DynamicParam.ps1

#Source: Reddit (http://www.reddit.com/r/PowerShell/comments/2oqk4k/using_validateset_with_exceptions/)

function test {
    # You have to use cmdletbinding for the dynamic parameter to work
    [CmdletBinding()]
    Param()
    # The cool thing about this is that you can actually get really good tab completion / intellisense from doing it this way...
    DynamicParam
    {
        [string[]]$ValidServices = get-content $PSScriptRoot\services.txt

        # Create a parameter attribute for the parameter..
        $paramAttribute = [System.Management.Automation.ParameterAttribute]::new()
        $paramAttribute.ParameterSetName = "__AllParameterSets"
        $paramAttribute.Mandatory = $true

        # Add the validateset attribute for the parameter with the values from services... this is the really cool part
        $validatesetAttribute = [System.Management.Automation.ValidateSetAttribute]::new($ValidServices)
        $validatesetAttribute.IgnoreCase = $true

        # Add the attributes to the collection of attributes
        $attributeCollection = [System.Collections.ObjectModel.Collection[System.Attribute]]::new()
        $attributeCollection.Add($paramAttribute)
        $attributeCollection.Add($validatesetAttribute)

        # Define the parameter itself
        $dynamicParam = [System.Management.Automation.RuntimeDefinedParameter]::new("Service", [string], $attributeCollection)

        # Create the parameter collection and return it
        $paramDictionary = [System.Management.Automation.RuntimeDefinedParameterDictionary]::new()
        $paramDictionary.Add("Service", $dynamicParam)
        return $paramDictionary
    }

    # If you use the DynamicParam block, you have to put your actual code inside a begin/process/end block
    End 
    {
        if ($PSBoundParameters.ContainsKey("Service"))
        {
            $Service = $PSBoundParameters["Service"]
        }

        $Service
    }
}

test -service asdf