 19px; width: 234px" 
            Text="Label"></asp:Label>
        <asp:ListBox ID="Item_Listbox" runat="server" 

            style="top: 204px; left: 443px; position: absolute; height: 136px; width: 732px" 
            AutoPostBack="True">
        </asp:ListBox>
        <asp:Button ID="Ascending_Button" runat="server" 
            style="top: 375px; left: 723px; position: absolute; height: 26px; width: 169px" 
            Text="Ascending Order" CommandName="Sort" CommandArgument="ASC" 
            OnCommand="Sort_Command" />
        <asp:Button ID="Descending_Button" runat="server" 
            style="top: 405px; left: 723px; position: absolute; height: 26px; width: 169px" 
            Text="Descending Order" CommandName="Sort" CommandArgument="DESC" 
            OnCommand="Sort_Command" />
        <asp:DropDownList ID="Cat_Menu" runat="server">
        </asp:DropDownList>
    </asp:Panel>

ASP.NET填充列表框问题

当您单击ASC或DESC按钮对列表进行排序时，它将转到排序处理程序，但此处books在页面回发时为空，因此空数据源将绑定到列表。

您应该重新绑定源或将其保持在视图状态，以便在页面回发时使用它。

试试这样的东西。

private List<String> books
{ 
   get{
         if(ViewState["books"] == null){
             List<String> books = new List<String>();
             books.Add("Title: The Old Man and The Sea | Decription: An epic novel. | Price: 10 USD | Quantity: 3");
            books.Add("Title: A Game of Thrones | Decription: A tale of fire and ice. | Price: 15 USD | Quantity: 6");
            books.Add("Title: Dracula | Decription: A book about vampires. | Price: 5 USD | Quantity: 7");
            books.Add("Title: Twilight | Decription: An awful book. | Price: Free | Quantity: 1000");
             ViewState["books"] = books;
         }
         return new List<String>((String[])ViewState["books"]);
   }
   set{
       ViewState["books"] = value;
   }
}
protected void Fiction_Click(object sender, EventArgs e)
{
        Header_Label.Text = "Fiction Section";
        Item_Listbox.DataSource = books;
        Item_Listbox.DataBind();
}
protected void Sort_Command(object sender, CommandEventArgs e)
{
    if (e.CommandName == "Sort")
    {
        switch (e.CommandArgument.ToString())
        {
            case "ASC":
                books.Sort(SortASC);
                break;
            case "DESC":
                books.Sort(SortDESC);
                break;
        }
    }
    Item_Listbox.DataSource = books;
    Item_Listbox.DataBind();  
}
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