How to count the checkboxes in a asp.net form

ASP.NET:

<asp:Panel ID="pnlFilter" runat="server">
    <div class="dvFilter">
        <input type="checkbox" id="cb01" checked="checked" />
        <label for="cb01">All</label>
    </div>
    <div class="dvFilter">
        <input type="checkbox" id="cb02" checked="checked" />
        <label for="cb02">None</label>
    </div>
</asp:Panel>

C#:

foreach (Control item in this.form1.Controls)
{
    System.Web.UI.HtmlControls.HtmlInputCheckBox _cbx = item as System.Web.UI.HtmlControls.HtmlInputCheckBox;
    if (_cbx != null)
    {
        if (_cbx.Checked)
        {
            //Do something: 
            Response.Write(_cbx.Name + " was checked.<br />");
        }
    }

}

I am getting a null value for the _cbx variable.

How can I update it so I am able to get the ID of all the checked input type checkboxes.

I tried this answer: Count the number of TextBoxes and CheckBoxes in a form but didn't work for me either.