Dropdownlist is not accepting new items when added dynamically

[jagdeep gupta]

On adding items in it ,shows an error: "Dropdownlist does not allow multiple selection."
But when i tries to print the selected item it shows null exception.

[C Sharper]

Could you include some of your code as an example?

[jagdeep gupta]

 
using System;
using System.Collections;
using System.Configuration;
using System.Data;
using System.Linq;
using System.Web;
using System.Web.Security;
using System.Web.UI;
using System.Web.UI.HtmlControls;
using System.Web.UI.WebControls;
using System.Web.UI.WebControls.WebParts;
using System.Xml.Linq;
 
public partial class Expt : System.Web.UI.Page
{
    protected void Page_Load(object sender, EventArgs e)
    {
 
 
    }
    protected void Bttnadd_Click(object sender, EventArgs e)
    {
 
       DropDownList11.Items.Add(ListBox1.SelectedItem);
        ListBox1.Items.Remove(ListBox1.SelectedItem);
 
 
    }
 

[C Sharper]

could you provide the page too, so I can see what the code behind is working with?

[jagdeep gupta]

Please check the attached files

[C Sharper]

By default the first item of a dropdown is selected. try setting the selectedindex of the dropdown to -1 before adding the new item...

protected void bttnaddck_Click(object sender, EventArgs e)
{
DropDownList1.SelectedIndex = -1;
DropDownList1.Items.Add(ListBox1.SelectedItem);
ListBox1.Items.Remove(ListBox1.SelectedItem);
}

[jagdeep gupta]

U try to print the selected item it will show Null Exception error why so?