如何在 asp.net 中即时创建动态控件；下拉列表中的 IndexChanged 事件在 post 返回后不会 persist/fire

Question

我有

several dynamic drop downs

将选定的索引更改事件附加到它们，但是一旦第一次触发索引更改事件并且一旦回发完成，它们就会丢失该索引更改事件，但不会丢失项目值

so its saving the state of the dropdowns but not attaching the selected index change event on a post back.

AspxPage:

 <div>
    <asp:PlaceHolder ID="mp_1" runat="server"></asp:PlaceHolder>
    </div>

C#:

public partial class WebForm1 : System.Web.UI.Page
    {
        protected void Page_Init(object sender, EventArgs e)
        {

        }

        private void ddl_IndexChanged(object sender, EventArgs e)
        {
            Response.Write("dsfsdf");
        }           

        protected void Page_Load(object sender, EventArgs e)
        {

 DropDownList ddl_1 = new DropDownList();
                ddl_1.ID = "ddl_1";
                ddl_1.AutoPostBack = true;
                ddl_1.EnableViewState = true;
                ddl_1.SelectedIndexChanged += new EventHandler(ddl_IndexChanged);
                this.mp_1.Controls.Add(ddl_1); 


            if(!IsPostBack)
            {
                DropDownList ddl_1 = (DropDownList)Page.FindControl("ddl_1");
                if (ddl_1 != null)
                {
                    List<ListItem> items = new List<ListItem>();
                    items.Add(new ListItem("Item 2", "Value 2"));
                    items.Add(new ListItem("Item 1", "Value 1"));
                    items.Add(new ListItem("Item 3", "Value 3"));
                    ddl_1.Items.AddRange(items.ToArray());
                }                
            }               
        }
    }

Answer 1

创建和绑定动态子控件的最稳定方法是覆盖 CreateChildControls 方法：

See official sample

Answer 2

AspxPage:

 <div>
    <asp:PlaceHolder ID="mp_1" runat="server"></asp:PlaceHolder>
    </div>

C#:

public partial class WebForm1 : System.Web.UI.Page
    {
        protected void Page_Init(object sender, EventArgs e)
        {
            DropDownList ddl_1 = new DropDownList();
            ddl_1.ID = "ddl_1";
            ddl_1.AutoPostBack = true;
            ddl_1.EnableViewState = true;
            ddl_1.SelectedIndexChanged += new EventHandler(ddl_IndexChanged);
            this.mp_1.Controls.Add(ddl_1); 
        }

        private void ddl_IndexChanged(object sender, EventArgs e)
        {
            Response.Write("dsfsdf");
        }           

        // add entries in page load method not in init method
        protected void Page_Load(object sender, EventArgs e)
        {
            if(!IsPostBack)
            {
                DropDownList ddl_1 = (DropDownList)Page.FindControl("ddl_1");
                if (ddl_1 != null)
                {
                    List<ListItem> items = new List<ListItem>();
                    items.Add(new ListItem("Item 2", "Value 2"));
                    items.Add(new ListItem("Item 1", "Value 1"));
                    items.Add(new ListItem("Item 3", "Value 3"));
                    ddl_1.Items.AddRange(items.ToArray());
                }                
            }               
        }
    }

如何在 asp.net 中即时创建动态控件；下拉列表中的 IndexChanged 事件在 post 返回后不会 persist/fire

How to create dynamic controls on the fly in asp.net;IndexChanged Event on dropdowns does not persist/fire after post back

c#

asp.net