ASP.NET Web窗体DropDownList具有SelectedValue,因为它不存在于

发布时间：2020-12-16 00:21:41 所属栏目：asp.Net 来源：网络整理

导读：首先有一些问题( DropDownList has a SelectedValue which is invalid because it does not exist in the list of items， DropDownList “has a SelectedValue which is invalid because it does not exist in the list of items”， asp:DropDownList Erro

首先有一些问题( DropDownList has a SelectedValue which is invalid because it does not exist in the list of items， DropDownList “has a SelectedValue which is invalid because it does not exist in the list of items”， asp:DropDownList Error: ‘DropDownList1’ has a SelectedValue which is invalid because it does not exist in the list of items)关于这个问题，并提出了解决方法，但我的问题是真的为什么发生这种情况。更重要的是，我对建议的解决方法不满意，我觉得他们很丑陋。

所以有一个页面有一个下拉列表和一个按钮：

<%@ Page Language="C#" AutoEventWireup="true" CodeBehind="WebForm2.aspx.cs" Inherits="TestWebApplication.WebForm2" ViewStateMode="Disabled" %>

<html lang="en" >
<body>
    <form id="form1" runat="server">
    <div>
        <asp:DropDownList ID="ddlTest" runat="server">
        </asp:DropDownList>
        <asp:Button Text="Test" ID="btnTest" runat="server" onclick="btnTest_Click" />
    </div>
    </form>
</body>
</html>

我将ddlTest绑定在Page_Init上的某些项目，然后在btnTest_Click中再次绑定：

using System;

namespace TestWebApplication
{
    public partial class WebForm2 : System.Web.UI.Page
    {
        protected void Page_Init(object sender,EventArgs e)
        {
            //SelectedIndex is -1,SelectedValue is "",SelectedItem is null
            ddlTest.DataSource = new[] { 1,2,3 };
            ddlTest.DataBind();
            ddlTest.SelectedValue = "3";
        }

        protected void btnTest_Click(object sender,EventArgs e)
        {
            //SelectedIndex is 2,SelectedValue is "3",SelectedItem is {3}
            ddlTest.ClearSelection();
            //SelectedIndex is 0,SelectedValue is "1",SelectedItem is {1}
            ddlTest.SelectedIndex = -1; //Nothing changes including SelectedIndex
            ddlTest.SelectedValue = ""; //Nothing changes including SelectedValue
            ddlTest.Items.Clear();
            //SelectedIndex is -1,SelectedItem is null
            ddlTest.DataSource = null; //Nothing changes except for the DataSource property
            ddlTest.DataSource = new[] { 1,2 };
            ddlTest.DataBind();//Exception!
            //'ddlTest' has a SelectedValue which is invalid because it does not exist in the list of items.
            //Parameter name: value
        }
    }
}

为什么我得到例外我尝试过不同版本的这些版本，它们都不工作。我尝试仅使用ClearSelection，但仍然有相同的异常。这个错误在控制或我想念的东西。其他问题的难处理解决方案是唯一的解决方案吗？

注意 – 即使删除按钮并且所有代码都在单个事件处理程序中移动，该错误也是可重现的。只要绑定一次设置所选值并再次绑定。

解决方法

我在Connect上提交了一个错误。它被解决为“不会修复”，这在我看来意味着它实际上是一个错误。提供了解决方法：

ddlTest.Items.Clear();
ddlTest.SelectedValue = null;

https://connect.microsoft.com/VisualStudio/feedback/details/666808/asp-net-dropdownlist-selectedvalue-is-persisted-which-results-in-exception-if-the-control-is-databound-second-time

我想这个答案。

（编辑：李大同）

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