Passing agruments to another program in Powershell - powershell

Question

I am trying to write a script that will parse a directory, find the 3 newest files of a given extension among other files with the same name but different extension and then pass the output to a command line program.
So far I have:
$file = dir test -Exclude *.md5 | sort -prop LastWriteTime | select fullname -last 3
\path\program argumentA $file[0] argumentB "argumentC"
\path\program argumentA $file[1] argumentB "argumentC"
\path\program argumentA $file[2] argumentB "argumentC"
Not sure if it matters a whole lot but argument2 needs to be in quotes for the program to use it. I will eventually either have the output written to a file or, preferably use the output in another line of code described in the bonus question.
The problem I am having is that when I use $file[0] as an argument nothing happens. When I use the file name stored in $file[0] instead of using the variable, it runs, but does not pass argumentB or argumentC. If I run it from cmd I get the proper output.
My questions are:
How do I get the program to recognize a variable?
How do I get the program to recognize the other arguments?
Bonus question:
The program will output a list of files. How do I delete everything in the given folder except for the files listed in the output of the program?

Answer 1

Try
$($file[0])
I find that I often have to wrap things in "magical" dollar sign brackets to get them working in PowerShell.

Answer 2

Since you are piping to Select-Object and selecting the FullName property you have 3 objects of type System.Management.Automation.PSCustomObject with a FullName property. You should access the FullName property on the object when using it as a command line argument for your program like this:
$file = dir test -Exclude *.md5 | sort -prop LastWriteTime | select fullname -last 3
\path\program argumentA $file[0].FullName argumentB "argumentC"
If you remove the fullname property from the Select-Object cmdlet Powershell will not create a PSCustomObject and you can access all properties and methods from the original System.IO.FileInfo object.
This is how you can see the difference:
PsCustomObject
$file = dir test -Exclude *.md5 | sort -prop LastWriteTime | select fullname -last 3
$file[0] | gm
System.IO.FileInfo
$file = dir test -Exclude *.md5 | sort -prop LastWriteTime | select -last 3
$file[0] | gm

Answer 3

I would actually use a loop with this to avoid duplication. (That way you'll avoid copy-and-paste errors and simplify the problem, especially if you add error-handling or anything else later).
$files = dir test -Exclude *.md5 | sort -prop LastWriteTime |
select fullname -last 3
foreach ($file in $files) {
& \path\program argumentA $file argumentB "argumentC"
}
As PS treats collections the same as normal objects anyway, this works even if you only select 1 object.