set all Labels Font before opening the Form

set all Labels Font before opening the Form

before opening the form I used following code to check if its label then
change the font
foreach (Label ctl in frm.Controls)
{
ctl.Font = usefontgrid;
}
but on first line return error because it check other control types such
as textbox or button,etc... how can I check if the object is only label
then go to for each?