In a custom PowerShell module, I have at the top of my module definition this code:

Update-FormatData -AppendPath (Join-Path $psscriptroot "*.ps1xml")

This is working fine as all .ps1xml files are loaded.

However, the module is sometimes loaded using Import-Module MyModule -Force (actually, this is in the install script of the module).

In this case, the call to Update-FormatData fails with this error :

Update-FormatData : There were errors in loading the format data file:
Microsoft.PowerShell, c:\pathto\myfile.Types.ext.ps1xml : File skipped because it was already present from "Microsoft.PowerShell".
At line:1 char:18
+ Update-FormatData <<<<  -AppendPath "c:\pathto\myfile.Types.ext.ps1xml"
    + CategoryInfo          : InvalidOperation: (:) [Update-FormatData], RuntimeException
    + FullyQualifiedErrorId : FormatXmlUpateException,Microsoft.PowerShell.Commands.UpdateFormatDataCommand

Is there a way to safely call this command?

I know I can call Update-FormatData with no parameters, and it will update any known .ps1xml file, but this would work only if the file has already been loaded.

Can I list somewhere the loaded format data files?

Here is a bit of background:

I'm building a custom module that is installed using a script. The install script looks like :

[CmdletBinding(SupportsShouldProcess=$true,ConfirmImpact="High")]
param()
process {
    $target = Join-Path $PSHOME "Modules\MyModule"
    if ($pscmdlet.ShouldProcess("$target","Deploying MyModule module"))
    {
        if(!(Test-Path $target)) { new-Item -ItemType Directory -Path $target | Out-Null }
        get-ChildItem -Path (Split-Path ((Get-Variable MyInvocation -Scope 0).Value).MyCommand.Path) | copy-Item -Destination $target -Force

        Write-Host -ForegroundColorWhite @"

The module has been installed. You can import it using :

    Import-Module MyModule

Or you can add it in your profile ($profile)
"@

        Write-Warning "To refresh any open PowerShell session, you should run ""Import-Module MyModule -Force"" to reload the module"

        Import-Module MyModule -Force

        Write-Warning "This session has been refreshed."
    }
}

MyModule defines, as first statement, this line :

Update-FormatData -AppendPath (Join-Path $psscriptroot "*.ps1xml")

As I updated my $profile to always load this module, the Update-Path command has been called when I run the install script. In the install script, I force import the module, which be fire again the module, and then, the Update-Path call