How count that how many checkbox is checked

You might think about whether your best strategy is to do something immediately whenever a CheckBox's 'CheckState Property is changed compared to taking deferred action based on reading the current state of a group of CheckBoxes. You often find the "deferred action" strategy used in configuration user-interfaces in combination with an "Apply" button.

A slight variation using 'OfType<> and Linq:

public int HowManyChecked(Control container)
{
    return container.Controls.OfType<CheckBox>().Where(cbx => cbx.Checked).Count();
}

Note: this will not throw an error if there are no CheckBoxes. 'OfType<> requires NET FrameWork 4.5, or later.

However, it seems logical to me that you would want to do something based on which CheckBoxes are checked; so:

public IEnumerable<CheckBox> GetCbxChecked(Control container)
{
    return container.Controls.OfType<CheckBox>().Where(cbx => cbx.Checked);
}

A use-case example:

private void button1_Click(object sender, EventArgs e)
{
    var chkd = GetCbxChecked(panel1);

    if(chkd.Count() > 0)
    {
        foreach (var cbx in chkd)
        {
            switch (cbx.Name)
            {
                case "checkBox1":
                    break;
                case "checkBox2":
                    break;
                case "checkBox3":
                    break;
            }
        }
    }
}

return container.Controls.OfType<CheckBox>().Where(cbx => cbx.Checked).Count();

return container.Controls.OfType<CheckBox>().Count(cbx => cbx.Checked);