c# - How to check null on passed PowerShell argument

Question

Below is the C# code where I am creating this powershell cmdlt which has two argument MaxAge and ContinuationToken. I would like

1. MaxAge to be mandatory when Continuation token is not provided.
2. ContinuationToken if provided to be mandatory and MaxAge to be optional.
3. ContinuationToken cannot be null whenever it is provided without MaxAge and it should be prompted to user to provide that value.

I am able to achieve my 1st two scenario but unable to achieve the last one. Below is my pwshell cmdlet and my C# code. Please advice:

[Cmdlet(VerbsCommon.Get, "ChangedRecordings", DefaultParameterSetName = GetChangedRecordingsCmd.ParamSetCloud)]
public class GetChangedRecordingsCmd : PwshCmd

{
protected const string ParamSetCloud = "Cloud";
protected const string ParamSetFile = "File";

[Parameter(Mandatory = true,
Position = 0, ParameterSetName = ParamSetCloud,
ValueFromPipeline = true,
ValueFromPipelineByPropertyName = true, HelpMessage = "Provide the maximum age of recordings from the change feed")]
[Parameter(Mandatory = false, ParameterSetName = ParamSetFile)]
public TimeSpan MaxAge { get; set; }

[Parameter(Position = 1,Mandatory = true,
ValueFromPipeline = true, ParameterSetName = ParamSetFile,
ValueFromPipelineByPropertyName = true, HelpMessage = "Provide a continuation token for the change feed")]

public string ContinuationToken { get; set; }

protected override void BeginProcessing()
{
if (ParameterSetName.Equals(ParamSetFile) && string.IsNullOrWhiteSpace(ContinuationToken))
{
WriteWarning("Continuation Token can't be null. Please pass a valid Continuation Token");
}
}

protected override void ProcessRecord()
{
//My opeartions
WriteObject(new { ContinuationToken = result, Recordings = recs });
}

Corresponding Powershell Cmndlet(Token File path contains the Continuation token. As per above scenario that can be blank and I need to handle this in C#)

param (
  $TokenFilePath = 'C:UsersDesktopContinuationToken.txt',
  $MaxAge = '2'
)
$existingToken = Get-Content -Path $TokenFilePath
$recordings = Get-ChangedRecordings -ContinuationToken $existingToken -MaxResults 10
Write-Host $recordings.Recordings.Count
$recordings.ContinuationToken|Set-Content -Path $TokenFilePath
$recs = $recordings.Recordings

Answer 1

For this, you'll probably want to use the ValidateNotNull attribute:

[ValidateNotNull()]
[Parameter(Position = 1,Mandatory = true,
ValueFromPipeline = true, ParameterSetName = ParamSetFile,
ValueFromPipelineByPropertyName = true, HelpMessage = "Provide a continuation token for the change feed")]
public string ContinuationToken { get; set; }