Moving value from one ListBox to another ListBox using C#.NET

First the ListBox declaration set the SelectionMode=”Multiple”.

For more details, you can visit on below link.
www.code-sample.com/2013/06/add-remove-items-from-one-listbox-to.html

   <tr>       

<td class="call2">

  <asp:ListBox ID=" ListBox1" runat="server" SelectionMode="Multiple"></asp:ListBox>

</td>

<td class="call1">

  <asp:Button ID="btnadd" runat="server" Text=">>" OnClick="btnadd_Click"    />

                    <br />

 <asp:Button ID="btnsubtract" runat="server" Text="<<" OnClick="btnsubtract_Click" />

 </td>

<td class="call2">

  <asp:ListBox ID=" ListBox2" runat="server" SelectionMode="Multiple"></asp:ListBox>

  </td>

    </tr>

For adding items in the list

   protected void btnadd_Click(object sender, EventArgs e)

    {

        for (int i = ListBox1.Items.Count - 1; i >= 0; i--)

        {

            if (ListBox1.Items[i].Selected)

            {

                ListBox2.Items.Add(ListBox1.Items[i]);

                ListBox1.Items.Remove(ListBox1.Items[i]);

            }

        }

    }

 For removing items in the list

    protected void btnsubtract_Click(object sender, EventArgs e)

    {

        for (int i = ListBox2.Items.Count - 1; i >= 0; i--)

        {

            if (listBoxSelectedEmp.Items[i].Selected)

            {

                ListBox1.Items.Add(ListBox2.Items[i]);

                ListBox2.Items.Remove(ListBox2.Items[i]);

            }

        }

    }