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C# 动态加载时，用户控件中的DropDownList在第一次回发时丢失SelectedValue

c# asp.net

C# 动态加载时，用户控件中的DropDownList在第一次回发时丢失SelectedValue,c#,asp.net,dynamic-usercontrols,C#,Asp.net,Dynamic Usercontrols,我有一个ASPX页面，其中有一个占位符，其中动态加载用户控件。HTML标记如下所示： <%@ Page Language="C#" AutoEventWireup="true" CodeBehind="Test.aspx.cs" Inherits="Test" %> <!DOCTYPE html> <html> <head runat="server"> </head> <body> <form id="f

我有一个ASPX页面，其中有一个占位符，其中动态加载用户控件。HTML标记如下所示：

<%@ Page Language="C#" AutoEventWireup="true" CodeBehind="Test.aspx.cs" Inherits="Test" %>

<!DOCTYPE html>

<html>
<head runat="server">
</head>
<body>
    <form id="form1" runat="server">

        <asp:Button ID="btnNumbers" Text="Select Numbers" OnClick="SelectNumbers" runat="server" />
        <br /><br />
        <asp:Button ID="btnLetters" Text="Select Letters" OnClick="SelectLetters" runat="server" />
        <br /><br />

        <asp:PlaceHolder ID="phContent" runat="server" />

    </form>
</body>
</html>

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;


public partial class Test : System.Web.UI.Page
{
    protected void Page_Init(object sender, EventArgs e)
    {
        if (!IsPostBack)
            HttpContext.Current.Session["control"] = "~/Controls/Numbers.ascx";
        LoadControl();        
    }

    protected void SelectNumbers(object sender, EventArgs e)
    {
        HttpContext.Current.Session["control"] = "~/Controls/Numbers.ascx";
        LoadControl();
    }

    protected void SelectLetters(object sender, EventArgs e)
    {
        HttpContext.Current.Session["control"] = "~/Controls/Letters.ascx";
        LoadControl();
    }

    private void LoadControl()
    {
        phContent.Controls.Clear();
        var control = Page.LoadControl((String)HttpContext.Current.Session["control"]);
        phContent.Controls.Add(control);
    }
}

<%@ Control Language="C#" AutoEventWireup="true" CodeBehind="Numbers.ascx.cs" Inherits="Numbers" %>


<asp:DropDownList ID="ddlSelectNumbers"
                  AutoPostBack="true" runat="server"
                  OnSelectedIndexChanged="SelectNumbers_IndexChanged" >
    <asp:ListItem Text="One" Value="1" />
    <asp:ListItem Text="Two" Value="2" />
    <asp:ListItem Text="Three" Value="3" />
</asp:DropDownList>

<br /><br />

<asp:Label ID="lblSelectedNumber" runat="server" />

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.IO;
using Superexpert.Controls;


public partial class Numbers : System.Web.UI.UserControl
{
    public override String ID
    {
        get { return "Numbers"; }
        set { }
    }

    protected void Page_Load(object sender, EventArgs e)
    {

    }

    protected void SelectNumbers_IndexChanged(object sender, EventArgs e)
    {
        lblSelectedNumber.Text = ((DropDownList)sender).SelectedItem.Text;
    }
}

<%@ Control Language="C#" AutoEventWireup="true" CodeBehind="Letters.ascx.cs" Inherits="Letters" %>



<asp:DropDownList ID="ddlSelectLetters"
                  AutoPostBack="true" runat="server"
                  OnSelectedIndexChanged="SelectLetters_IndexChanged" >
    <asp:ListItem Text="A" Value="a" />
    <asp:ListItem Text="B" Value="b" />
    <asp:ListItem Text="C" Value="c" />
</asp:DropDownList>

<br /><br />

<asp:Label ID="lblSelectedLetter" runat="server" />

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.IO;
using Superexpert.Controls;


public partial class Letters : System.Web.UI.UserControl
{
    public override String ID
    {
        get { return "Letters"; }
        set { }
    }

    protected void Page_Load(object sender, EventArgs e)
    {

    }

    protected void SelectLetters_IndexChanged(object sender, EventArgs e)
    {
        lblSelectedLetter.Text = ((DropDownList)sender).SelectedItem.Text;
    }
}
第一个用户控件有一个下拉列表来选择数字。它具有如下标记：

<%@ Page Language="C#" AutoEventWireup="true" CodeBehind="Test.aspx.cs" Inherits="Test" %>

<!DOCTYPE html>

<html>
<head runat="server">
</head>
<body>
    <form id="form1" runat="server">

        <asp:Button ID="btnNumbers" Text="Select Numbers" OnClick="SelectNumbers" runat="server" />
        <br /><br />
        <asp:Button ID="btnLetters" Text="Select Letters" OnClick="SelectLetters" runat="server" />
        <br /><br />

        <asp:PlaceHolder ID="phContent" runat="server" />

    </form>
</body>
</html>

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;


public partial class Test : System.Web.UI.Page
{
    protected void Page_Init(object sender, EventArgs e)
    {
        if (!IsPostBack)
            HttpContext.Current.Session["control"] = "~/Controls/Numbers.ascx";
        LoadControl();        
    }

    protected void SelectNumbers(object sender, EventArgs e)
    {
        HttpContext.Current.Session["control"] = "~/Controls/Numbers.ascx";
        LoadControl();
    }

    protected void SelectLetters(object sender, EventArgs e)
    {
        HttpContext.Current.Session["control"] = "~/Controls/Letters.ascx";
        LoadControl();
    }

    private void LoadControl()
    {
        phContent.Controls.Clear();
        var control = Page.LoadControl((String)HttpContext.Current.Session["control"]);
        phContent.Controls.Add(control);
    }
}

<%@ Control Language="C#" AutoEventWireup="true" CodeBehind="Numbers.ascx.cs" Inherits="Numbers" %>


<asp:DropDownList ID="ddlSelectNumbers"
                  AutoPostBack="true" runat="server"
                  OnSelectedIndexChanged="SelectNumbers_IndexChanged" >
    <asp:ListItem Text="One" Value="1" />
    <asp:ListItem Text="Two" Value="2" />
    <asp:ListItem Text="Three" Value="3" />
</asp:DropDownList>

<br /><br />

<asp:Label ID="lblSelectedNumber" runat="server" />

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.IO;
using Superexpert.Controls;


public partial class Numbers : System.Web.UI.UserControl
{
    public override String ID
    {
        get { return "Numbers"; }
        set { }
    }

    protected void Page_Load(object sender, EventArgs e)
    {

    }

    protected void SelectNumbers_IndexChanged(object sender, EventArgs e)
    {
        lblSelectedNumber.Text = ((DropDownList)sender).SelectedItem.Text;
    }
}

<%@ Control Language="C#" AutoEventWireup="true" CodeBehind="Letters.ascx.cs" Inherits="Letters" %>



<asp:DropDownList ID="ddlSelectLetters"
                  AutoPostBack="true" runat="server"
                  OnSelectedIndexChanged="SelectLetters_IndexChanged" >
    <asp:ListItem Text="A" Value="a" />
    <asp:ListItem Text="B" Value="b" />
    <asp:ListItem Text="C" Value="c" />
</asp:DropDownList>

<br /><br />

<asp:Label ID="lblSelectedLetter" runat="server" />

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.IO;
using Superexpert.Controls;


public partial class Letters : System.Web.UI.UserControl
{
    public override String ID
    {
        get { return "Letters"; }
        set { }
    }

    protected void Page_Load(object sender, EventArgs e)
    {

    }

    protected void SelectLetters_IndexChanged(object sender, EventArgs e)
    {
        lblSelectedLetter.Text = ((DropDownList)sender).SelectedItem.Text;
    }
}
第二个用户控件有一个下拉列表来选择字母。它具有如下标记：

<%@ Page Language="C#" AutoEventWireup="true" CodeBehind="Test.aspx.cs" Inherits="Test" %>

<!DOCTYPE html>

<html>
<head runat="server">
</head>
<body>
    <form id="form1" runat="server">

        <asp:Button ID="btnNumbers" Text="Select Numbers" OnClick="SelectNumbers" runat="server" />
        <br /><br />
        <asp:Button ID="btnLetters" Text="Select Letters" OnClick="SelectLetters" runat="server" />
        <br /><br />

        <asp:PlaceHolder ID="phContent" runat="server" />

    </form>
</body>
</html>

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;


public partial class Test : System.Web.UI.Page
{
    protected void Page_Init(object sender, EventArgs e)
    {
        if (!IsPostBack)
            HttpContext.Current.Session["control"] = "~/Controls/Numbers.ascx";
        LoadControl();        
    }

    protected void SelectNumbers(object sender, EventArgs e)
    {
        HttpContext.Current.Session["control"] = "~/Controls/Numbers.ascx";
        LoadControl();
    }

    protected void SelectLetters(object sender, EventArgs e)
    {
        HttpContext.Current.Session["control"] = "~/Controls/Letters.ascx";
        LoadControl();
    }

    private void LoadControl()
    {
        phContent.Controls.Clear();
        var control = Page.LoadControl((String)HttpContext.Current.Session["control"]);
        phContent.Controls.Add(control);
    }
}

<%@ Control Language="C#" AutoEventWireup="true" CodeBehind="Numbers.ascx.cs" Inherits="Numbers" %>


<asp:DropDownList ID="ddlSelectNumbers"
                  AutoPostBack="true" runat="server"
                  OnSelectedIndexChanged="SelectNumbers_IndexChanged" >
    <asp:ListItem Text="One" Value="1" />
    <asp:ListItem Text="Two" Value="2" />
    <asp:ListItem Text="Three" Value="3" />
</asp:DropDownList>

<br /><br />

<asp:Label ID="lblSelectedNumber" runat="server" />

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.IO;
using Superexpert.Controls;


public partial class Numbers : System.Web.UI.UserControl
{
    public override String ID
    {
        get { return "Numbers"; }
        set { }
    }

    protected void Page_Load(object sender, EventArgs e)
    {

    }

    protected void SelectNumbers_IndexChanged(object sender, EventArgs e)
    {
        lblSelectedNumber.Text = ((DropDownList)sender).SelectedItem.Text;
    }
}

<%@ Control Language="C#" AutoEventWireup="true" CodeBehind="Letters.ascx.cs" Inherits="Letters" %>



<asp:DropDownList ID="ddlSelectLetters"
                  AutoPostBack="true" runat="server"
                  OnSelectedIndexChanged="SelectLetters_IndexChanged" >
    <asp:ListItem Text="A" Value="a" />
    <asp:ListItem Text="B" Value="b" />
    <asp:ListItem Text="C" Value="c" />
</asp:DropDownList>

<br /><br />

<asp:Label ID="lblSelectedLetter" runat="server" />

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.IO;
using Superexpert.Controls;


public partial class Letters : System.Web.UI.UserControl
{
    public override String ID
    {
        get { return "Letters"; }
        set { }
    }

    protected void Page_Load(object sender, EventArgs e)
    {

    }

    protected void SelectLetters_IndexChanged(object sender, EventArgs e)
    {
        lblSelectedLetter.Text = ((DropDownList)sender).SelectedItem.Text;
    }
}
当页面第一次加载（！IsPostBack）时，一切都正常。您可以从下拉列表中选择数字，它的SelectedItemChanged事件触发，它的SelectedValue更新，标签更新

当您单击其中一个按钮以动态加载其中一个用户控件时，就会出现此问题。之后，第一次从DropDownList中选择内容时，SelectedItemChanged事件不会触发，它的SelectedValue不会更新，因此标签不会更新。之后，一切正常

是什么导致了这种情况

提前谢谢

你很封闭。您只需要将ID分配给动态创建的控件

基本上，若要在回发时检索控件，需要将ID分配给动态创建的控件

请看3