Checkboxlist validation inside the RadListView using javascript

Let's analyze...

When CheckBoxList is rendered in Browser, it will be a Table having many CheckBox inside one one td like...

<table class="horizontalListbox" id="Listbox_Permission">
    <tbody>
        <tr>
            <td>
                <input type="checkbox" value="1" name="Listbox_Permission$0" id="Listbox_Permission_0">
                <label for="Listbox_Permission_0">Apples</label>
            </td>
            <td>
                <input type="checkbox" value="2" name="Listbox_Permission$1" id="Listbox_Permission_1">
                <label for="Listbox_Permission_1">Oranges</label>
            </td>
            <td>
                <input type="checkbox" value="3" name="Listbox_Permission$2" id="Listbox_Permission_2">
                <label for="Listbox_Permission_2">Mangoes</label>
            </td>
        </tr>
    </tbody>
</table>

Now, let's add OnClientClick attribute to the Button to call a JavaScript function...

<asp:Button runat="server" ID="Button1" Text="Submit" OnClick="OnButton1_Click" OnClientClick="return someJavaScriptFunction();" />

After that, it will render as...

<input type="submit" id="Button1" onclick="return someJavaScriptFunction();" value="Submit" name="Button1">

So, what's Next?

In that function, we will just try to fetch all the CheckBoxes and try to loop through each of them to see if anyone is checked or not. If checked, we will return true and show one alert.

function someJavaScriptFunction() {
    var isAnyCheckBoxChecked = false;

    // Let's get all the CheckBoxes inside the CheckBoxList
    var checkBoxes = document.getElementById("Listbox_Permission").getElementsByTagName("input");
    // For jsfiddle demo I have directly used the ID, otherwise you might have to use ClientID like below...
    // document.getElementById("<%= Listbox_Permission.ClientID %>").getElementsByTagName("input");

    // Now let's Loop through the Children.
    for (var i = 0; i < checkBoxes.length; i++) {
        if (checkBoxes[i].type == "checkbox") {
            if (checkBoxes[i].checked) {
                // If current CheckBox is checked, then return true.
                isAnyCheckBoxChecked = true;
                alert("Atleast one CheckBox is checked");
                break;
            }
        }
    }

    if (!isAnyCheckBoxChecked) {
        alert("No CheckBox is Checked.");
    }

    return isAnyCheckBoxChecked;
}

Please read the Comments inside code to know what is happening inside it.

Demo

Check the Demo. See how it is working. Customize it according to your requirements.

[Demo] CheckBoxList Validation in ASP.NET[^]