在 ASP.NET Webform 中扩展 DropDownList

Question

我正在尝试创建自己的下拉列表以在网页上使用，但我无法让它按照我想要的方式工作。

在最基本的形式中，我可以创建一个类，从 System.Web.UI.WebControls.DropDownList 继承，在页面上注册它，添加标记，它至少会在页面上显示一个控件：

我的班级：

Imports System
Imports System.Collections.Generic
Imports System.ComponentModel
Imports System.Text
Imports System.Web
Imports System.Web.UI
Imports System.Web.UI.WebControls

Namespace CustomDropdown
    <DefaultProperty("Text"), ToolboxData("<{0}:DropDownListWithAttributes runat=server></{0}:DropDownListWithAttributes>")>
    Public Class DropDownListWithAttributes
        Inherits System.Web.UI.WebControls.DropDownList

    End Class
End Namespace

页面注册：

<%@ Register TagPrefix="myControls" Namespace="CustomDropdown" %>

标记：

<myControls:DropDownListWithAttributes ID="ddlMyTestCustomDDL" runat="server"></myControls:DropDownListWithAttributes>

但就是这样。这是我所能得到的。我根本无法从后面的代码访问它。就好像它不存在一样。我需要能够用项目、触发事件等填充它...普通下拉列表可以做的所有事情。

标记似乎认为它可以有事件：

但是当我添加一个时，设计师会中断：

所以我没有获得正常的 ddl 功能，我无法从后面的代码访问控件，添加任何类型的事件都会破坏东西......我不知道如何使这项工作发挥作用:(

Answer 1

嗯，根据 Andrew 的建议并将其转移到自己的项目中，它现在似乎正在运行。这是它的样子：

C# 类库：带有单个 .cs 文件的 DropDownWithAttributes：DropDownListWithAttributes.cs

using System;
using System.ComponentModel;
using System.Security.Permissions;
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;

[assembly: TagPrefix("DropDownWithAttributes", "myControl")]
namespace DropDownWithAttributes
{
    [AspNetHostingPermission(SecurityAction.Demand, Level = AspNetHostingPermissionLevel.Minimal), AspNetHostingPermission(SecurityAction.InheritanceDemand, Level = AspNetHostingPermissionLevel.Minimal)]
    [DefaultProperty("Text")]
    [ToolboxData("<{0}:ServerControl1 runat=server></{0}:ServerControl1>")]
    public class DropDownListWithAttributes : DropDownList
    {
        [Bindable(true)]
        [Category("Appearance")]
        [DefaultValue("")]
        [Localizable(true)]
        protected override object SaveViewState()
        {
            // create object array for Item count + 1
            object[] allStates = new object[this.Items.Count + 1];

            // the +1 is to hold the base info
            object baseState = base.SaveViewState();
            allStates[0] = baseState;

            Int32 i = 1;
            // now loop through and save each Style attribute for the List
            foreach (ListItem li in this.Items)
            {
                Int32 j = 0;
                string[][] attributes = new string[li.Attributes.Count][];
                foreach (string attribute in li.Attributes.Keys)
                {
                    attributes[j++] = new string[] { attribute, li.Attributes[attribute] };
                }
                allStates[i++] = attributes;
            }
            return allStates;
        }

        protected override void LoadViewState(object savedState)
        {
            if (savedState != null)
            {
                object[] myState = (object[])savedState;

                // restore base first
                if (myState[0] != null)
                    base.LoadViewState(myState[0]);

                Int32 i = 1;
                foreach (ListItem li in this.Items)
                {
                    // loop through and restore each style attribute
                    foreach (string[] attribute in (string[][])myState[i++])
                    {
                        li.Attributes[attribute[0]] = attribute[1];
                    }
                }
            }
        }
    }
}

我将项目添加到我的解决方案中，并从我的 Web 项目中添加了对 DropDownWithAttributes 的引用：

在我的页面上注册：

<%@ Register TagPrefix="myControl" Namespace="DropDownWithAttributes" Assembly="DropDownWithAttributes" %>

在页面上添加控件：

<myControl:DropDownListWithAttributes ID="ddlWithAttribute" runat="server" OnSelectedIndexChanged="ddlWithAttribute_SelectedIndexChanged"></myControl:DropDownListWithAttributes>

这就是之前发生故障的地方......现在看起来好多了：

 '''<summary>
    '''ddlWithAttribute control.
    '''</summary>
    '''<remarks>
    '''Auto-generated field.
    '''To modify move field declaration from designer file to code-behind file.
    '''</remarks>
    Protected WithEvents ddlWithAttribute As Global.DropDownWithAttributes.DropDownListWithAttributes