Shahed Khan (MVP C#)

ASP.NET Tips: DropDownList.ClearSelection() to avoid "Cannot have multiple items selected in DropDownList"

Problem
I was facing the following Exception when I was programmatically assigning Selected Value of a DropDownList with the following piece of code.

ListItem item = DropDownListTest.Items.FindByValue("Test");
  if (item != null)
     item.Selected = true;

Exception

Cannot have multiple items selected in a DropDownList.

Description: An unhandled exception occurred during the execution of the current web request. Please review the stack trace for more information about the error and where it originated in the code.
Exception Details: System.Web.HttpException: Cannot have multiple items selected in a DropDownList.
Source Error:

An unhandled exception was generated during the execution of the current web request. Information regarding the origin and location of the exception can be identified using the exception stack trace below.

Stack Trace:

[HttpException (0x80004005): Cannot have multiple items selected in a DropDownList.]
   System.Web.UI.WebControls.DropDownList.VerifyMultiSelect() +107
   System.Web.UI.WebControls.ListControl.RenderContents(HtmlTextWriter writer) +1825978
   System.Web.UI.WebControls.WebControl.Render(HtmlTextWriter writer) +29
   System.Web.UI.Control.RenderControlInternal(HtmlTextWriter writer, ControlAdapter adapter) +25
   System.Web.UI.Control.RenderControl(HtmlTextWriter writer, ControlAdapter adapter) +121
   System.Web.UI.Control.RenderControl(HtmlTextWriter writer) +22
   System.Web.UI.Control.RenderChildrenInternal(HtmlTextWriter writer, ICollection children) +199
   System.Web.UI.Control.RenderChildren(HtmlTextWriter writer) +20
   System.Web.UI.Control.Render(HtmlTextWriter writer) +7
   System.Web.UI.Control.RenderControlInternal(HtmlTextWriter writer, ControlAdapter adapter) +25
   System.Web.UI.Control.RenderControl(HtmlTextWriter writer, ControlAdapter adapter) +121
   System.Web.UI.Control.RenderControl(HtmlTextWriter writer) +22
   CSSFriendly.WizardAdapter.RenderStep(HtmlTextWriter writer, Wizard wizard) +137
   CSSFriendly.WizardAdapter.RenderContents(HtmlTextWriter writer) +93
   System.Web.UI.WebControls.Adapters.WebControlAdapter.Render(HtmlTextWriter writer) +23
   System.Web.UI.Control.RenderControlInternal(HtmlTextWriter writer, ControlAdapter adapter) +2116097
   System.Web.UI.Control.RenderControl(HtmlTextWriter writer, ControlAdapter adapter) +121
   System.Web.UI.Control.RenderControl(HtmlTextWriter writer) +22
   System.Web.UI.Control.RenderChildrenInternal(HtmlTextWriter writer, ICollection children) +199
   System.Web.UI.Control.RenderChildren(HtmlTextWriter writer) +20
   System.Web.UI.Control.Render(HtmlTextWriter writer) +7
   System.Web.UI.Control.RenderControlInternal(HtmlTextWriter writer, ControlAdapter adapter) +25
   System.Web.UI.Control.RenderControl(HtmlTextWriter writer, ControlAdapter adapter) +121
   System.Web.UI.Control.RenderControl(HtmlTextWriter writer) +22
   System.Web.UI.Control.RenderChildrenInternal(HtmlTextWriter writer, ICollection children) +199
   System.Web.UI.Control.RenderChildren(HtmlTextWriter writer) +20
   System.Web.UI.UpdatePanel.RenderChildren(HtmlTextWriter writer) +236
 

Solution

The solution is to remember to call ClearSelection() Method before calling FindByValue("") or FindByText("") Method.

The following piece of code will work fine.

DropDownListTest.ClearSelection();

ListItem item = DropDownListTest.Items.FindByValue("Test");
  if (item != null)
     item.Selected = true;

I also noticed, I do not get the "Cannot have multiple items" exception when I use SelectedIndex instead. i.e.

DropDownListTest.SelectedIndex = 12;

Hope this saves some of your time. Happy Coding.