Using Dynamic Parameters

Most PowerShell functions use static parameters. They are defined in a param() block and are always present. A little-known fact is that you can also add dynamic parameters programmatically on the fly. The big advantage of dynamic parameters is that you are completely free as to when they should appear, and what kind of values they should accept. The drawback is that you need to use a lot of low level code to “program” the parameter attributes.

Here is a sample function. It has only one static parameter called “Company”. Only when you choose a company will the function add a new dynamic parameter called “Department”. The new and dynamic -Department parameter exposes a list of valid values depending on which company was chosen. In essence, the -Department parameter is assigned an individual ValidateSet attribute based in the chosen company:

function Test-Department
{
    [CmdletBinding()]
    param
    (
        [Parameter(Mandatory=$true)]
        [ValidateSet('Microsoft','Amazon','Google','Facebook')]
        $Company
    )

    dynamicparam
    {
        # this hash table defines the departments available in each company
        $data = @{
            Microsoft = 'CEO', 'Marketing', 'Delivery'
            Google = 'Marketing', 'Delivery'
            Amazon = 'CEO', 'IT', 'Carpool'
            Facebook = 'CEO', 'Facility', 'Carpool'
        }

        # check to see whether the user already chose a company
        if ($Company)
        {
            # yes, so create a new dynamic parameter
            $paramDictionary = New-Object -TypeName System.Management.Automation.RuntimeDefinedParameterDictionary
            $attributeCollection = New-Object -TypeName System.Collections.ObjectModel.Collection[System.Attribute]

            # define the parameter attribute
            $attribute = New-Object System.Management.Automation.ParameterAttribute
            $attribute.Mandatory = $false
            $attributeCollection.Add($attribute)

            # create the appropriate ValidateSet attribute, listing the legal values for
            # this dynamic parameter
            $attribute = New-Object System.Management.Automation.ValidateSetAttribute($data.$Company)
            $attributeCollection.Add($attribute)

            # compose the dynamic -Department parameter
            $Name = 'Department'
            $dynParam = New-Object -TypeName System.Management.Automation.RuntimeDefinedParameter($Name,
            [string], $attributeCollection)
            $paramDictionary.Add($Name, $dynParam)

            # return the collection of dynamic parameters
            $paramDictionary
        }
    }

    end
    {
        # take the dynamic parameters from $PSBoundParameters
        $Department = $PSBoundParameters.Department
    
        "Chosen department for $Company : $Department"
    }
}

Check it out! When you type Test-Department in your editor of choice, initially there is only one parameter: -Company. Once you select one of the four companies available, a second parameter, -Department, is available and shows the departments available for the chosen company.

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