function Get-RSFileHash {
<#
.Synopsis
   Returns an MD5 filehash when given a file path
.DESCRIPTION
   Returns an MD5 filehash when given a file path
.EXAMPLE
   Get-RSFileHash -Filename c:\temp\filetohash.txt
.EXAMPLE
   Get-ChildItem c:\temp\*.txt | get-rsfilehash
#>

    [CmdletBinding()]
    [OutputType([string])]
    Param
    (
        [Parameter(Mandatory=$true,
                   ValueFromPipeline=$true,
                   Position=0,
                   ParameterSetName="String")]
        [string[]]$Path,
        [Parameter(Mandatory=$true,
                   ValueFromPipeline=$true,
                   Position=0,
                   ParameterSetName="FileInfo")]
        [IO.FileInfo[]]$InputObject
    )

    BEGIN {"------------"}
    Process
    {
        if($PSCmdlet.ParameterSetName -eq "string") {
            $file = $Path
        } else {
            $file = $InputObject.FullName
        }
        foreach($file in $file){
            if (test-path -Path $file -PathType Leaf) {
                try {
                    $md5 = New-Object -TypeName System.Security.Cryptography.MD5CryptoServiceProvider
                    $stream = [System.IO.File]::Open($file, [System.IO.FileMode]::Open, [System.IO.FileAccess]::Read)
                    $hash = [System.BitConverter]::ToString($md5.ComputeHash($stream))
                    $stream.Close()
                    Write-Debug "$file - $hash"

                    Write-Output $hash.Replace('-',$null)
                }
                catch {
                    throw "Unable to generate hash for $file - $($_)"
                }
            }
        }
    }
}

$someFilePath = "C:\ps\test.txt","C:\ps\test.tx3"
$files =  get-childitem -Path $someFilePath
$files | Get-RSFileHash
Get-RSFileHash -path $someFilePath[0]
Get-RSFileHash -path $someFilePath
Get-RSFileHash -InputObject $files[0]
Get-RSFileHash -InputObject $files
$someFilePath | Get-RSFileHash