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POASP.NET Web Forms DropDownList has a SelectedValue which is invalid because it does not exist in the list of items
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  1. POASP.NET Web Forms DropDownList has a SelectedValue which is invalid because it does not exist in the list of items
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    copied!<p>First of all there has been questions ( <a href="https://stackoverflow.com/questions/1869150/dropdownlist-has-a-selectedvalue-which-is-invalid-because-it-does-not-exist-in-th">DropDownList has a SelectedValue which is invalid because it does not exist in the list of items</a> , <a href="https://stackoverflow.com/questions/5704651/dropdownlist-has-a-selectedvalue-which-is-invalid-because-it-does-not-exist-in-t">DropDownList &quot;has a SelectedValue which is invalid because it does not exist in the list of items&quot;</a> , <a href="https://stackoverflow.com/questions/38612/aspdropdownlist-error-dropdownlist1-has-a-selectedvalue-which-is-invalid-beca">asp:DropDownList Error: &#39;DropDownList1&#39; has a SelectedValue which is invalid because it does not exist in the list of items</a> ) about this and there are proposed workarounds but my question is really WHY this happens. What is more I am not satisfied with the suggested workarounds and I find them quite ugly.</p> <p>So there is a page with a drop down list and a button:</p> <pre><code>&lt;%@ Page Language="C#" AutoEventWireup="true" CodeBehind="WebForm2.aspx.cs" Inherits="TestWebApplication.WebForm2" ViewStateMode="Disabled" %&gt; &lt;html lang="en" &gt; &lt;body&gt; &lt;form id="form1" runat="server"&gt; &lt;div&gt; &lt;asp:DropDownList ID="ddlTest" runat="server"&gt; &lt;/asp:DropDownList&gt; &lt;asp:Button Text="Test" ID="btnTest" runat="server" onclick="btnTest_Click" /&gt; &lt;/div&gt; &lt;/form&gt; &lt;/body&gt; &lt;/html&gt; </code></pre> <p>I bind ddlTest with some items on Page_Init and then in btnTest_Click I bind again:</p> <pre><code>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, SelectedValue is "", 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 } } } </code></pre> <p>Why do I get the exception. I tried different versions of these and none of them works. I tried using only ClearSelection but still got the same exception. Is this bug in the control or something I miss. Are the ugly workarounds from the other questions the only solution?</p> <p>Note - the bug is reproduceable even if the button is removed and the all the code is moved in a single event handler. Just bind once set selected value and bind again.</p>
 

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