This example shows how to limit the number of Listitems to be selected in the ListBox control.

ASPX

 

<html xmlns="http://www.w3.org/1999/xhtml" >
<head runat="server">
    <title>Untitled Page</title>
        <script type="text/javascript" language="javascript">
        function GetCurrentItemsSelected(obj)
        {
          var o = obj;
          document.getElementById('<%= HiddenField1.ClientID %>').value = o;
        }
    </script>
</head>
<body>
    <form id="form1" runat="server">
        <asp:ListBox ID="ListBox1" runat="server" AutoPostBack="True" Height="109px" SelectionMode="Multiple" 
                           OnSelectedIndexChanged="ListBox1_SelectedIndexChanged" Width="141px">
        <asp:ListItem onclick ="GetCurrentItemsSelected('0');">A</asp:ListItem>
        <asp:ListItem onclick ="GetCurrentItemsSelected('1');">B</asp:ListItem>
        <asp:ListItem onclick ="GetCurrentItemsSelected('2');">C</asp:ListItem>
        <asp:ListItem onclick ="GetCurrentItemsSelected('3');">D</asp:ListItem>
        <asp:ListItem onclick ="GetCurrentItemsSelected('4');">E</asp:ListItem>
        <asp:ListItem onclick ="GetCurrentItemsSelected('5');">F</asp:ListItem>
        </asp:ListBox>
        <asp:HiddenField ID="HiddenField1" runat="server" />
    </form>
</body>
</html>

 
 

RELEVANT CODES:

 

protected void Page_Load(object sender, EventArgs e)
{
        if (!Page.IsPostBack)
        {
            ListBox1.ClearSelection();
        }
}

protected void ListBox1_SelectedIndexChanged(object sender, EventArgs e)
{
        int count = 0;
        int limit = 0;
        int lastSelectedIndex = 0;
        for (int i = 0; i < ListBox1.Items.Count; i++)
        {
            if (ListBox1.Items[i].Selected)
            {
                if (ViewState["CountLimit"] != null)
                {
                    count++;
                    ViewState["CountLimit"] = count;
                    limit = (int)ViewState["CountLimit"];
                }
                else
                {
                    count++;
                    ViewState["CountLimit"] = count;
                }
                if (limit > 3) // Selected Item Limit is up to 3
                {
                    Page.ClientScript.RegisterStartupScript(this.GetType(), "ShowBox", "alert('You are only allowed to Select 3 items!');", true);
                    //Get the last selected items selected when the items selected exceeds in 3
                    lastSelectedIndex = int.Parse(HiddenField1.Value.ToString());
                    //Unselect the 4th items selected
                    ListBox1.Items[lastSelectedIndex].Selected = false;
                }
            }

}
 
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