.net - PowerShell - Why "Divide By Zero Exception" is not being Caught?

Question

On my Machine each one of the following code snippets throws and exception instead of printing to the standard output "1" and "2" Why the exception is not being Caught?

try {
    [int]$a = 1/0
}
catch {
    write 1
}
finally {
    write 2
}

try {
    [int]$a = 1/0
}
catch [System.Exception] {
    write 1
}
finally {
    write 2
}

Answer 1

As you are using constants, the interpreter tries to precompute the result and fails with a division by zero error. Your code does not even get executed so there's nothing to trap.

You can verify this for yourself by changing your code to use variables, forcing it to be executed.

try {
    $divisor = 0
    [int]$a = 1/$divisor
}
catch {
    write 1
}
finally {
    write 2
}

From Windows PowerShell in Action (p.257)

The example here uses 1/$null. The reason for doing this instead of simply 1/0 is because the PowerShell interpreter does something called constant expression folding.

It looks at expressions that contain only constant values. When it sees one, it evaluates that expression once at compile time so it doesn’t have to waste time doing it again at runtime.

This means that impossible expressions, such as division by zero, are caught and treated as parsing errors. Parsing errors can’t be caught and don’t get logged when they’re entered interactively, so they don’t make for a good example. (If one script calls another script and that script has one of these errors, the calling script can catch it, but the script being parsed cannot.)