Passing arguments to a function

I found this link for creating vss Shadow copies under Windows. But the code uses fixed strings for the parameters:

function createVssSnapshot{
[cmdletbinding()]
param(
    [string]$targetVolume="C:\",
    [string]$accessPath='C:\vssSnapshot',
    [bool]$openSnapshotAfter=$true
)
[..]

I'd like to modify it in order to be more flexible:

function [String]createVssSnapshot{
[cmdletbinding()]
param(
    [String]$targetVolume,
    [String]$accessPath,
    [Bool]$openSnapshotAfter
)
[..]

and call it using

$result = createVssSnapshot("C:\", "C:\vssSnapshot", $false)

But I get the following error:

createVssSnapshot : Die Argumenttransformation für den Parameter "targetVolume" kann nicht verarbeitet werden. Der Wert kann nicht in den Typ "System.String" konvertiert werden.
In F:\Powershell_4_Pure\Object_based.ps1:143 Zeichen:28
+ $result = createVssSnapshot("C:\", "C:\vssSnapshot", $false)
+                            ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+ CategoryInfo          : InvalidData: (:) [createVssSnapshot], ParameterBindingArgumentTransformationException
+ FullyQualifiedErrorId : ParameterArgumentTransformationError,createVssSnapshot

Sorry for German error message, but it seems targetVolume isn't of type System.String.

What am I missing here?

For future questions: How can I modify PowerShell ISE to have english error messages?

In PowerShell, parameter arguments are passed to command by name or position, but arguments for separate parameters are NOT separated by comma (unlike many C-like languages).

The correct way to pass the arguments shown would be:

$result = createVssSnapshot "C:\" "C:\vssSnapshot" -openSnapshotAfter:$false

I'd strongly suggest changing the type constraint on the $openSnapshotAfter parameter to a [switch] instead of a [bool] - it will always default to $false and you can then specify it by just doing -openSnapshotAfter instead of -openSnapshotAfter:$true.

Since you'll no longer be providing default values for the first 2 parameters, I'd also recommend marking those Mandatory - this way PowerShell will refuse to een attempt executing your function if the caller doesn't pass arguments for those:

function New-VssSnapshot {
  [CmdletBinding()]
  param(
    [Parameter(Mandatory)]
    [string]$TargetVolume,

    [Parameter(Mandatory)]
    [string]$AccessPath,

    [switch]$OpenSnapshotAfter
  )

  # ...
}