Ensures values match the case given in validateset by correcting to match instead of throwing exceptions. Trick thanks to MKlement: https://stackoverflow.com/a/42699260/361842

Convert-StringToValidateSetParameterCase.ps1

Function Convert-StringToValidateSetParameterCase {
    Param (
        [Parameter(Mandatory)]
        [System.Management.Automation.InvocationInfo]$InvocationInfo
        ,
        [Parameter(Mandatory)]
        [string]$ParameterName
        ,
        [Parameter(Mandatory)]
        [AllowEmptyString()]
        [AllowNull()]
        [string]$ParameterValue
    )
    if ($ParameterValue) {
        (
            $InvocationInfo.MyCommand.Parameters[$ParameterName].Attributes |
            Where-Object { $_ -is [ValidateSet] }
        ).ValidValues | Where-Object{$_ -eq $ParameterValue};
    } else {
        $ParameterValue
    }
}

# Example

Function Test-CorrectingParameterCase {
    Param (
        [Parameter(Mandatory)]
        [ValidateSet('One', 'Two', 'Three', IgnoreCase = $true)]
        [string]$Demo
    )
    Convert-StringToValidateSetParameterCase -InvocationInfo $MyInvocation -ParameterName 'Demo' -ParameterValue $Demo
}
Test-CorrectingParameterCase two
# Outputs: Two