[研究]DropDownList下拉選單的恩怨情仇(四)項目不存在的預選
2022-3-19
環境：Visual Studio 2022 + ASP.NET + Web Application + WebForm + C#
續這篇
[研究]DropDownList下拉選單的恩怨情仇(一)'DropDownList1' 擁有的 SelectedValue 無效，因為它不在項目清單中。
https://shaurong.blogspot.com/2022/03/dropdownlistdropdownlist1-selectedvalue.html
[研究]DropDownList下拉選單的恩怨情仇(二)使用料庫儲存選項清單http://shaurong.blogspot.com/2022/03/dropdownlist.html
[研究]DropDownList下拉選單的恩怨情仇(三)已儲存項目不再提供於資料庫選項名單
http://shaurong.blogspot.com/2022/03/dropdownlist_19.html
之前下拉選單一定只出現第一項，改進一下；
若已存資料庫中項目不再存在，希望「編輯」畫面下拉選單預選空白；
若已存資料庫中項目存在，希望「編輯」畫面下拉選單預選該項目。
Default.aspx
<%@ Page Language="C#" AutoEventWireup="true" CodeBehind="Default.aspx.cs" Inherits="WebApplication3.Default" %>
<!DOCTYPE html>
<html xmlns="http://www.w3.org/1999/xhtml">
<head runat="server">
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
<title></title>
</head>
<body>
<form id="form1" runat="server">
<div>
<asp:SqlDataSource ID="SqlDataSource1" runat="server"
ConnectionString="<%$ ConnectionStrings:TestDBConnectionString %>"
DeleteCommand="DELETE FROM [FruitRecord] WHERE [sn] = @sn"
InsertCommand="INSERT INTO [FruitRecord] ([FruitBuy]) VALUES (@FruitBuy)"
SelectCommand="SELECT * FROM [FruitRecord]"
UpdateCommand="UPDATE [FruitRecord] SET [FruitBuy] = @FruitBuy WHERE [sn] = @sn">
<DeleteParameters>
<asp:Parameter Name="sn" Type="Int32" />
</DeleteParameters>
<InsertParameters>
<asp:Parameter Name="FruitBuy" DefaultValue="" ConvertEmptyStringToNull="false" Type="String" />
</InsertParameters>
<UpdateParameters>
<asp:Parameter Name="FruitBuy" DefaultValue="" ConvertEmptyStringToNull="false" Type="String" />
<asp:Parameter Name="sn" Type="Int32" />
</UpdateParameters>
</asp:SqlDataSource>
<asp:GridView ID="GridView1" runat="server" AllowPaging="True"
AllowSorting="True" AutoGenerateColumns="False"
DataKeyNames="sn" DataSourceID="SqlDataSource1"
OnRowDataBound="GridView1_RowDataBound"
>
<Columns>
<asp:CommandField ShowDeleteButton="True" ShowEditButton="True" />
<asp:BoundField DataField="sn" HeaderText="sn" InsertVisible="False" ReadOnly="True" SortExpression="sn" />
<asp:TemplateField HeaderText="FruitBuy" SortExpression="FruitBuy">
<EditItemTemplate>
<asp:TextBox ID="TextBox_FruitBuy" runat="server" ReadOnly="true"
Text='<%# Bind("FruitBuy") %>'></asp:TextBox>
<asp:SqlDataSource ID="SqlDataSource_FruitName" runat="server"
ConnectionString="<%$ ConnectionStrings:TestDBConnectionString %>"
SelectCommand="SELECT * FROM [FruitList]"></asp:SqlDataSource>
<asp:DropDownList ID="DropDownList_FruitBuy" runat="server"
AutoPostBack="true"
DataSourceID="SqlDataSource_FruitName"
AppendDataBoundItems="true"
DataTextField="FruitName" DataValueField="FruitName"
OnSelectedIndexChanged="DropDownList1_SelectedIndexChanged">
<asp:ListItem></asp:ListItem>
</asp:DropDownList>
</EditItemTemplate>
<ItemTemplate>
<asp:Label ID="Label1" runat="server" Text='<%# Bind("FruitBuy") %>'></asp:Label>
</ItemTemplate>
</asp:TemplateField>
</Columns>
</asp:GridView>
</div>
</form>
</body>
</html> |
Default.aspx.cs 中部分
using System;
using System.Web.UI.WebControls;
namespace WebApplication3
{
public partial class Default : System.Web.UI.Page
{
protected void Page_Load(object sender, EventArgs e)
{
}
protected void DropDownList1_SelectedIndexChanged(object sender, EventArgs e)
{
var row = GridView1.Rows[GridView1.EditIndex];
TextBox TextBox_FruitBuy = (TextBox)row.FindControl("TextBox_FruitBuy");
DropDownList DropDownList_FruitBuy = (DropDownList)row.FindControl("DropDownList_FruitBuy");
TextBox_FruitBuy.Text = DropDownList_FruitBuy.SelectedValue;
}
protected void GridView1_RowDataBound(object sender, GridViewRowEventArgs e)
{
if (e.Row.RowType == DataControlRowType.DataRow
&& e.Row.RowState == (DataControlRowState.Edit | DataControlRowState.Alternate))
{
if (e.Row.RowIndex == GridView1.EditIndex)
{
TextBox TextBox_FruitBuy = (TextBox)e.Row.FindControl("TextBox_FruitBuy");
DropDownList DropDownList_FruitBuy = (DropDownList)e.Row.FindControl("DropDownList_FruitBuy");
if (DropDownList_FruitBuy.Items.Contains(new ListItem(TextBox_FruitBuy.Text)))
{
DropDownList_FruitBuy.SelectedValue = TextBox_FruitBuy.Text;
}
else
{
DropDownList_FruitBuy.SelectedValue = "";
}
}
}
}
}
} |
按下「編輯」按鈕後出現
(下圖)芭樂不在下拉選單中，下拉選單預選空白
別忘了GridView1_RowUpdating()要檢查按下「更新」按鈕後，新值是否屬於新名單中選項。
甚至按下「取消」觸發的GridView1_RowCancelingEdit()也進行檢查。但也或許可以不用，看該欄位是否允許不選擇，也就是輸入空字串。
敝人遇過某種下拉選單，允許不選，選項有數千項，隨時增、減、改名，非敝人管理，也不會通知敝人，就合適這種情況。
(完)
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