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;
我想这个答案。
