C# Web app "The name 'CheckBox2' does not exist in the current context"

So I am adding check boxes to an HTML page. However, when I try to reference my CheckBox2, I get the error. Here is the checkbox in my html. Default.aspx:

<asp:CheckBox ID="CheckBox1" runat="server" Text="Yes" OnCheckedChanged="CheckBox1_CheckChanged" />

This is no problem at the moment, however the error is thrown here in my "C#" Default.aspx.cs:

protected void CheckBox1_CheckChanged(object source, System.Web.UI.WebControls.DataListCommandEventArgs e)
    {
        CheckBox2.Checked = false;
        CheckBox3.Checked = false;
        CheckBox4.Checked = false;
        CheckBox5.Checked = false;

    }

Apparently the check boxes dont exist in the current context, also, double clicking them did not create the event handler, I had to do so myself. Ive looked at many other similar questions and the answers helped everyone else but still none of them have fixed this problem for me