Vista - Test for parameters in functions

param($p1, $p2, $p3, $p4)
function Usage {
@'
Syntax: powershell.exe .\testhello.ps1 p1 p2 p3 p4
-parameter p1, p2, p3, p4 are mandatory
'@
exit 1
}

# test for parameters function
if (!$p1)
{
cls
usage
}

==================================================
How can i enforce parameter p1 p2 p3 p4 needs to be there ?

I don't want to write multiple function for testing p1 to p4

Just use following:

$p1 = $(Throw "-p1 is mandatory parameter")

Martin