"Maximum-Count of Selected Items"-Validator for ListBox

i have asked myself if there is an e开发者_如何学Pythonasy way to check if a ListBox has a maximum of 5 selected items. There must be at least one and at most 5 items selected.

Do i need a CustomValidator with server-side validation?

Many thanks in advance...

You can do it with a CustomValidator routine.

<asp:CustomValidator ID="ListBox5ItemsValidator" runat="server"
    OnServerValidate="ListBox5ItemsValidator_ServerValidate"
    ClientValidationFunction="ListBox5ItemsValidator_ClientValidate"
    ControlToValidate="MyListBox">
</asp:CustomValidator>

Server-side code:

protected void ListBox5ItemsValidator_ServerValidate(
        object source, ServerValidateEventArgs args) {

    int selectionCount = 0;
    foreach (ListItem item in MyListBox.Items) {
        if (item.Selected) selectionCount++;
    }
    args.IsValid = (selectionCount >= 1 && selectionCount <= 5);
}

Client-side code:

function ListBox5ItemsValidator_ClientValidate(sender, args) {
    var selectionCount = $('#<% =MyListBox.ClientID %> option:selected').length;
    args.IsValid = (selectionCount >= 1 && selectionCount <= 5);
};

Replace MyListBox with the actual name of your ListBox that you want to validate. If your ListBox is contained inside of other container controls, you may need a little more work to reference the control both on the server and client side. For instance, if it is contained in a FormView control called FormView1, you would use

ListBox MyListBox = (ListBox)FormView1.FindControl("MyListBox");