Get-child item Lastwritetime is not returning properly

Question

I am trying to get the CreationTime and Lastwritetime using get-childitem command and comparing it with the same assembly time to check if there is any difference for every 5 minutes.

Strangely there is a difference in milliseconds for every time when I fetch the records if the assembly did not change and it is causing the compare-object to fail.

Is there any way to fix this, other than converting the time to string format?

below is the command to fetch the details of the Assemblies

Get-ChildItem -Path 'C:\Windows\Microsoft.NET\assembly\GAC_MSIL' -include *.dll -Recurse|select @{Label=”AssemblyName”;Expression={$_.Name}},@{Label=”Size”; Expression={$_.Length}},@{label="Version";expression={if($_.versioninfo.productversion -eq $null) { 
              "NULL"
            } else { 
              $_.versioninfo.productversion
            }
          }},@{Label=”CreationTime”; Expression={$_.CreationTime}},@{Label=”LastWriteTimeUtc”; Expression={$_.LastWriteTimeUtc}}

and running this command after 5 minutes and comparing with compare-object

compare-object -$oldobject $newObject -property Name,Lastwritetime

What do you mean? What causes `Compare-Object` to fail? You need to post a specific and concrete example and explain what you want to do. (Remember that nobody can see your screen. You must explain all details. Don't make people guess.) — Bill_Stewart, Jan 12 '18 at 22:58
One thing to note is that you have `$.somepropertyname` in many places in your code, which should be replaced with `$_.somepropertyname`. Also note that Compare-Object will fail a comparison if the value is $NULL. You may want to change your results in those instances to an empty string to avoid the cmdlet breaking, or adding is a `try{} catch [Exception]{}` block to handle instances where you have null results — trebleCode, Jan 13 '18 at 00:38

wp78de · Accepted Answer · 2018-01-13T19:44:23.077

I do not know the exact reasons why there is a difference of some milliseconds (most probably an inaccurate floating point number).

The usual solution for a problem like this is to use an epsilon value in your comparisons (pretty common in game programming among others).

Set-Variable -name EPSILON -value ([double]0.00001) #-option Constant
function Is-Same {
    Param(
    [Parameter(Mandatory = $true)] [double]$a,
    [Parameter(Mandatory = $true)] [double]$b
    )
    #return $a - $b;
    return [math]::abs($a - $b) -le $EPSILON;
}

Is-Same 1.000001 1.0000
Is-Same 1.00001 1.0000
Is-Same 1.0001 1.0000

True
False
False

Adjust the epsilon value as needed. (adhering numerical limits)

Get-child item Lastwritetime is not returning properly

1 Answers1